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\title Scene Graph - Simple Material
\brief Shows how to define a scene graph material to fill a shape.
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
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
\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
\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
usecase, where all the colors are different, the updateState 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
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
\snippet scenegraph/simplematerial/main.qml 1
In the QML file, we import our custom type so we can instantiate
\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.