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/*!
    \example custommaterial
    \ingroup quick3d-examples
    \title Qt Quick 3D - Custom Materials Example
    \brief Demonstrates writing shaded custom materials.
    \image custommaterial-example.jpg

    This example shows how to write \e shaded \l{CustomMaterial}{custom materials}.
    With shaded materials, we don't have to write complete shader
    programs. Instead, we write functions that modify Qt's standard shaders.
    This way the resulting material will by default participate in lighting,
    shadow mapping, and be compatible with light probes. We only have to
    write custom logic for the cases where we want special behavior.  This is
    achieved by effectively augmenting the shader code that would be
    generated for a PrincipledMaterial with our own custom functions that get
    called at certain stages in the vertex and fragment shaders.

    To make a shaded custom material, set the
    \l{CustomMaterial::shadingMode}{shadingMode} property to
    \c CustomMaterial.Shaded.


    \section1 A simple material

    The first model uses a simple material that does not add any custom logic. We
    set the custom material on the model just like any other material:

    \snippet custommaterial/main.qml simple

    In addition to setting the \c shadingMode and \c fragmentShader, we also add two
    properties to the material: \c uDiffuse and \c Specular. These will get picked up by
    the fragment shader.


    The code for the fragment shader is short:

    \quotefile custommaterial/material_simple.frag

    All shaders have to implement the \c MAIN function. In this one, we use the properties
    defined in the materials to set values that will be used by Qt's standard shader
    code. Note that we do not have to declare these as uniforms: all we need to do is to
    make sure the names match. We would get a shader compile error if the material did not
    have matching properties.

    The special variables \c SPECULAR_AMOUNT and \c BASE_COLOR correspond to
    \l {PrincipledMaterial::specularAmount}{specularAmount} and
    \l {PrincipledMaterial::baseColor}{baseColor} of PrincipledMaterial. These are
    then used by the standard shader code to perform lighting calculations just as if we had used
    a PrincipledMaterial.

    \section1 Custom handling of lights

    The next object uses a more complex material that implements custom lighting. The
    material has different uniform names, but otherwise we use it in the same way:

    \snippet custommaterial/main.qml custom lights

    The fragment shader implements custom logic for all the different types of light:

    \quotefile custommaterial/material_customlights.frag

    Here we use many new special keywords referring to properties of the various light
    types. See the CustomMaterial documentation for a description each keyword. Note that
    each light type has its own function. Any function not implemented will use the
    default implementation, behaving like PrincipledMaterial. For example: in this shader, we have not
    implemented \c {SPECULAR_LIGHT()}, so we will get the built-in specular reflection.

    \section1 Adding a vertex shader

    A custom material can also use a vertex shader to modify the geometry of the model. Here we
    specify both the fragment and vertex shaders, and add several more properties that
    will be picked up as uniform values:

    \snippet custommaterial/main.qml custom vertex

    The vertex shader is very short:
    \quotefile custommaterial/material_distortion.vert

    This deforms the model by displacing each vertex according to a sine-wave that changes with time.

    \section1 A transparent material

    Finally, we add a sphere with a material that is transparent. For performance reasons,
    Qt does not implement transparency in a completely realistic way. Instead, Qt renders
    all the opaque object in the scene to a texture, and then transparent materials read
    from this texture. This means that transparent materials will give the best result
    when placed in front of other objects:

    \snippet custommaterial/main.qml transparent

    For this example, we implement a simplistic distortion function that does not try to
    do real physical refraction:

    \quotefile custommaterial/material_transparent.frag

    \c SCREEN_TEXTURE refers to the texture showing all the opaque objects in the
    scene. We first calculate the uv coordinates inside this texture that matches the
    screen position of the current vertex. We then add an offset to this position,
    simulating a refraction effect, before doing a texture lookup.

    Finally, we blend in 20% white to get a slight cloudiness. Note that the output is
    assigned to \c BASE_COLOR, so Qt will add lighting on top of this. This is why we can
    see reflections on the surface of the sphere.

    \section1 Unshaded materials

    It is also possible to have custom materials that use complete shader
    programs (while still using the convenience keywords). The
    \l{Qt Quick 3D - Custom Shaders Example}{customshaders example}
    demonstrates the other group of custom materials: \e unshaded custom
    materials.

*/
