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Please review the following information to ensure ** the GNU Free Documentation License version 1.3 requirements ** will be met: http://www.gnu.org/copyleft/fdl.html. ** $QT_END_LICENSE$ ** ****************************************************************************/ /*! \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. */