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/*!
    \page qt3drender-framegraph.html
    \title Qt 3D Render Framegraph

    \brief A framegraph is the data structure that controls how a scene is
    rendered.

    The Qt 3D Render aspect allows for the rendering algorithm to be entirely
    data-driven. The controlling data structure is known as the \e framegraph.
    Similar to how the Qt 3D ECS (entity component system) allows you to define
    a so-called Scenegraph by building a scene from a tree of Entities and
    Components, the framegraph is also a tree structure but one used for a
    different purpose. Namely, controlling \e how the scene is rendered.

    Over the course of rendering a single frame, a 3D renderer will likely
    change state many times. The number and nature of these state changes
    depends upon not only which materials (shaders, mesh geometry, textures and
    uniform variables) are found within the scene, but also upon which high
    level rendering scheme you are using.

    For example, using a traditional simple \e{forward rendering} scheme is
    very different to using a \e{deferred rendering} approach. Other features
    such as reflections, shadows, multiple viewports, and early z-fill passes
    all change which states a renderer needs to set over the course of a frame
    and when those state changes need to occur.

    As a comparison, the \l {Qt Quick Scene Graph}{Qt Quick 2
    scenegraph renderer} responsible for drawing Qt Quick 2 scenes is
    hard-wired in C++ to do things like batching of primitives and rendering
    opaque items followed by rendering of transparent items. In the case of Qt
    Quick 2 that is perfectly fine as that covers all of the requirements. As
    you can see from some of the examples listed above, such a hard-wired
    renderer is not likely to be flexible enough for generic 3D scenes given
    the multitude of rendering methods available. Or if a renderer could be
    made flexible enough to cover all such cases, its performance would likely
    suffer from being too general. To make matters worse, more rendering
    methods are being researched all of the time. We therefore needed an
    approach that is \e {both flexible and extensible} whilst being simple to
    use and maintain. Enter the framegraph!

    Each node in the framegraph defines a part of the configuration the
    renderer will use to render the scene. The position of a node in the
    framegraph tree determines when and where the subtree rooted at that node
    will be the active configuration in the rendering pipeline. As we will see
    later, the renderer traverses this tree in order to build up the state
    needed for your rendering algorithm at each point in the frame.

    Obviously if you just want to render a simple cube onscreen you may think
    this is overkill. However, as soon as you want to start doing slightly more
    complex scenes this comes in handy. For the common cases, Qt 3D provides
    some example framegraphs that are ready to use out of the box.

    We will demonstrate the flexibility of the framegraph concept by presenting a few
    examples and the resulting framegraphs.

    Please note that unlike the Scenegraph which is composed of Entities and
    Components, the framegraph is only composed of nested nodes which are all
    subclasses of Qt3DRender::QFrameGraphNode. This is because the framegraph nodes
    are not simulated objects in our virtual world, but rather supporting
    information.

    We will soon see how to
    construct our first simple framegraph but before that we will introduce
    the framegraph nodes available to you. Also as with the Scenegraph tree,
    the QML and C++ APIs are a 1 to 1 match so you can favor the one you like
    best. For the sake of readability and conciseness, the QML API was chosen
    for this article.

    \omit
            TODO: Add list of framegraph node types
    \endomit

    The beauty of the framegraph is that combining these simple node types, it
    is possible to configure the renderer to suit your specific needs without
    touching any hairy, low-level C/C++ rendering code at all.

    \section1 FrameGraph Rules

    In order to construct a correctly functioning framegraph tree,
    you should know a few rules about how it is traversed and how to feed it to
    the Qt 3D renderer.

    \section2 Setting the Framegraph

    The FrameGraph tree should be assigned to the activeFrameGraph property of
    a QRenderSettings component, itself being a component of the root entity in
    the Qt 3D scene. This is what makes it the active framegraph for the
    renderer. Of course, since this is a QML property binding, the active
    framegraph (or parts of it) can be changed on the fly at runtime. For
    example, if you want to use different rendering approaches for indoor and
    outdoor scenes or to enable or disable some special effect.

    \badcode
    Entity {
        id: sceneRoot
        components: RenderSettings {
             activeFrameGraph: ... // FrameGraph tree
        }
    }
    \endcode

    \note activeFrameGraph is the default property of the FrameGraph component
    in QML.

    \badcode
    Entity {
        id: sceneRoot
        components: RenderSettings {
             ... // FrameGraph tree
        }
    }
    \endcode

    \section2 How the Framegraph Is Used

    \list
        \li The Qt 3D renderer performs a \e{depth first traversal} of the
            framegraph tree. Note that, because the traversal is depth first,
            the \e {order in which you define nodes is important}.
        \li When the renderer reaches a leaf node of the framegraph, it
            collects together all of the state specified by the path from the
            leaf node to the root node. This defines the state used to render
            a section of the frame. If you are interested in the internals of
            Qt 3D, this collection of state is called a \e RenderView.
        \li Given the configuration contained in a RenderView, the renderer
            collects together all of the Entities in the Scenegraph to be
            rendered, and from them builds a set of \e RenderCommands and
            associates them with the RenderView.
        \li The combination of RenderView and set of RenderCommands is passed
            over for submission to OpenGL.
        \li When this is repeated for each leaf node in the framegraph, the
            frame is complete and the renderer calls
            QOpenGLContext::swapBuffers() to display the frame.
    \endlist

    At its heart, the framegraph is a data-driven method for configuring the
    Qt 3D renderer. Due to its data-driven nature, we can change configuration
    at runtime, allow non-C++ developers or designers to change the structure
    of a frame, and try out new rendering approaches without having to write
    thousands of lines of boiler plate code.


    \section1 Framegraph Examples

    Now that you know the rules to abide by when writing a framegraph tree, we
    will go over a few examples and break them down.

    \section2 A Simple Forward Renderer

    Forward rendering is when you use OpenGL in its traditional manner and
    render directly to the backbuffer one object at a time shading each one as
    we go. This is opposed to \l {Deferred Renderer}{deferred rendering} where
    we render to an intermediate \e G-buffer. Here is a simple FrameGraph that
    can be used for forward rendering:

    \badcode
    Viewport {
         rect: Qt.rect(0.0, 0.0, 1.0, 1.0)
         property alias camera: cameraSelector.camera

         ClearBuffers {
              buffers: ClearBuffers.ColorDepthBuffer

              CameraSelector {
                   id: cameraSelector
              }
         }
    }
    \endcode

    As you can see, this tree has a single leaf and is composed of 3 nodes in
    total as shown in the following diagram.

    \image simple-framegraph.png

    Using the rules defined \l {Framegraph Rules}{above}, this framegraph tree yields a single
    RenderView with the following configuration:

    \list
        \li Leaf Node -> RenderView
        \list
            \li Viewport that fills the entire screen (uses normalized
                coordinates to make it easy to support nested viewports)
            \li Color and Depth buffers are set to be cleared
            \li Camera specified in the exposed camera property
        \endlist
    \endlist

    Several different FrameGraph trees can produce the same rendering result.
    As long as the state collected from leaf to root is the same, the result
    will also be the same. It is best to put state that remains constant longest
    nearer to the root of the framegraph as this will result in fewer leaf
    nodes, and hence, fewer RenderViews overall.

    \badcode
    Viewport {
         rect: Qt.rect(0.0, 0.0, 1.0, 1.0)
         property alias camera: cameraSelector.camera

         CameraSelector {
              id: cameraSelector

              ClearBuffers {
                   buffers: ClearBuffers.ColorDepthBuffer
              }
         }
    }
    \endcode

    \badcode
    CameraSelector {
          Viewport {
               rect: Qt.rect(0.0, 0.0, 1.0, 1.0)

               ClearBuffers {
                    buffers: ClearBuffers.ColorDepthBuffer
               }
          }
    }
    \endcode

    \section2 A Multi Viewport FrameGraph

    Let us move on to a slightly more complex example that renders a Scenegraph
    from the point of view of 4 virtual cameras into the 4 quadrants of the
    window. This is a common configuration for 3D CAD or modelling tools or
    could be adjusted to help with rendering a rear-view mirror in a car racing
    game or a CCTV camera display.

    \image multiviewport.png

    \badcode
    Viewport {
         id: mainViewport
         rect: Qt.rect(0, 0, 1, 1)
         property alias Camera: cameraSelectorTopLeftViewport.camera
         property alias Camera: cameraSelectorTopRightViewport.camera
         property alias Camera: cameraSelectorBottomLeftViewport.camera
         property alias Camera: cameraSelectorBottomRightViewport.camera

         ClearBuffers {
              buffers: ClearBuffers.ColorDepthBuffer
         }

         Viewport {
              id: topLeftViewport
              rect: Qt.rect(0, 0, 0.5, 0.5)
              CameraSelector { id: cameraSelectorTopLeftViewport }
         }

         Viewport {
              id: topRightViewport
              rect: Qt.rect(0.5, 0, 0.5, 0.5)
              CameraSelector { id: cameraSelectorTopRightViewport }
         }

         Viewport {
              id: bottomLeftViewport
              rect: Qt.rect(0, 0.5, 0.5, 0.5)
              CameraSelector { id: cameraSelectorBottomLeftViewport }
         }

         Viewport {
              id: bottomRightViewport
              rect: Qt.rect(0.5, 0.5, 0.5, 0.5)
              CameraSelector { id: cameraSelectorBottomRightViewport }
         }
    }
    \endcode

    This tree is a bit more complex with 5 leaves. Following the same rules as
    before we construct 5 RenderView objects from the FrameGraph. The following
    diagrams show the construction for the first two RenderViews. The remaining
    RenderViews are very similar to the second diagram just with the other
    sub-trees.

    \image multiviewport-1.png

    \image multiviewport-2.png

    In full, the RenderViews created are:

    \list
        \li RenderView (1)
        \list
            \li Fullscreen viewport defined
            \li Color and Depth buffers are set to be cleared
        \endlist

        \li RenderView (2)
        \list
            \li Fullscreen viewport defined
            \li Sub viewport defined (rendering viewport will be scaled relative to its parent)
            \li CameraSelector specified
        \endlist

        \li RenderView (3)
        \list
            \li Fullscreen viewport defined
            \li Sub viewport defined (rendering viewport will be scaled relative to its parent)
            \li CameraSelector specified
        \endlist

        \li RenderView (4)
        \list
            \li Fullscreen viewport defined
            \li Sub viewport defined (rendering viewport will be scaled relative to its parent)
            \li CameraSelector specified
        \endlist

        \li RenderView (5)
        \list
            \li Fullscreen viewport defined
            \li Sub viewport defined (rendering viewport will be scaled relative to its parent)
            \li CameraSelector specified
        \endlist
    \endlist

    However, in this case the \e {order is important}. If the ClearBuffers node
    were to be the last instead of the first, this would result in a black
    screen for the simple reason that everything would be cleared right after
    having been so carefully rendered. For a similar reason, it could not be
    used as the root of the FrameGraph as that would result in a call to clear
    the whole screen for each of our viewports.

    Although the declaration order of the FrameGraph is important, Qt 3D is able
    to process each RenderView in parallel as each RenderView is independent of
    the others for the purposes of generating a set of RenderCommands to be
    submitted whilst the RenderView's state is in effect.

    Qt 3D uses a task-based approach to parallelism which naturally scales up
    with the number of available cores. This is shown in the following diagram
    for the previous example.

    \image framegraph-parallel-build.png

    The RenderCommands for the RenderViews can be generated in parallel across
    many cores, and as long as we take care to submit the RenderViews in the
    correct order on the dedicated OpenGL submission thread, the resulting
    scene will be rendered correctly.

    \section2 Deferred Renderer

    When it comes to rendering, deferred rendering is a different beast in
    terms of renderer configuration compared to forward rendering. Instead of
    drawing each mesh and applying a shader effect to shade it, deferred
    rendering adopts a \e {two render pass} method.

    First all the meshes in the scene are drawn using the same shader that will
    output, usually for each fragment, at least four values:

    \list
        \li World normal vector
        \li Color (or some other material properties)
        \li Depth
        \li World position vector
    \endlist

    Each of these values will be stored in a texture. The normal, color, depth,
    and position textures form what is called the G-Buffer. Nothing is drawn
    onscreen during the first pass, but rather drawn into the G-Buffer ready
    for later use.

    Once all the meshes have been drawn, the G-Buffer is filled with all the
    meshes that can currently be seen by the camera. The second render pass is
    then used to render the scene to the back buffer with the final color
    shading by reading the normal, color, and position values from the G-buffer
    textures and outputting a color onto a full screen quad.

    The advantage of that technique is that the heavy computing power required
    for complex effects is only used during the second pass only on the
    elements that are actually being seen by the camera. The first pass does
    not cost much processing power as every mesh is being drawn with a simple
    shader. Deferred rendering, therefore, decouples shading and lighting from
    the number of objects in a scene and instead couples it to the resolution
    of the screen (and G-Buffer). This is a technique that has been used in
    many games due to the ability to use large numbers of dynamic lights at
    the expense of additional GPU memory usage.

    \badcode
    Viewport {
         rect: Qt.rect(0.0, 0.0, 1.0, 1.0)

         property alias gBuffer: gBufferTargetSelector.target
         property alias camera: sceneCameraSelector.camera

         LayerFilter {
              layers: "scene"

              RenderTargetSelector {
                    id: gBufferTargetSelector

                    ClearBuffers {
                         buffers: ClearBuffers.ColorDepthBuffer

                         RenderPassFilter {
                               id: geometryPass
                               includes: Annotation { name: "pass"; value: "geometry" }

                               CameraSelector {
                                     id: sceneCameraSelector
                               }
                         }
                    }
              }
         }

         LayerFilter {
              layers: "screenQuad"

              ClearBuffers {
                   buffers: ClearBuffers.ColorDepthBuffer

                   RenderPassFilter {
                         id: finalPass
                         includes: Annotation { name: "pass"; value: "final" }
                   }
             }
         }
    }
    \endcode

    Graphically, the resulting framegraph looks like:

    \image deferred-framegraph.png

    And the resulting RenderViews are:

    \list
        \li RenderView (1)
        \list
            \li Define a viewport that fills the whole screen
            \li Select all Entities that have a Layer component matching
                \c "scene"
            \li Set the \c gBuffer as the active render target
            \li Clear the color and depth on the currently bound render target
                (the \c gBuffer)
            \li Select only Entities in the scene that have a Material and
                Technique matching the annotations in the RenderPassFilter
            \li Specify which camera should be used
        \endlist

        \li RenderView (2)
        \list
            \li Define a viewport that fills the whole screen
            \li Select all Entities that have a Layer component matching
                \c "screenQuad"
            \li Clear the color and depth buffers on the currently bound
                framebuffer (the screen)
            \li Select only Entities in the scene that have a Material and
                Technique matching the annotations in the RenderPassFilter
        \endlist
    \endlist

    \section1 Other Benefits of the framegraph

    Since the FrameGraph tree is entirely data-driven and can be modified dynamically at runtime, you can:

    \list
        \li Have different framegraph trees for different platforms and
            hardware and select the most appropriate at runtime
        \li Easily add and enable visual debugging in a scene
        \li Use different FrameGraph trees depending on the nature of what
            you need to render for a particular region of the scene
        \li Implement a new rendering technique without having to
            modify Qt 3D's internals
    \endlist

    \section1 Conclusion

    We have introduced the FrameGraph and the node types that compose it. We
    then went on to discuss a few examples to illustrate the framegraph
    building rules and how the Qt 3D engine uses the framegraph behind the
    scenes. By now you should have a pretty good overview of the FrameGraph and
    how it can be used (perhaps to add an \l {early z-fill pass} to a
    forward renderer). Also you should always keep in mind that the FrameGraph
    is a tool for you to use so that you are not tied down to the provided
    renderer and materials that Qt 3D provides out of the box.
*/
