QSGMaterialShader Class

The QSGMaterialShader class represents an OpenGL shader program in the renderer. More...

Header: #include <QSGMaterialShader>
qmake: QT += quick
Inherited By:

QSGMaterialRhiShader and QSGSimpleMaterialShader

Public Types

class RenderState

Public Functions

QSGMaterialShader()
virtual void activate()
virtual const char *const *attributeNames() const = 0
virtual void deactivate()
QOpenGLShaderProgram *program()
virtual void updateState(const QSGMaterialShader::RenderState &state, QSGMaterial *newMaterial, QSGMaterial *oldMaterial)

Protected Functions

virtual void compile()
virtual const char *fragmentShader() const
virtual void initialize()
void setShaderSourceFile(QOpenGLShader::ShaderType type, const QString &sourceFile)
void setShaderSourceFiles(QOpenGLShader::ShaderType type, const QStringList &sourceFiles)
virtual const char *vertexShader() const

Detailed Description

The QSGMaterialShader API is relatively low-level. A more convenient API, which provides almost all the same features, is available through QSGSimpleMaterialShader.

Warning: This class is only functional when running with the legacy OpenGL renderer of the Qt Quick scenegraph.

The QSGMaterial and QSGMaterialShader form a tight relationship. For one scene graph (including nested graphs), there is one unique QSGMaterialShader instance which encapsulates the QOpenGLShaderProgram the scene graph uses to render that material, such as a shader to flat coloring of geometry. Each QSGGeometryNode can have a unique QSGMaterial containing the how the shader should be configured when drawing that node, such as the actual color used to render the geometry.

An instance of QSGMaterialShader is never created explicitly by the user, it will be created on demand by the scene graph through QSGMaterial::createShader(). The scene graph will make sure that there is only one instance of each shader implementation through a scene graph.

The source code returned from vertexShader() is used to control what the material does with the vertiex data that comes in from the geometry. The source code returned from the fragmentShader() is used to control what how the material should fill each individual pixel in the geometry. The vertex and fragment source code is queried once during initialization, changing what is returned from these functions later will not have any effect.

The activate() function is called by the scene graph when a shader is is starting to be used. The deactivate function is called by the scene graph when the shader is no longer going to be used. While active, the scene graph may make one or more calls to updateState() which will update the state of the shader for each individual geometry to render.

The attributeNames() returns the name of the attributes used in the vertexShader(). These are used in the default implementation of activate() and deactivate() to decide whice vertex registers are enabled.

The initialize() function is called during program creation to allow subclasses to prepare for use, such as resolve uniform names in the vertexShader() and fragmentShader().

A minimal example:

class Shader : public QSGMaterialShader
{
public:
    const char *vertexShader() const {
        return
        "attribute highp vec4 vertex;          \n"
        "uniform highp mat4 matrix;            \n"
        "void main() {                         \n"
        "    gl_Position = matrix * vertex;    \n"
        "}";
    }

    const char *fragmentShader() const {
        return
        "uniform lowp float opacity;                            \n"
        "void main() {                                          \n"
                "    gl_FragColor = vec4(1, 0, 0, 1) * opacity; \n"
        "}";
    }

    char const *const *attributeNames() const
    {
        static char const *const names[] = { "vertex", 0 };
        return names;
    }

    void initialize()
    {
        QSGMaterialShader::initialize();
        m_id_matrix = program()->uniformLocation("matrix");
        m_id_opacity = program()->uniformLocation("opacity");
    }

    void updateState(const RenderState &state, QSGMaterial *newMaterial, QSGMaterial *oldMaterial)
    {
        Q_ASSERT(program()->isLinked());
        if (state.isMatrixDirty())
            program()->setUniformValue(m_id_matrix, state.combinedMatrix());
        if (state.isOpacityDirty())
            program()->setUniformValue(m_id_opacity, state.opacity());
    }

private:
    int m_id_matrix;
    int m_id_opacity;
};

Note: All classes with QSG prefix should be used solely on the scene graph's rendering thread. See Scene Graph and Rendering for more information.

Member Function Documentation

QSGMaterialShader::QSGMaterialShader()

Creates a new QSGMaterialShader.

[virtual] void QSGMaterialShader::activate()

This function is called by the scene graph to indicate that geometry is about to be rendered using this shader.

State that is global for all uses of the shader, independent of the geometry that is being drawn, can be setup in this function.

[pure virtual] const char *const *QSGMaterialShader::attributeNames() const

Returns a zero-terminated array describing the names of the attributes used in the vertex shader.

This function is called when the shader is compiled to specify which attributes exist. The order of the attribute names defines the attribute register position in the vertex shader.

[virtual protected] void QSGMaterialShader::compile()

This function is called when the shader is initialized to compile the actual QOpenGLShaderProgram. Do not call it explicitly.

The default implementation will extract the vertexShader() and fragmentShader() and bind the names returned from attributeNames() to consecutive vertex attribute registers starting at 0.

[virtual] void QSGMaterialShader::deactivate()

This function is called by the scene graph to indicate that geometry will no longer to be rendered using this shader.

[virtual protected] const char *QSGMaterialShader::fragmentShader() const

Called when the shader is being initialized to get the fragment shader source code.

The contents returned from this function should never change.

[virtual protected] void QSGMaterialShader::initialize()

Reimplement this function to do one-time initialization when the shader program is compiled. The OpenGL shader program is compiled and linked, but not bound, when this function is called.

QOpenGLShaderProgram *QSGMaterialShader::program()

Returns the shader program used by this QSGMaterialShader.

[protected] void QSGMaterialShader::setShaderSourceFile(QOpenGLShader::ShaderType type, const QString &sourceFile)

Sets the GLSL source file for the shader stage type to sourceFile. The default implementation of the vertexShader() and fragmentShader() functions will load the source files set by this function.

This function is useful when you have a single source file for a given shader stage. If your shader consists of multiple source files then use setShaderSourceFiles()

See also setShaderSourceFiles(), vertexShader(), and fragmentShader().

[protected] void QSGMaterialShader::setShaderSourceFiles(QOpenGLShader::ShaderType type, const QStringList &sourceFiles)

Sets the GLSL source files for the shader stage type to sourceFiles. The default implementation of the vertexShader() and fragmentShader() functions will load the source files set by this function in the order given.

See also setShaderSourceFile(), vertexShader(), and fragmentShader().

[virtual] void QSGMaterialShader::updateState(const QSGMaterialShader::RenderState &state, QSGMaterial *newMaterial, QSGMaterial *oldMaterial)

This function is called by the scene graph before geometry is rendered to make sure the shader is in the right state.

The current rendering state is passed from the scene graph. If the state indicates that any state is dirty, the updateState implementation must update accordingly for the geometry to render correctly.

The subclass specific state, such as the color of a flat color material, should be extracted from newMaterial to update the color uniforms accordingly.

The oldMaterial can be used to minimze state changes when updating material states. The oldMaterial is 0 if this shader was just activated.

See also activate() and deactivate().

[virtual protected] const char *QSGMaterialShader::vertexShader() const

Called when the shader is being initialized to get the vertex shader source code.

The contents returned from this function should never change.

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