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/*!
    \example scenegraph/simplematerial
    \title Scene Graph - Simple Material
    \ingroup qtquickexamples
    \brief Shows how to define a scene graph material to fill a shape.

    \image simplematerial-example.jpg

    In this example, we will make use of the \l
    QSGSimpleMaterialShader class to fill a shape in the scene
    graph. This is a convenience class intended to avoid a lot of the
    boilerplate code required when creating materials with the \l
    QSGMaterial, \l QSGMaterialShader and \l QSGMaterialType classes
    directly.

    A simple material consists of two parts: the material state and
    the material shader. The material shader has one instance per
    scene graph and contains the actual OpenGL shader program and
    information about which attributes and uniforms it uses. The
    material state is what we assign to each individual node; in this
    case to give them different colors.

    \snippet scenegraph/simplematerial/simplematerial.cpp 1

    The first thing we do when creating custom materials with the
    simplified scheme is to create a state class. In this case the
    state class contains only one member, a QColor. It also defines a
    compare function which the scene graph can use to reorder the node
    rendering.

    \snippet scenegraph/simplematerial/simplematerial.cpp 2

    Next we define the material shader, by subclassing a template
    instantiation of \l QSGSimpleMaterialShader with our \c State.

    Then we use the macro \l QSG_DECLARE_SIMPLE_COMPARABLE_SHADER()
    which will generate some boilerplate code for us. Since our \c
    State class has a compare function, we declare that the states can
    be compared. It would have been possible to remove the \c
    State::compare() function and instead declare the shader with \l
    QSG_DECLARE_SIMPLE_SHADER(), but this could then reduce performance
    in certain use cases.

    The state struct is used as a template parameter to
    automatically generate a \l QSGMaterialType for us, so it is
    crucial that the pair of shader and state are made up of unique
    classes. Using the same \c State class in multiple shaders will
    will lead to undefined behavior.

    \snippet scenegraph/simplematerial/simplematerial.cpp 3

    Next comes the declaration of the shader source code, where we
    define a vertex and fragment shader. The simple material assumes
    the presence of \c qt_Matrix in the vertex shader and \c
    qt_Opacity in the fragment shader.

    \snippet scenegraph/simplematerial/simplematerial.cpp 4

    We reimplement the \c attributes function to return the name of
    the \c aVertex and \c aTexCoord attributes. These attributes
    will be mapped to attribute indices 0 and 1 in the node's
    geometry.

    \snippet scenegraph/simplematerial/simplematerial.cpp 6

    Uniforms can be accessed either by name or by index, where index
    is faster than name. We reimplement the \c resolveUniforms()
    function to find the index of the \c color uniform. We do not have
    to worry about resolving \c qt_Opacity or \c qt_Matrix as these
    are handled by the baseclass.

    \snippet scenegraph/simplematerial/simplematerial.cpp 5

    The \c updateState() function is called once for every unique
    state and we use it to update the shader program with the current
    color. The previous state is passed in as a second parameter so
    that the user can update only that which has changed. In our
    use case, where all the colors are different, the updateState()
    function will be called once for every node.

    \snippet scenegraph/simplematerial/simplematerial.cpp 7

    The \c ColorNode class is supposed to draw something, so it needs
    to be a subclass of \l QSGGeometryNode.

    Since our shader expects both a position and a texture coordinate,
    we use the default attribute set \l
    QSGGeometry::defaultAttributes_TexturedPoint2D() and declare that
    the geometry consists of a total of four vertices. To avoid the
    allocation, we make the QSGGeometry a member of the
    QSGGeometryNode.

    When we used the macro \l QSG_DECLARE_SIMPLE_COMPARABLE_SHADER() above,
    it defined the \c createMaterial() function which we use to
    instantiate materials for our \c State struct.

    As we will be making use of opacity in our custom material, we
    need to set the \l QSGMaterial::Blending flag. The scene graph may
    use this flag to either disable or enable \c GL_BLEND when drawing
    the node or to reorder the drawing of the node.

    Finally, we tell the node to take ownership of the material, so we
    do not have to explicitly memory-manage it.

    \snippet scenegraph/simplematerial/simplematerial.cpp 8

    Since the Item is providing its own graphics to the scene graph,
    we set the flag \l QQuickItem::ItemHasContents.

    \snippet scenegraph/simplematerial/simplematerial.cpp 9

    Whenever the Item has changed graphically, the \l
    QQuickItem::updatePaintNode() function is called.

    \note The scene graph may be rendered in a different thread than the
    GUI thread and \l QQuickItem::updatePaintNode() is one of the few
    places where it is safe to access properties of the QML
    object. Any interaction with the scene graph from a custom \l
    QQuickItem should be contained within this function. The function is
    called on the rendering thread while the GUI thread is blocked.

    The first time this function is called for an \c Item instance,
    the node will be 0, and so we create a new one. For every consecutive
    call, the node will be what we returned previously. There are
    scenarios where the scene graph will be removed and rebuilt from
    scratch however, so one should always check the node and recreate
    it if required.

    Once we have a \c ColorNode, we update its geometry and material
    state. Finally, we notify the scene graph that the node has
    undergone changes to its geometry and material.

    \snippet scenegraph/simplematerial/simplematerial.cpp 11

    The \c main() function of the application adds the custom QML type
    using \l qmlRegisterType() and opens up a \l QQuickView with our
    QML file.

    \snippet scenegraph/simplematerial/main.qml 1

    In the QML file, we import our custom type so we can instantiate
    it.

    \snippet scenegraph/simplematerial/main.qml 2

    Then we create a column containing three instances of our custom item,
    each with a different color.

    \snippet scenegraph/simplematerial/main.qml 3

    And finally we overlay a short descriptive text.

 */
