/****************************************************************************
**
** Copyright (C) 2020 The Qt Company Ltd.
** Contact: https://www.qt.io/licensing/
**
** This file is part of the documentation of the Qt Toolkit.
**
** $QT_BEGIN_LICENSE:FDL$
** Commercial License Usage
** Licensees holding valid commercial Qt licenses may use this file in
** accordance with the commercial license agreement provided with the
** Software or, alternatively, in accordance with the terms contained in
** a written agreement between you and The Qt Company. For licensing terms
** and conditions see https://www.qt.io/terms-conditions. For further
** information use the contact form at https://www.qt.io/contact-us.
**
** GNU Free Documentation License Usage
** Alternatively, this file may be used under the terms of the GNU Free
** Documentation License version 1.3 as published by the Free Software
** Foundation and appearing in the file included in the packaging of
** this file. Please review the following information to ensure
** the GNU Free Documentation License version 1.3 requirements
** will be met: https://www.gnu.org/licenses/fdl-1.3.html.
** $QT_END_LICENSE$
**
****************************************************************************/

/*!
\page qtquick3d-custom.html
\title Programmable Materials, Effects, Geometry, and Texture data
\brief Custom materials, effects, geometry and texture data providers in Qt Quick 3D

While the built-in materials of Qt Quick 3D, \l DefaultMaterial and \l PrincipledMaterial,
allow a wide degree of customization via their properties, they do not provide
programmability on the vertex and fragment shader level. To allow that, the \l
CustomMaterial type is provided.

\table
\header
\li A model with PrincipledMaterial
\li With a CustomMaterial transforming the vertices
\row
\li \image quick3d-custom-mat1.jpg
\li \image quick3d-custom-mat2.jpg
\endtable

Post-processing effects, where one or more passes of processing on the color buffer are
performed, optionally taking the depth buffer into account, before the View3D's output is
passed on to Qt Quick, also exist in two varieties:
\list
\li a \l{Qt Quick 3D Effects QML Types}{set of built-in, pre-made effects}, such as blur,
depth of field, distortion, and others,
\li \c custom effects implemented by the application in form of fragment shader code and a
specification of the processing passes in an \l Effect object.
\endlist

\table
\header
\li Scene without effect
\li The same scene with a custom post-processing effect applied
\row
\li \image quick3d-custom-effect1.jpg
\li \image quick3d-custom-effect2.jpg
\endtable

In addition to programmable materials and post-processing, there are two types of data that is
normally provided in form of files (\c{.mesh} files or images such as \c{.png}):

\list

\li vertex data, including the geometry for the mesh to be rendered, texture coordinates,
normals, colors, and other data,

\li the content for textures that are then used as texture maps for the rendered
objects, or used with skybox or image based lighting.

\endlist

If they so wish, applications can provide such data from C++ in form of a QByteArray. Such
data can also be changed over time, allowing to procedurally generate and later alter the
data for a \l Model or \l Texture.

\table
\header
\li A grid, rendered by specifying vertex data dynamically from C++
\li A cube textured with image data generated from C++
\row
\li \image quick3d-custom-geom.jpg
\li \image quick3d-custom-tex.jpg
\endtable

These four approaches to customizing and making materials, effects, geometry, and textures
dynamic enable the programmability of shading and procedural generation of the data the
shaders get as their input. The following sections provide an overview of these
features. The full reference is available in the documentation pages for the respective
types:

\table
\header
\li Feature
\li Reference Documentation
\li Relevant Examples
\row
\li Custom materials
\li \l CustomMaterial
\li \l {Qt Quick 3D - Custom Shaders Example}, \l {Qt Quick 3D - Custom Materials
Example}
\row
\li Custom post-processing effects
\li \l Effect
\li \l {Qt Quick 3D - Custom Effect Example}
\row
\li Custom geometry
\li \l QQuick3DGeometry, \l{Model::geometry}
\li \l {Qt Quick 3D - Custom Geometry Example}
\row
\li Custom texture data
\li \l QQuick3DTextureData, \l{Texture::textureData}
\li \l {Qt Quick 3D - Procedural Texture Example}
\endtable

\section1 Programmability for Materials

Let's have a scene with a cube, and start with a default \l PrincipledMaterial and
\l CustomMaterial:

\table
\header
\li PrincipledMaterial
\li CustomMaterial
\row
\li
  \qml
  import QtQuick
  import QtQuick3D
  Item {
      View3D {
          anchors.fill: parent
          environment: SceneEnvironment {
              backgroundMode: SceneEnvironment.Color
              clearColor: "black"
          }
          PerspectiveCamera { z: 600 }
          DirectionalLight { }
          Model {
              source: "#Cube"
              scale: Qt.vector3d(2, 2, 2)
              eulerRotation.x: 30
              materials: PrincipledMaterial { }
           }
      }
  }
  \endqml
\li
  \qml
  import QtQuick
  import QtQuick3D
  Item {
      View3D {
          anchors.fill: parent
          environment: SceneEnvironment {
              backgroundMode: SceneEnvironment.Color
              clearColor: "black"
          }
          PerspectiveCamera { z: 600 }
          DirectionalLight { }
          Model {
              source: "#Cube"
              scale: Qt.vector3d(2, 2, 2)
              eulerRotation.x: 30
              materials: CustomMaterial { }
           }
      }
  }
  \endqml
\endtable

These both lead to the exact same result, because a \l CustomMaterial is effectively a \l
PrincipledMaterial, when no vertex or fragment shader code is added to it.

\image quick3d-custom-cube1.jpg

\note Properties, such as, \l{PrincipledMaterial::baseColor}{baseColor},
\l{PrincipledMaterial::metalness}{metalness},
\l{PrincipledMaterial::baseColorMap}{baseColorMap}, and many others, have no equivalent
properties in the \l CustomMaterial QML type. This is by design: customizing the material
is done via shader code, not by merely providing a few fixed values.

\section2 Our first vertex shader

Let's add a custom vertex shader snippet. This is done by referencing a file in the
\l{CustomMaterial::vertexShader}{vertexShader} property. The approach will be the same for
fragment shaders. These references work like \l{Image::source}{Image.source} or
\l{ShaderEffect::vertexShader}{ShaderEffect.vertexShader}: they are local or \c qrc URLs,
and a relative path is treated relative to the \c{.qml} file's location. The common
approach is therefore to place the \c{.vert} and \c{.frag} files into the Qt resource
system (\c qt_add_resources when using CMake) and reference them using a relative path.

In Qt 6.0 inline shader strings are no longer supported, neither in Qt Quick nor in Qt
Quick 3D. (make note of the fact that these properties are URLs, not strings) However, due
to their intrinsically dynamic nature, custom materials and post-processing effects in Qt
Quick 3D still provide shader snippets in source form in the referenced files. This is a
difference to \l ShaderEffect where the shaders are complete on their own, with no further
amending by the engine, and so are expected to be provided as pre-conditioned \c{.qsb}
shader packs.

\note In Qt Quick 3D URLs can only refer to local resources. Schemes for remote content
are not supported.

\note The shading language used is Vulkan-compatible GLSL. The \c{.vert} and \c{.frag}
files are not complete shaders on their own, hence being often called \c snippets. That is
why there are no uniform blocks, input and output variables, or sampler uniforms provided
directly by these snippets. Rather, the Qt Quick 3D engine will amend them as appropriate.

\table
\header
\li Change in main.qml, material.vert
\li Result
\row
  \li \qml
  materials: CustomMaterial {
      vertexShader: "material.vert"
  }
  \endqml
  \badcode
  void MAIN()
  {
  }
  \endcode
  \li \image quick3d-custom-cube1-small.jpg
\endtable

A custom vertex or fragment shader snippet is expected to provide one or more functions
with pre-defined names, such as \c MAIN, \c DIRECTIONAL_LIGHT, \c POINT_LIGHT, \c
SPOT_LIGHT, \c AMBIENT_LIGHT, \c SPECULAR_LIGHT. For now let's focus on \c MAIN.

As shown here, the end result with an empty MAIN() is exactly the same as before.

Before making it more interesting, let's look at an overview of the most commonly used
special keywords in custom vertex shader snippets. This is not the full list. For a full
reference, check the \l CustomMaterial page.

\table
\header
\li Keyword
\li Type
\li Description
\row
\li MAIN
\li
\li void MAIN() is the entry point. This function must always be present in a custom
vertex shader snippet, there is no point in providing one otherwise.
\row
\li VERTEX
\li vec3
\li The vertex position the shader receives as input. A common use case for vertex shaders
in custom materials is to change (displace) the x, y, or z values of this vector, by simply
assigning a value to the whole vector, or some of its components.
\row
\li NORMAL
\li vec3
\li The vertex normal from the input mesh data, or all zeroes if there were no normals provided.
As with VERTEX, the shader is free to alter the value as it sees fit. The altered value is then
used by the rest of the pipeline, including the lighting calculations in the fragment stage.
\row
\li UV0
\li vec2
\li The first set of texture coordinates from the input mesh data, or all zeroes if there
were no UV values provided. As with VERTEX and NORMAL, the value can altered.
\row
\li MODELVIEWPROJECTION_MATRIX
\li mat4
\li The model-view-projection matrix. To unify the behavior regardless of which graphics API
rendering happens with, all vertex data and transformation matrices follow OpenGL conventions
on this level. (Y axis pointing up, OpenGL-compatible projection matrix) Read only.
\row
\li MODEL_MATRIX
\li mat4
\li The model (world) matrix. Read only.
\row
\li NORMAL_MATRIX
\li mat3
\li The transposed inverse of the top-left 3x3 slice of the model matrix. Read only.
\row
\li CAMERA_POSITION
\li vec3
\li The camera position in world space. In the examples on this page this is \c{(0, 0, 600)}. Read only.
\row
\li CAMERA_DIRECTION
\li vec3
\li The camera direction vector. In the examples on this page this is \c{(0, 0, -1)}. Read only.
\row
\li CAMERA_PROPERTIES
\li vec2
\li The near and far clip values of the camera. In the examples on this page this is \c{(10, 10000)}. Read only.
\row
\li POINT_SIZE
\li float
\li Relevant only when rendering with a topology of points, for example because the
\l{QQuick3DGeometry}{custom geometry} provides such a geometry for the mesh. Writing to
this value is equivalent to setting \l{PrincipledMaterial::pointSize}{pointSize on a
PrincipledMaterial}.
\row
\li POSITION
\li vec4
\li Like \c gl_Position. When not present, a default assignment statement is generated
automatically using \c MODELVIEWPROJECTION_MATRIX and \c VERTEX. This is why an empty
MAIN() is functional, and in most cases there will be no need to assign a custom value to
it.
\endtable

Let's make a custom material that displaces the vertices according to some pattern. To
make it more interesting, have some animated QML properties, the values of which end up
being exposed as uniforms in the shader code. (to be precise, most properties are going to
be mapped to members in a uniform block, backed by a uniform buffer at run time, but Qt
Quick 3D conveniently makes such details transparent to the custom material author)

\table
\header
\li Change in main.qml, material.vert
\li Result
\row
  \li \qml
  materials: CustomMaterial {
     vertexShader: "material.vert"
     property real uAmplitude: 0
     NumberAnimation on uAmplitude {
         from: 0; to: 100; duration: 5000; loops: -1
     }
     property real uTime: 0
     NumberAnimation on uTime {
         from: 0; to: 100; duration: 10000; loops: -1
     }
  }
  \endqml
  \badcode
  void MAIN()
  {
      VERTEX.x += sin(uTime + VERTEX.y) * uAmplitude;
  }
  \endcode
  \li \image quick3d-custom-cube2-anim.gif
\endtable

\section2 Uniforms from QML properties

Custom properties in the CustomMaterial object get mapped to uniforms. In the above
example this includes \c uAmplitude and \c uTime. Any time the values change, the updated
value will become visible in the shader. This concept may already be familiar from \l
ShaderEffect.

The name of the QML property and the GLSL variable must match. There is no separate
declaration in the shader code for the individual uniforms. Rather, the QML property name
can be used as-is. This is why the example above can just reference \c uTime and \c
uAmplitude in the vertex shader snippet without any previous declaration for them.

The following table lists how the types are mapped:

\table
\header
\li QML Type
\li Shader Type
\li Notes
\row
\li real, int, bool
\li float, int, bool
\li
\row
\li color
\li vec4
\li sRGB to linear conversion is performed implicitly
\row
\li vector2d
\li vec2
\li
\row
\li vector3d
\li vec3
\li
\row
\li vector4d
\li vec4
\li
\row
\li matrix4x4
\li mat4
\li
\row
\li quaternion
\li vec4
\li scalar value is \c w
\row
\li rect
\li vec4
\li
\row
\li point, size
\li vec2
\li
\row
\li TextureInput
\li sampler2D
\li
\endtable

\section2 Improving the example

Before moving further, let's make the example somewhat better looking. By adding a rotated
rectangle mesh and making the \l DirectionalLight cast shadows, we can verify that the
alteration to the cube's vertices is correctly reflected in all rendering passes,
including shadow maps. To get a visible shadow, the light is now placed a bit higher on
the Y axis, and a rotation is applied to have it pointing partly downwards. (this being a
\c directional light, the rotation matters)

\table
\header
\li main.qml, material.vert
\li Result
\row \li \qml
import QtQuick
import QtQuick3D
Item {
    View3D {
        anchors.fill: parent
        environment: SceneEnvironment { backgroundMode: SceneEnvironment.Color; clearColor: "black" }
        PerspectiveCamera { z: 600 }
        DirectionalLight {
            y: 200
            eulerRotation.x: -45
            castsShadow: true
        }
        Model {
            source: "#Rectangle"
            y: -250
            scale: Qt.vector3d(5, 5, 5)
            eulerRotation.x: -45
            materials: PrincipledMaterial { baseColor: "lightBlue" }
        }
        Model {
            source: "#Cube"
            scale: Qt.vector3d(2, 2, 2)
            eulerRotation.x: 30
            materials: CustomMaterial {
                vertexShader: "material.vert"
                property real uAmplitude: 0
                NumberAnimation on uAmplitude {
                    from: 0; to: 100; duration: 5000; loops: -1
                }
                property real uTime: 0
                NumberAnimation on uTime {
                    from: 0; to: 100; duration: 10000; loops: -1
                }
            }
        }
    }
}
\endqml
\badcode
void MAIN()
{
    VERTEX.x += sin(uTime + VERTEX.y) * uAmplitude;
}
\endcode
\li \image quick3d-custom-cube3-anim.gif
\endtable

\section2 Adding a fragment shader

Many custom materials will want to have a fragment shader as well. In fact, many will want
only a fragment shader. If there is no extra data to be passed from the vertex to fragment
stage, and the default vertex transformation is sufficient, setting the \c vertexShader
property can be left out from the \l CustomMaterial.

\table
\header
\li Change in main.qml, material.frag
\li Result
\row \li \qml
materials: CustomMaterial {
    fragmentShader: "material.frag"
}
\endqml
\badcode
void MAIN()
{
}
\endcode
\li \image quick3d-custom-cube4.jpg
\endtable

Our first fragment shader contains an empty MAIN() function. This is no different than not
specifying a fragment shader snippet at all: what we get looks like what we get with a
default PrincipledMaterial.

Let's look at some of the commonly used keywords in fragment shaders. This is not the full
list, refer to the \l CustomMaterial documentation for a complete reference. Many of these
are read-write, meaning they have a default value, but the shader can, and often will want
to, assign a different value to them.

As the names suggest, many of these map to similarly named \l PrincipledMaterial
properties, with the same meaning and semantics, following the
\l{https://github.com/KhronosGroup/glTF/tree/master/specification/2.0#metallic-roughness-material}{metallic-roughness
material model}. It is up the custom material implementation to decide how these values
are calculated: for example, a value for BASE_COLOR can be hard coded in the shader, can
be based on sampling a texture, or can be calculated based on QML properties exposed as
uniforms or on interpolated data passed along from the vertex shader.

\table
\header
\li Keyword
\li Type
\li Description
\row
\li BASE_COLOR
\li vec4
\li The base color and alpha value. Corresponds to \l{PrincipledMaterial::baseColor}. The
final alpha value of the fragment is the model opacity multiplied by the base color
alpha. The default value is \c{(1.0, 1.0, 1.0, 1.0)}.
\row
\li EMISSIVE_COLOR
\li vec3
\li The color of self-illumination. Corresponds to
\l{PrincipledMaterial::emissiveColor}. The default value is \c{(0.0, 0.0, 0.0)}.
\row
\li METALNESS
\li float
\li \l{PrincipledMaterial::metalness}{Metalness} value in range 0-1. Default to 0, which
means the material is dielectric (non-metallic).
\row
\li ROUGHNESS
\li float
\li \l{PrincipledMaterial::roughness}{Roughness} value in range 0-1. The default value is
0. Larger values soften specular highlights and blur reflections.
\row
\li SPECULAR_AMOUNT
\li float
\li \l{PrincipledMaterial::specularAmount}{The strength of specularity} in range 0-1. The
default value is \c 0.5. For metallic objects with \c metalness set to \c 1 this value
will have no effect. When both \c SPECULAR_AMOUNT and \c METALNESS have values larger than
0 but smaller than 1, the result is a blend between the two material models.
\row
\li NORMAL
\li vec3
\li The interpolated normal in world space, adjusted for double-sidedness when face culling is disabled. Read only.
\row
\li UV0
\li vec2
\li The interpolated texture coordinates. Read only.
\row
\li VAR_WORLD_POSITION
\li vec3
\li Interpolated vertex position in world space. Read only.
\endtable

Let's make the cube's base color red:

\table
\header
\li Change in main.qml, material.frag
\li Result
\row \li \qml
materials: CustomMaterial {
    fragmentShader: "material.frag"
}
\endqml
\badcode
void MAIN()
{
    BASE_COLOR = vec4(1.0, 0.0, 0.0, 1.0);
}
\endcode
\li \image quick3d-custom-cube5.jpg
\endtable

Now strengthen the level of self-illumination a bit:

\table
\header
\li Change in main.qml, material.frag
\li Result
\row \li \qml
materials: CustomMaterial {
    fragmentShader: "material.frag"
}
\endqml
\badcode
void MAIN()
{
    BASE_COLOR = vec4(1.0, 0.0, 0.0, 1.0);
    EMISSIVE_COLOR = vec3(0.4);
}
\endcode
\li \image quick3d-custom-cube6.jpg
\endtable

Instead of having values hardcoded in the shader, we could also use QML properties exposed
as uniforms, even animated ones:

\table
\header
\li Change in main.qml, material.frag
\li Result
\row \li \qml
materials: CustomMaterial {
    fragmentShader: "material.frag"
    property color baseColor: "black"
    ColorAnimation on baseColor {
        from: "black"; to: "purple"; duration: 5000; loops: -1
    }
}
\endqml
\badcode
void MAIN()
{
    BASE_COLOR = vec4(baseColor.rgb, 1.0);
    EMISSIVE_COLOR = vec3(0.4);
}
\endcode
\li \image quick3d-custom-cube7-anim.gif
\endtable

Let's do something less trivial, something that is not implementable with a
PrincipledMaterial and its standard, built-in properties. The following material
visualizes the texture UV coordinates of the cube mesh. U runs 0 to 1, so from black to
red, while V is also 0 to 1, black to green.

\table
\header
\li Change in main.qml, material.frag
\li Result
\row \li \qml
materials: CustomMaterial {
    fragmentShader: "material.frag"
}
\endqml
\badcode
void MAIN()
{
    BASE_COLOR = vec4(UV0, 0.0, 1.0);
}
\endcode
\li \image quick3d-custom-cube8.jpg
\endtable

While we are at it, why not visualize normals as well, this time on a sphere. Like with
UVs, if a custom vertex shader snippet were to alter the value of NORMAL, the interpolated
per-fragment value in the fragment shader, also exposed under the name NORMAL, would
reflect those adjustments.

\table
\header
\li Change in main.qml, material.frag
\li Result
\row \li \qml
Model {
    source: "#Sphere"
    scale: Qt.vector3d(2, 2, 2)
    materials: CustomMaterial {
    fragmentShader: "material.frag"
}
\endqml
\badcode
void MAIN()
{
    BASE_COLOR = vec4(NORMAL, 1.0);
}
\endcode
\li \image quick3d-custom-cube9.jpg
\endtable

\section2 Colors

Let's switch over to a teapot model for a moment, make the material a blend of metallic
and dielectric, and try to set a green base color for it. The \c green QColor value maps
to \c{(0, 128, 0)}, based on which our first attempt could be:

\table
\header
\li main.qml, material.frag
\row \li \qml
import QtQuick
import QtQuick3D
Item {
    View3D {
        anchors.fill: parent
        environment: SceneEnvironment { backgroundMode: SceneEnvironment.Color; clearColor: "black" }
        PerspectiveCamera { z: 600 }
        DirectionalLight { }
        Model {
            source: "teapot.mesh"
            scale: Qt.vector3d(60, 60, 60)
            eulerRotation.x: 30
            materials: CustomMaterial {
                fragmentShader: "material.frag"
            }
        }
    }
}
\endqml
\badcode
void MAIN()
{
    BASE_COLOR = vec4(0.0, 0.5, 0.0, 1.0);
    METALNESS = 0.6;
    SPECULAR_AMOUNT = 0.4;
    ROUGHNESS = 0.4;
}
\endcode
\endtable

\image quick3d-custom-color1.jpg

This does not look entirely right. Compare with the second approach:

\table
\header
\li Change in main.qml, material.frag
\li Result
\row \li \qml
materials: CustomMaterial {
    fragmentShader: "material.frag"
    property color uColor: "green"
}
\endqml
\badcode
void MAIN()
{
    BASE_COLOR = vec4(uColor.rgb, 1.0);
    METALNESS = 0.6;
    SPECULAR_AMOUNT = 0.4;
    ROUGHNESS = 0.4;
}
\endcode
\li \image quick3d-custom-color2.jpg
\endtable

Switching to a PrincipledMaterial, we can confirm that setting the
\l{PrincipledMaterial::baseColor} to "green" and following the metalness and other
properties, the result is identical to our second approach:

\table
\header
\li Change in main.qml
\li Result
\row \li \qml
materials: PrincipledMaterial {
    baseColor: "green"
    metalness: 0.6
    specularAmount: 0.4
    roughness: 0.4
}
\endqml
\li \image quick3d-custom-color3.jpg
\endtable

If the type of the \c uColor property was changed to \c vector4d, or any type other than
\c color, the results would suddenly change and become identical to our first approach.

Why is this?

The answer lies in the sRGB to linear conversion that is performed implicitly for color
properties of DefaultMaterial, PrincipledMaterial, and also for custom properties with a
\c color type in a CustomMaterial. Such conversion is not performed for any other value,
so if the shader hardcodes a color value, or bases it on a QML property with a type
different from \c color, it will be up to the shader to perform linearization in case the
source value was in sRGB color space. Converting to linear is important since Qt Quick 3D
performs \l{SceneEnvironment::tonemapMode}{tonemapping} on the results of fragment
shading, and that process assumes values in the sRGB space as its input.

The built-in QColor constants, such as, \c{"green"}, are all given in sRGB
space. Therefore, just assigning \c{vec4(0.0, 0.5, 0.0, 1.0)} to BASE_COLOR in the first
attempt is insufficient if we wanted a result that matches an RGB value \c{(0, 128, 0)} in
the sRGB space. See the \c BASE_COLOR documentation in \l CustomMaterial for a formula for
linearizing such color values. The same applies to color values retrieved by sampling
textures: if the source image data is not in the sRGB color space, a conversion is needed
(unless \l{SceneEnvironment::tonemapMode}{tonemapping} is disabled).

\section2 Blending

Just writing a value less than \c 1.0 to \c{BASE_COLOR.a} is not sufficient if the
expectation is to get alpha blending. Such materials will very often change the values of
\l{CustomMaterial::sourceBlend}{sourceBlend} and
\l{CustomMaterial::destinationBlend}{destinationBlend} properties to get the desired
results.

Also keep in mind that the combined alpha value is the \l{Node::opacity}{Node opacity}
multiplied by the material alpha.

To visualize, let's use a shader that assigns red with alpha \c 0.5 to \c BASE_COLOR:

\table
\header
\li main.qml, material.frag
\li Result
\row \li \qml
import QtQuick
import QtQuick3D
Item {
    View3D {
        anchors.fill: parent
        environment: SceneEnvironment {
            backgroundMode: SceneEnvironment.Color
            clearColor: "white"
        }
        PerspectiveCamera {
            id: camera
            z: 600
        }
        DirectionalLight { }
        Model {
            source: "#Cube"
            x: -150
            eulerRotation.x: 60
            eulerRotation.y: 20
            materials: CustomMaterial {
                fragmentShader: "material.frag"
            }
        }
        Model {
            source: "#Cube"
            eulerRotation.x: 60
            eulerRotation.y: 20
            materials: CustomMaterial {
                sourceBlend: CustomMaterial.SrcAlpha
                destinationBlend: CustomMaterial.OneMinusSrcAlpha
                fragmentShader: "material.frag"
            }
        }
        Model {
            source: "#Cube"
            x: 150
            eulerRotation.x: 60
            eulerRotation.y: 20
            materials: CustomMaterial {
                sourceBlend: CustomMaterial.SrcAlpha
                destinationBlend: CustomMaterial.OneMinusSrcAlpha
                fragmentShader: "material.frag"
            }
            opacity: 0.5
        }
    }
}
\endqml
\badcode
void MAIN()
{
    BASE_COLOR = vec4(1.0, 0.0, 0.0, 0.5);
}
\endcode
\li \image quick3d-custom-blend.jpg
\endtable

The first cube is writing 0.5 to the alpha value of the color but it does not bring
visible results since alpha blending is not enabled. The second cube enables simple alpha
blending via the CustomMaterial properties. The third one also assigns an opacity of 0.5
to the Model, which means that the effective opacity is 0.25.

\section2 Passing data between the vertex and fragment shader

Calculating a value per vertex (for example, assuming a single triangle, for the 3 corners
of the triangle), and then passing it on to the fragment stage, where for each fragment
(for example, every fragment covered by the rasterized triangle) an interpolated value is
made accessible. In custom material shader snippets this is made possible by the \c
VARYING keyword. This provides a syntax similar to GLSL 120 and GLSL ES 100, but will work
regardless of the graphics API used at run time. The engine will take care of rewriting
the varying declaration as appropriate.

Let's see how the classic texture sampling with UV coordinates would look like. Textures
are going to be covered in an upcoming section, for now let's focus on how we get the UV
coordinates that can be passed to the \c{texture()} function in the shader.

\table
\header
\li main.qml, material.vert, material.frag
\row \li \qml
import QtQuick
import QtQuick3D
Item {
    View3D {
        anchors.fill: parent
        environment: SceneEnvironment { backgroundMode: SceneEnvironment.Color; clearColor: "black" }
        PerspectiveCamera { z: 600 }
        DirectionalLight { }
        Model {
            source: "#Sphere"
            scale: Qt.vector3d(4, 4, 4)
            eulerRotation.x: 30
            materials: CustomMaterial {
                vertexShader: "material.vert"
                fragmentShader: "material.frag"
                property TextureInput someTextureMap: TextureInput {
                    texture: Texture {
                        source: "qt_logo_rect.png"
                    }
                }
            }
        }
    }
}
\endqml
\badcode
VARYING vec2 uv;
void MAIN()
{
    uv = UV0;
}
\endcode
\badcode
VARYING vec2 uv;
void MAIN()
{
    BASE_COLOR = texture(someTextureMap, uv);
}
\endcode
\endtable

\table
\header
\li qt_logo_rect.png
\li Result
\row \li \image quick3d-custom-varying-map.png
\li \image quick3d-custom-varying1.jpg
\endtable

Note that \c VARYING declarations. The name and type must match, \c uv in the fragment
shader will expose the interpolated UV coordinate for the current fragment.

Any other type of data can be passed on to the fragment stage in a similar manner. It is
worth noting that in many cases setting up the material's own varyings is not necessary
because there are builtins provided that cover many of typical needs. This includes making
the (interpolated) normals, UVs, world position (\c VAR_WORLD_POSITION), or the vector
pointing towards the camera (\c VIEW_VECTOR).

The above example can in fact be simplified to the following as \c UV0 is automatically
available in the fragment stage as well:

\table
\header
\li Change in main.qml, material.frag
\li Result
\row \li \qml
materials: CustomMaterial {
    fragmentShader: "material.frag"
    property TextureInput someTextureMap: TextureInput {
        texture: Texture {
        source: "qt_logo_rect.png"
    }
}
\endqml
\badcode
void MAIN()
{
    BASE_COLOR = texture(someTextureMap, UV0);
}
\endcode
\li \image quick3d-custom-varying1.jpg
\endtable

\section2 Textures

A \l CustomMaterial has no built-in texture maps, meaning there is no equivalent of, for
example, \l{PrincipledMaterial::baseColorMap}. This is because implementing the same is
often trivial, while giving a lot more flexibility than what DefaultMaterial and
PrincipledMaterial has built in. Besides simply sampling a texture, custom fragment shader
snippets are free to combine and blend data from various sources when calculating the
values they assign to \c BASE_COLOR, \c EMISSIVE_COLOR, \c ROUGHNESS, etc. They can base
these calculations on data provided via QML properties, interpolated data sent on from the
vertex stage, values retrieved from sampling textures, and on hardcoded values.

As the previous example shows, exposing a texture to the vertex, fragment, or both shaders
is very similar to scalar and vector uniform values: a QML property with the type \l
TextureInput will automatically get associated with a \c sampler2D in the shader code. As
always, there is no need to declare this sampler in the shader code.

A \l TextureInput references a \l Texture, with an additional
\l{TextureInput::enabled}{enabled} property.  A \l Texture can source its data in three
ways: \l{Texture::source}{from an image file}, \l{Texture::sourceItem}{from a texture with
live Qt Quick content}, or \l{Texture::textureData}{can be provided from C++} via
QQuick3DTextureData.

\note When it comes to \l Texture properties, the source, tiling, and filtering related
ones are the only ones that are taken into account implicitly with custom materials, as
the rest (such as, UV transformations) is up to the custom shaders to implement as they
see fit.

Let's see an example where a model, a sphere in this case, is textured using live Qt Quick
content:

\table
\header
\li main.qml, material.frag
\row \li \qml
import QtQuick
import QtQuick3D
Item {
    View3D {
        anchors.fill: parent
        environment: SceneEnvironment { backgroundMode: SceneEnvironment.Color; clearColor: "black" }
        PerspectiveCamera { z: 600 }
        DirectionalLight { }
        Model {
            source: "#Sphere"
            scale: Qt.vector3d(4, 4, 4)
            eulerRotation.x: 30
            materials: CustomMaterial {
                fragmentShader: "material.frag"
                property TextureInput someTextureMap: TextureInput {
                    texture: Texture {
                        sourceItem: Rectangle {
                            width: 512; height: 512
                            color: "red"
                            Rectangle {
                                width: 32; height: 32
                                anchors.horizontalCenter: parent.horizontalCenter
                                y: 150
                                color: "gray";
                                NumberAnimation on rotation { from: 0; to: 360; duration: 3000; loops: -1 }
                            }
                            Text {
                                anchors.centerIn: parent
                                text: "Texture Map"
                                font.pointSize: 16
                            }
                        }
                    }
                }
            }
        }
    }
}
\endqml
\badcode
void MAIN()
{
    vec2 uv = vec2(UV0.x, 1.0 - UV0.y);
    vec4 c = texture(someTextureMap, uv);
    BASE_COLOR = c;
}
\endcode
\endtable

\image quick3d-custmat-tex1-anim.gif

Here the 2D subtree (Rectangle with two children: another Rectangle and the Text) is
rendered in to an 512x512 2D texture every time this mini-scene changes. The texture is
then exposed to the custom material under the name of \c someTextureMap.

Note the flipping of the V coordinate in the shader. As noted above, custom materials,
where there is full programmability on shader level, do not offer the "fixed" features of
\l Texture and \l PrincipledMaterial. This means that any transformations to the UV
coordinates will need to be applied by the shader. Here we know that the texture is
generated via \l{Texture::sourceItem} and so V needs to be flipped to get something that
matches the UV set of the mesh we are using.

What this example shows is possible to do with a \l PrincipledMaterial too. Let's make it
more interesting by doing a simple emboss effect in addition:

\table
\header
\li material.frag
\li Result
\row \li \badcode
void MAIN()
{
    vec2 uv = vec2(UV0.x, 1.0 - UV0.y);
    vec2 size = vec2(textureSize(someTextureMap, 0));
    vec2 d = vec2(1.0 / size.x, 1.0 / size.y);
    vec4 diff = texture(someTextureMap, uv + d) - texture(someTextureMap, uv - d);
    float c = (diff.x + diff.y + diff.z) + 0.5;
    BASE_COLOR = vec4(c, c, c, 1.0);
}
\endcode
\li \image quick3d-custmat-tex2-anim.gif
\endtable

With the features covered so far a wide range of possibilities are open for creating
materials that shade the meshes in visually impressive ways. To finish the basic tour,
let's look at an example that applies height and normal maps to a plane mesh. (a dedicated
\c{.mesh} file is used here because the builtin \c{#Rectangle} does not have enough
subdivisions) For better lighting results, we will use image based lighting with a 360
degree HDR image. The image is also set as the skybox to make it more clear what is
happening.

First let's start with an empty CustomMaterial:

\table
\header
\li main.qml
\li Result
\row \li \qml
import QtQuick
import QtQuick3D
Item {
    View3D {
        anchors.fill: parent
        environment: SceneEnvironment {
            backgroundMode: SceneEnvironment.SkyBox
            lightProbe: Texture {
                source: "00489_OpenfootageNET_snowfield_low.hdr"
            }
        }
        PerspectiveCamera {
            z: 600
        }
        Model {
            source: "plane.mesh"
            scale: Qt.vector3d(400, 400, 400)
            z: 400
            y: -50
            eulerRotation.x: -90
            materials: CustomMaterial { }
        }
    }
}
\endqml
\li \image quick3d-custom-tex3.jpg
\endtable

Now let's make some shaders that apply a height and normal map to the mesh:

\table
\header
\li Height map
\li Normap map
\row
\li \image quick3d-custom-heightmap.png
\li \image quick3d-custom-normalmap.jpg
\endtable

\table
\header
\li material.vert, material.frag
\row
\li \badcode
float getHeight(vec2 pos)
{
    return texture(heightMap, pos).r;
}

void MAIN()
{
    const float offset = 0.004;
    VERTEX.y += getHeight(UV0);
    TANGENT = normalize(vec3(0.0, getHeight(UV0 + vec2(0.0, offset)) - getHeight(UV0 + vec2(0.0, -offset)), offset * 2.0));
    BINORMAL = normalize(vec3(offset * 2.0, getHeight(UV0 + vec2(offset, 0.0)) - getHeight(UV0 + vec2(-offset, 0.0)), 0.0));
    NORMAL = cross(TANGENT, BINORMAL);
}
\endcode
\badcode
void MAIN()
{
    vec3 normalValue = texture(normalMap, UV0).rgb;
    normalValue.xy = normalValue.xy * 2.0 - 1.0;
    normalValue.z = sqrt(max(0.0, 1.0 - dot(normalValue.xy, normalValue.xy)));
    NORMAL = normalize(mix(NORMAL, TANGENT * normalValue.x + BINORMAL * normalValue.y + NORMAL * normalValue.z, 1.0));
}
\endcode
\endtable

\table
\header
\li Change in main.qml
\li Result
\row
\li \qml
materials: CustomMaterial {
    vertexShader: "material.vert"
    fragmentShader: "material.frag"
    property TextureInput normalMap: TextureInput {
        texture: Texture { source: "normalmap.jpg" }
    }
    property TextureInput heightMap: TextureInput {
        texture: Texture { source: "heightmap.png" }
    }
}
\endqml
\li \image quick3d-custom-tex4.jpg
\endtable

\note The \l WasdController object can be immensely helpful during development and
troubleshooting as it allows navigating and looking around in the scene with the keyboard
and mouse in a familiar manner. Having a camera controlled by the WasdController is as
simple as:

\qml
import QtQuick3D.Helpers
View3D {
    PerspectiveCamera {
        id: camera
    }
    // ...
}
WasdController {
    controlledObject: camera
}
\endqml

\section2 Depth and screen textures

When a custom shader snippet uses the \c DEPTH_TEXTURE or \c SCREEN_TEXTURE keywords, it
opts in to generating the corresponding textures in a separate render pass, which is not
necessarily a cheap operation, but allows implementing a variety of techniques, such as
refraction for glass-like materials.

\c DEPTH_TEXTURE is a \c sampler2D that allows sampling a texture with the contents of the
depth buffer with all the \c opaque objects in the scene rendered. Similarly, \c
SCREEN_TEXTURE is a \c sampler2D that allows sampling a texture that has all \c opaque
objects in the scene rendered to it, without anything else, meaning no skybox, no shadows,
no effects, and no objects that have any transparency in some form (\l{Node::opacity}
smaller than 1, textures maps for the material from images with semi-transparent pixels in
them, etc.)  are present. The texture is cleared to \c{vec4(0.0)} meaning it has an alpha
value of 0 for fragments that are not covered by objects in the scene. The size of these
textures matches the size of the View3D in pixels.

Let's have a simple demonstration by visualizing the depth buffer contents via \c
DEPTH_TEXTURE. The camera's \l{PerspectiveCamera::clipFar}{far clip value} is reduced here from the
default 10000 to 2000, in order to have a smaller range, and so have the visualized depth
value differences more obvious. The result is a rectangle that happens to visualize the
depth buffer for the scene over its surface.

\table
\header
\li main.qml, material.frag
\li Result
\row
\li \qml
import QtQuick
import QtQuick3D
import QtQuick3D.Helpers
Item {
    View3D {
        anchors.fill: parent
        environment: SceneEnvironment {
            backgroundMode: SceneEnvironment.Color
            clearColor: "black"
        }
        PerspectiveCamera {
            id: camera
            z: 600
            clipNear: 1
            clipFar: 2000
        }
        DirectionalLight { }
        Model {
            source: "#Cube"
            scale: Qt.vector3d(2, 2, 2)
            position: Qt.vector3d(150, 200, -1000)
            eulerRotation.x: 60
            eulerRotation.y: 20
            materials: PrincipledMaterial { }
        }
        Model {
            source: "#Cylinder"
            scale: Qt.vector3d(2, 2, 2)
            position: Qt.vector3d(400, 200, -1000)
            materials: PrincipledMaterial { }
            opacity: 0.3
        }
        Model {
            source: "#Sphere"
            scale: Qt.vector3d(2, 2, 2)
            position: Qt.vector3d(-150, 200, -600)
            materials: PrincipledMaterial { }
        }
        Model {
            source: "#Cone"
            scale: Qt.vector3d(2, 2, 2)
            position: Qt.vector3d(0, 400, -1200)
            materials: PrincipledMaterial { }
        }
        Model {
            source: "#Rectangle"
            scale: Qt.vector3d(3, 3, 3)
            y: -150
            materials: CustomMaterial {
                fragmentShader: "material.frag"
            }
        }
    }
    WasdController {
        controlledObject: camera
    }
}
\endqml
\badcode
void MAIN()
{
    float zNear = CAMERA_PROPERTIES.x;
    float zFar = CAMERA_PROPERTIES.y;
    float zRange = zFar - zNear;
    vec4 depthSample = texture(DEPTH_TEXTURE, vec2(UV0.x, 1.0 - UV0.y));
    float zn = 2.0 * depthSample.r - 1.0;
    float d = 2.0 * zNear * zFar / (zFar + zNear - zn * zRange);
    d /= zFar;
    BASE_COLOR = vec4(d, d, d, 1.0);
}
\endcode
\li \image quick3d-custom-depth-anim.gif
\endtable

Note how the cylinder is not present in \c DEPTH_TEXTURE due to its reliance on
semi-transparency, which puts it into a different category than the other objects that are
all opaque. These objects do not write into the depth buffer, although they do test
against the depth values written by opaque objects, and rely on being rendered in back to
front order. Hence they are not present in \c DEPTH_TEXTURE either.

What happens if we switch the shader to sample \c SCREEN_TEXTURE instead?

\table
\header
\li material.frag
\li Result
\row \li \badcode
void MAIN()
{
    vec4 c = texture(SCREEN_TEXTURE, vec2(UV0.x, 1.0 - UV0.y));
    if (c.a == 0.0)
        c.rgb = vec3(0.2, 0.1, 0.3);
    BASE_COLOR = c;
}
\endcode
\li \image quick3d-custom-screen.jpg
\endtable

Here the rectangle is textured with \c SCREEN_TEXTURE, while replacing transparent pixels
with purple.

\section2 Light processor functions

An advanced feature of \l CustomMaterial is the ability to define functions in the
fragment shader that reimplement the lighting equations that are used to calculate the
fragment color. A light processor function, when present, is called once per each light in
the scene, for each fragment. There is a dedicated function for different light types, as
well as the ambient and specular contribution. When no corresponding light processor
function is present, the standard calculations are used, just like a PrincipledMaterial
would do. When a light processor is present, but the function body is empty, it means
there will be no contribution from a given type of lights in the scene.

Refer to the \l CustomMaterial documentation for details on functions such as \c
DIRECTIONAL_LIGHT, \c POINT_LIGHT, \c SPOT_LIGHT, \c AMBIENT_LIGHT, and \c SPECULAR_LIGHT.

\section2 Unshaded custom materials

There is another type of \l CustomMaterial: \c unshaded custom materials. All the example
so far used \c shaded custom materials, with the
\l{CustomMaterial::shadingMode}{shadingMode} property left at its default
CustomMaterial.Shaded value.

What happens if we switch this property to CustomMaterial.Unshaded?

First of all, keywords like \c BASE_COLOR, \c EMISSIVE_COLOR, \c METALNESS, etc. no longer
have the desired effect. This is because an unshaded material, as the name suggests, does
not automatically get amended with much of the standard shading code, thus ignoring
lights, image based lighting, shadows, and ambient occlusion in the scene. Rather, an
unshaded material gives full control to the shader via the \c FRAGCOLOR keyword. This is
similar to gl_FragColor: the color assigned to \c FRAGCOLOR is the result and the final
color of the fragment, without any further adjustments by Qt Quick 3D.

\table
\header
\li main.qml, material.frag, material2.frag
\li Result
\row \li \qml
import QtQuick
import QtQuick3D
Item {
    View3D {
        anchors.fill: parent
        environment: SceneEnvironment {
            backgroundMode: SceneEnvironment.Color
            clearColor: "black"
        }
        PerspectiveCamera { z: 600 }
        DirectionalLight { }
        Model {
            source: "#Cylinder"
            x: -100
            eulerRotation.x: 30
            materials: CustomMaterial {
                fragmentShader: "material.frag"
            }
        }
        Model {
            source: "#Cylinder"
            x: 100
            eulerRotation.x: 30
            materials: CustomMaterial {
                shadingMode: CustomMaterial.Unshaded
                fragmentShader: "material2.frag"
            }
        }
    }
}
\endqml
\badcode
void MAIN()
{
    BASE_COLOR = vec4(1.0);
}
\endcode
\badcode
void MAIN()
{
    FRAGCOLOR = vec4(1.0);
}
\endcode
\li \image quick3d-custom-unshaded1.jpg
\endtable

Notice how the right cylinder ignores the DirectionalLight in the scene. Its shading knows
nothing about scene lighting, the final fragment color is all white.

The vertex shader in an unshaded material still has the typical inputs available: \c
VERTEX, \c NORMAL, \c MODELVIEWPROJECTION_MATRIX, etc. and can write to \c POSITION.  The
fragment shader no longer has the similar conveniences available, however: \c NORMAL, \c
UV0, or \c VAR_WORLD_POSITION are not available in an unshaded material's fragment
shader. Rather, it is now up to the shader code to calculate and pass on using \c VARYING
everything it needs to determine the final fragment color.

Let's look at an example that has both a vertex and fragment shader. The altered vertex
position is passed on to the fragment shader, with an interpolated value made available to
every fragment.

\table
\header
\li main.qml, material.vert, material.frag
\row \li \qml
import QtQuick
import QtQuick3D
Item {
    View3D {
        anchors.fill: parent
        environment: SceneEnvironment {
            backgroundMode: SceneEnvironment.Color
            clearColor: "black"
        }
        PerspectiveCamera { z: 600 }
        Model {
            source: "#Sphere"
            scale: Qt.vector3d(3, 3, 3)
            materials: CustomMaterial {
                property real time: 0.0
                NumberAnimation on time { from: 0; to: 100; duration: 20000; loops: -1 }
                property real amplitude: 10.0
                shadingMode: CustomMaterial.Unshaded
                vertexShader: "material.vert"
                fragmentShader: "material.frag"
            }
        }
    }
}
\endqml
\badcode
VARYING vec3 pos;
void MAIN()
{
    pos = VERTEX;
    pos.x += sin(time * 4.0 + pos.y) * amplitude;
    POSITION = MODELVIEWPROJECTION_MATRIX * vec4(pos, 1.0);
}
\endcode
\badcode
VARYING vec3 pos;
void MAIN()
{
    FRAGCOLOR = vec4(vec3(pos.x * 0.02, pos.y * 0.02, pos.z * 0.02), 1.0);
}
\endcode
\endtable

\image quick3d-custom-unshaded-anim.gif

Unshaded materials are useful when interacting with scene lighting is not necessary or
desired, and the material needs full control on the final fragment color. Notice how the
example above has neither a DirectionalLight nor any other lights, but the sphere with the
custom material shows up as expected.

\note An unshaded material that only has a vertex shader snippet, but does not specify the
fragmentShader property, will still be functional but the results are as if the
shadingMode was set to Shaded. Therefore it makes little sense to switch shadingMode for
materials that only have a vertex shader.

\section1 Programmability for Effects

Post-processing effects apply one or more fragment shaders to the result of a \l
View3D. The output from these fragment shaders is then displayed instead of the original
rendering results. This is conceptually very similar to Qt Quick's \l ShaderEffect and \l
ShaderEffectSource.

\note Post-processing effects are only available when the
\l{View3D::renderMode}{renderMode} for the View3D is set to View3D.Offscreen.

Custom vertex shader snippets can also be specified for an effect, but they have limited
usefulness and therefore are expected to be used relatively rarely. The vertex input for a
post-processing effect is a quad (either two triangles or a triangle strip), transforming
or displacing the vertices of that is often not helpful. It can however make sense to have
a vertex shader in order to calculate and pass on data to the fragment shader using the \c
VARYING keyword. As usual, the fragment shader will then receive an interpolated value
based on the current fragment coordinate.

The syntax of the shader snippets associated with a \l Effect is identical to the shaders
for an unshaded \l CustomMaterial. When it comes to the built-in special keywords, \c
VARYING, \c MAIN, \c FRAGCOLOR (fragment shader only), \c POSITION (vertex shader only), \c
VERTEX (vertex shader only), and \c MODELVIEWPROJECTION_MATRIX work identically to \l
CustomMaterial.

The most important special keywords for \l Effect fragment shaders are the following:

\table
\header
\li Name
\li Type
\li Description
\row
\li INPUT
\li sampler2D
\li The sampler for the input texture. An effect will typically sample this using \c INPUT_UV.
\row
\li INPUT_UV
\li vec2
\li UV coordinates for sampling \c INPUT.
\row
\li INPUT_SIZE
\li vec2
\li The size of the \c INPUT texture, in pixels. This is a convenient alternative to calling textureSize().
\row
\li OUTPUT_SIZE
\li vec2
\li The size of the output texture, in pixels. Equal to \c INPUT_SIZE in many cases, but a multi-pass effect
may have passes that output to intermediate textures with different sizes.
\row
\li DEPTH_TEXTURE
\li sampler2D
\li Depth texture with the depth buffer contents with the opaque objects in the scene. Like with CustomMaterial,
the presence of this keyword in the shader triggers generating the depth texture automatically.
\endtable

\section2 A post-processing effect

Let's start with a simple scene, this time using a few more objects, including a textured
rectangle that uses a checkerboard texture as its base color map.

\table
\header
\li main.qml
\li Result
\row \li \qml
import QtQuick
import QtQuick3D
Item {
    View3D {
        anchors.fill: parent
        environment: SceneEnvironment {
            backgroundMode: SceneEnvironment.Color
            clearColor: "black"
        }

        PerspectiveCamera { z: 400 }

        DirectionalLight { }

        Texture {
            id: checkerboard
            source: "checkerboard.png"
            scaleU: 20
            scaleV: 20
            tilingModeHorizontal: Texture.Repeat
            tilingModeVertical: Texture.Repeat
        }

        Model {
            source: "#Rectangle"
            scale: Qt.vector3d(10, 10, 1)
            eulerRotation.x: -45
            materials: PrincipledMaterial {
                baseColorMap: checkerboard
            }
        }

        Model {
            source: "#Cone"
            position: Qt.vector3d(100, -50, 100)
            materials: PrincipledMaterial { }
        }

        Model {
            source: "#Cube"
            position.y: 100
            eulerRotation.y: 20
            materials: PrincipledMaterial { }
        }

        Model {
            source: "#Sphere"
            position: Qt.vector3d(-150, 200, -100)
            materials: PrincipledMaterial { }
        }
    }
}
\endqml
\li \image quick3d-custom-effect-section-scene.jpg
\endtable

Now let's apply an affect to the entire scene. More precisely, to the View3D. When there
are multiple View3D items in the scene, each has its own SceneEnvironment and therefore
have their own post-processing effect chain. In the example there is one single View3D
covering the entire window.

\table
\header
\li Change in main.qml
\li effect.frag
\row \li \qml
        environment: SceneEnvironment {
            backgroundMode: SceneEnvironment.Color
            clearColor: "black"
            effects: redEffect
        }

        Effect {
            id: redEffect
            property real uRed: 1.0
            NumberAnimation on uRed { from: 1; to: 0; duration: 5000; loops: -1 }
            passes: Pass {
                shaders: Shader {
                    stage: Shader.Fragment
                    shader: "effect.frag"
                }
            }
        }
\endqml
\li \badcode
void MAIN()
{
    vec4 c = texture(INPUT, INPUT_UV);
    c.r = uRed;
    FRAGCOLOR = c;
}
\endcode
\endtable

This simple effect alters the red color channel value. Exposing QML properties as uniforms
works the same way with effects as with custom materials. The shader starts with a line
that is going to be very common when writing fragment shaders fro effects: sampling \c
INPUT at the UV coordinates \c INPUT_UV. It then performs its desired calculations, and
assigns the final fragment color to \c FRAGCOLOR.

\image quick3d-custom-first-effect-anim.gif

Many properties set in the example are in plural (effects, passes, shaders). While the
list \c{[ ]} syntax can be omitted when having a single element only, all these properties
are lists, and can hold more than one element. Why is this?

\list

\li \l{SceneEnvironment::effects}{effects} is a list, because View3D allows chaining
multiple effects together. The effects are applied in the order in which they are added to
the list. This allows easily applying two or more effects together to the View3D, and is
similar to what one can achieve in Qt Quick by nesting \l ShaderEffect items. The \c INPUT
texture of the next effect is always a texture that contains the previous effect's
output. The output of the last effect in what gets used as the final output of the View3D.

\li \l{Effect::passes}{passes} is a list, because unlike ShaderEffect, Effect has built-in
support for multiple passes. A multi-pass effect is more powerful than chaining together
multiple, independent effects in \l{SceneEnvironment::effects}{effects}: a pass can output
to a temporary, intermediate texture, which can then be used as input to subsequent
passes, in addition to the original input texture of the effect. This allows creating
complex effects that calculate, render, and blend together multiple textures in order to
get to the final fragment color. This advanced use case is not going to be covered
here. Refer to the \l Effect documentation page for details.

\li \l{Pass::shaders}{shaders} is a list, because an effect may have both a vertex and a
fragment shader associated.

\endlist

\section2 Chaining multiple effects

Let's look at an example where the effect from the previous example gets complemented by
another effect similar to the built-in \l DistortionSpiral effect.

\table
\header
\li Change in main.qml
\li effect2.frag
\row \li \qml
        environment: SceneEnvironment {
            backgroundMode: SceneEnvironment.Color
            clearColor: "black"
            effects: [redEffect, distortEffect]
        }

        Effect {
            id: redEffect
            property real uRed: 1.0
            NumberAnimation on uRed { from: 1; to: 0; duration: 5000; loops: -1 }
            passes: Pass {
                shaders: Shader {
                    stage: Shader.Fragment
                    shader: "effect.frag"
                }
            }
        }

        Effect {
            id: distortEffect
            property real uRadius: 0.1
            NumberAnimation on uRadius { from: 0.1; to: 1.0; duration: 5000; loops: -1 }
            passes: Pass {
                shaders: Shader {
                    stage: Shader.Fragment
                    shader: "effect2.frag"
                }
            }
        }
\endqml
\li \badcode
void MAIN()
{
    vec2 center_vec = INPUT_UV - vec2(0.5, 0.5);
    center_vec.y *= INPUT_SIZE.y / INPUT_SIZE.x;
    float dist_to_center = length(center_vec) / uRadius;
    vec2 texcoord = INPUT_UV;
    if (dist_to_center <= 1.0) {
        float rotation_amount = (1.0 - dist_to_center) * (1.0 - dist_to_center);
        float r = radians(360.0) * rotation_amount / 4.0;
        float cos_r = cos(r);
        float sin_r = sin(r);
        mat2 rotation = mat2(cos_r, sin_r, -sin_r, cos_r);
        texcoord = vec2(0.5, 0.5) + rotation * (INPUT_UV - vec2(0.5, 0.5));
    }
    vec4 c = texture(INPUT, texcoord);
    FRAGCOLOR = c;
}
\endcode
\endtable

\image quick3d-custom-chained-effect-anim.gif

Now the perhaps surprising question: why is this a bad example?

More precisely, it is not bad, but rather shows a pattern that can often be beneficial to
avoid.

Chaining effects this way can be useful, but it is important to keep in mind the
performance implications: doing two render passes (one to generate a texture with the
adjusted red color channel, and then another one two calculate the distortion) is quite
wasteful when one would be enough. If the fragment shader snippets were combined, the same
result could have been achieved with one single effect.

\section1 Defining Mesh and Texture Data from C++

Procedurally generating mesh and texture image data both follow similar steps:

\list
\li Subclass \l QQuick3DGeometry or \l QQuick3DTextureData
\li Set the desired vertex or image data upon construction by calling the protected member functions
from the base class
\li If dynamic changes are needed afterwards at some point, set the new data and call update()
\li Once the implementation is done, the class needs to be registered to make it visible in QML
\li \l Model and \l Texture objects in QML can now use the custom vertex or image data provider by
setting the \l{Model::geometry} or \l{Texture::textureData} property
\endlist

\section2 Custom vertex data

Vertex data refers to the sequence of (typically \c float) values that make up a
mesh. Instead of loading \c{.mesh} files, a custom geometry provider is responsible for
providing the same data. The vertex data consist of \c attributes, such as position,
texture (UV) coordinates, or normals. The specification of attributes describes what kind
of attributes are present, the component type (for example, a 3 component float vector for
vertex position consisting of x, y, z values), which offset they start at in the provided
data, and what the stride (the increment that needs to be added to the offset to point to
the next element for the same attribute) is.

This may seem familiar if one has worked with graphics APIs, such as OpenGL or Vulkan
directly, because the way vertex input is specified with those APIs maps loosely to what a
\c{.mesh} file or a \l QQuick3DGeometry instance defines.

In addition, the mesh topology (primitive type) must be specified too. For indexed
drawing, the data for an index buffer must be provided as well.

There is one built-in custom geometry implementation: the QtQuick3D.Helpers module
includes a \l GridGeometry type. This allows rendering a grid in the scene with line
primitives, without having to implement a custom \l QQuick3DGeometry subclass.

One other common use cases is rendering points. This is fairly simple to do since the
attribute specification is going to be minimal: we provide three floats (x, y, z) for each
vertex, nothing else. A QQuick3DGeometry subclass could implement a geometry consisting of
2000 points similarly to the following:

\badcode
    clear();
    const int N = 2000;
    const int stride = 3 * sizeof(float);
    QByteArray v;
    v.resize(N * stride);
    float *p = reinterpret_cast<float *>(v.data());
    QRandomGenerator *rg = QRandomGenerator::global();
    for (int i = 0; i < N; ++i) {
        const float x = float(rg->bounded(200.0f) - 100.0f) / 20.0f;
        const float y = float(rg->bounded(200.0f) - 100.0f) / 20.0f;
        *p++ = x;
        *p++ = y;
        *p++ = 0.0f;
    }
    setVertexData(v);
    setStride(stride);
    setPrimitiveType(QQuick3DGeometry::PrimitiveType::Points);
    addAttribute(QQuick3DGeometry::Attribute::PositionSemantic, 0, QQuick3DGeometry::Attribute::F32Type);
\endcode

Combined with a material of

\qml
DefaultMaterial {
    lighting: DefaultMaterial.NoLighting
    cullMode: DefaultMaterial.NoCulling
    diffuseColor: "yellow"
    pointSize: 4
}
\endqml

the end result is similar to this (here viewed from an altered camera angle, with the help
of \l WasdController):

\image quick3d-custom-points.jpg

\note Be aware that point sizes and line widths other than 1 may not be supported at run
time, depending on the underlying graphics API. This is not something Qt has control
over. Therefore, it can become necessary to implement alternative techniques instead of
relying on point and line drawing.

\section2 Custom texture data

With textures, the data that needs to be provided is a lot simpler structurally: it is the
raw pixel data, with a varying number of bytes per pixel, depending on the texture
format. For example, an \c RGBA texture expects four bytes per pixel, whereas \c RGBA16F
is four half-floats per pixel. This is similar to what a \l QImage stores
internally. However, Qt Quick 3D textures can have formats the data for which cannot be
represented by a QImage. For example, floating point HDR textures, or compressed
textures. Therefore the data for \l QQuick3DTextureData is always provided as a raw
sequence of bytes. This may seem familiar if one has worked with graphics APIs, such as
OpenGL or Vulkan directly.

For details, refer to the \l QQuick3DGeometry and \l QQuick3DTextureData documentation pages.

\sa CustomMaterial, Effect, QQuick3DGeometry, QQuick3DTextureData, {Qt Quick 3D - Custom
Effect Example}, {Qt Quick 3D - Custom Shaders Example}, {Qt Quick 3D - Custom Materials
Example}, {Qt Quick 3D - Custom Geometry Example}, {Qt Quick 3D - Procedural Texture
Example}

*/
