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QSGMaterialShader Class

The QSGMaterialShader class represents a graphics API independent shader program. More...

Header: #include <QSGMaterialShader>
CMake: find_package(Qt6 REQUIRED COMPONENTS Quick)
target_link_libraries(mytarget PRIVATE Qt6::Quick)
qmake: QT += quick

Public Types

struct GraphicsPipelineState
class RenderState
enum Flag { UpdatesGraphicsPipelineState }
flags Flags

Public Functions

QSGMaterialShader()
(since 6.4) int combinedImageSamplerCount(int binding) const
QSGMaterialShader::Flags flags() const
void setFlag(QSGMaterialShader::Flags flags, bool on = true)
void setFlags(QSGMaterialShader::Flags flags)
virtual bool updateGraphicsPipelineState(QSGMaterialShader::RenderState &state, QSGMaterialShader::GraphicsPipelineState *ps, QSGMaterial *newMaterial, QSGMaterial *oldMaterial)
virtual void updateSampledImage(QSGMaterialShader::RenderState &state, int binding, QSGTexture **texture, QSGMaterial *newMaterial, QSGMaterial *oldMaterial)
virtual bool updateUniformData(QSGMaterialShader::RenderState &state, QSGMaterial *newMaterial, QSGMaterial *oldMaterial)

Protected Functions

void setShader(QSGMaterialShader::Stage stage, const QShader &shader)
void setShaderFileName(QSGMaterialShader::Stage stage, const QString &filename)

Detailed Description

QSGMaterialShader represents a combination of vertex and fragment shaders, data that define the graphics pipeline state changes, and logic that updates graphics resources, such as uniform buffers and textures.

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.

The QSGMaterial and QSGMaterialShader form a tight relationship. For one scene graph (including nested graphs), there is one unique QSGMaterialShader instance that encapsulates the shaders and other data the scene graph uses to render an object with that material. Each QSGGeometryNode can have a unique QSGMaterial that defines how the graphics pipeline must be configured while drawing the node. 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 creates an instance of QSGMaterialShader by calling the QSGMaterial::createShader() method, ensuring that there is only one instance of each shader implementation.

In Qt 5, QSGMaterialShader was tied to OpenGL. It was built directly on QOpenGLShaderProgram and had functions like updateState() that could issue arbitrary OpenGL commands. This is no longer the case in Qt 6. QSGMaterialShader is not strictly data-oriented, meaning it provides data (shaders and the desired pipeline state changes) together with logic that updates data in a uniform buffer. Graphics API access is not provided. This means that a QSGMaterialShader cannot make OpenGL, Vulkan, Metal, or Direct 3D calls on its own. Together with the unified shader management, this allows a QSGMaterialShader to be written once, and be functional with any of the supported graphics APIs at run time.

The shaders set by calling the protected setShaderFileName() function control what material does with the vertex data from the geometry, and how the fragments are shaded. A QSGMaterialShader will typically set a vertex and a fragment shader during construction. Changing the shaders afterwards may not lead to the desired effect and must be avoided.

In Qt 6, the default approach is to ship .qsb files with the application, typically embedded via the resource system, and referenced when calling setShaderFileName(). The .qsb files are generated offline, or at latest at application build time, from Vulkan-style GLSL source code using the qsb tool from the Qt Shader Tools module.

There are three virtuals that can be overridden. These provide the data, or the logic to generate the data, for uniform buffers, textures, and pipeline state changes.

updateUniformData() is the function that is most commonly reimplemented in subclasses. This function is expected to update the contents of a QByteArray that will then be exposed to the shaders as a uniform buffer. Any QSGMaterialShader that has a uniform block in its vertex or fragment shader must reimplement updateUniformData().

updateSampledImage() is relevant when the shader code samples textures. The function will be invoked for each sampler (or combined image sampler, in APIs where relevant), giving it the option to specify which QSGTexture should be exposed to the shader.

The shader pipeline state changes are less often used. One use case is materials that wish to use a specific blend mode. The relevant function is updateGraphicsPipelineState(). This function is not called unless the QSGMaterialShader has opted in by setting the flag UpdatesGraphicsPipelineState. The task of the function is to update the GraphicsPipelineState struct instance that is passed to it with the desired changes. Currently only blending and culling-related features are available, other states cannot be controlled by materials.

A minimal example, that also includes texture support, could be the following. Here we assume that Material is the QSGMaterial that creates an instance of Shader in its createShader(), and that it holds a QSGTexture we want to sample in the fragment shader. The vertex shader relies only on the modelview-projection matrix.

class Shader : public QSGMaterialShader
{
public:
    Shader()
    {
        setShaderFileName(VertexStage, QLatin1String(":/materialshader.vert.qsb"));
        setShaderFileName(FragmentStage, QLatin1String(":/materialshader.frag.qsb"));
    }

    bool updateUniformData(RenderState &state, QSGMaterial *, QSGMaterial *)
    {
        bool changed = false;
        QByteArray *buf = state.uniformData();
        if (state.isMatrixDirty()) {
            const QMatrix4x4 m = state.combinedMatrix();
            memcpy(buf->data(), m.constData(), 64);
            changed = true;
        }
        return changed;
    }

    void updateSampledImage(RenderState &, int binding, QSGTexture **texture, QSGMaterial *newMaterial, QSGMaterial *)
    {
        Material *mat = static_cast<Material *>(newMaterial);
        if (binding == 1)
            *texture = mat->texture();
    }
};

The Vulkan-style GLSL source code for the shaders could look like the following. These are expected to be preprocessed offline using the qsb tool, which generates the .qsb files referenced in the Shader() constructor.

#version 440
layout(location = 0) in vec4 aVertex;
layout(location = 1) in vec2 aTexCoord;
layout(location = 0) out vec2 vTexCoord;
layout(std140, binding = 0) uniform buf {
    mat4 qt_Matrix;
} ubuf;
out gl_PerVertex { vec4 gl_Position; };
void main() {
    gl_Position = ubuf.qt_Matrix * aVertex;
    vTexCoord = aTexCoord;
}
#version 440
layout(location = 0) in vec2 vTexCoord;
layout(location = 0) out vec4 fragColor;
layout(binding = 1) uniform sampler2D srcTex;
void main() {
    vec4 c = texture(srcTex, vTexCoord);
    fragColor = vec4(c.rgb * 0.5, 1.0);
}

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.

See also QSGMaterial, Scene Graph - Custom Material, Scene Graph - Two Texture Providers, and Scene Graph - Graph.

Member Type Documentation

enum QSGMaterialShader::Flag
flags QSGMaterialShader::Flags

Flag values to indicate special material properties.

ConstantValueDescription
QSGMaterialShader::UpdatesGraphicsPipelineState0x0001Setting this flag enables calling updateGraphicsPipelineState().

The Flags type is a typedef for QFlags<Flag>. It stores an OR combination of Flag values.

Member Function Documentation

QSGMaterialShader::QSGMaterialShader()

Constructs a new QSGMaterialShader.

[since 6.4] int QSGMaterialShader::combinedImageSamplerCount(int binding) const

Returns the number of elements in the combined image sampler variable at binding. This value is introspected from the shader code. The variable may be an array, and may have more than one dimension.

The count reflects the total number of combined image sampler items in the variable. In the following example, the count for srcA is 1, srcB is 4, and srcC is 6.

layout (binding = 0) uniform sampler2D srcA;
layout (binding = 1) uniform sampler2D srcB[4];
layout (binding = 2) uniform sampler2D srcC[2][3];

This count is the number of QSGTexture pointers in the texture parameter of QSGMaterialShader::updateSampledImage.

This function was introduced in Qt 6.4.

See also QSGMaterialShader::updateSampledImage.

QSGMaterialShader::Flags QSGMaterialShader::flags() const

Returns the currently set flags for this material shader.

See also setFlags().

void QSGMaterialShader::setFlag(QSGMaterialShader::Flags flags, bool on = true)

Sets the flags on this material shader if on is true; otherwise clears the specified flags.

void QSGMaterialShader::setFlags(QSGMaterialShader::Flags flags)

Sets the flags for this material shader.

See also flags().

[protected] void QSGMaterialShader::setShader(QSGMaterialShader::Stage stage, const QShader &shader)

Sets the shader for the specified stage.

[protected] void QSGMaterialShader::setShaderFileName(QSGMaterialShader::Stage stage, const QString &filename)

Sets the filename for the shader for the specified stage.

The file is expected to contain a serialized QShader.

[virtual] bool QSGMaterialShader::updateGraphicsPipelineState(QSGMaterialShader::RenderState &state, QSGMaterialShader::GraphicsPipelineState *ps, QSGMaterial *newMaterial, QSGMaterial *oldMaterial)

This function is called by the scene graph to enable the material to provide a custom set of graphics state. The set of states that are customizable by material is limited to blending and related settings.

Note: This function is only called when the UpdatesGraphicsPipelineState flag was enabled via setFlags(). By default it is not set, and so this function is never called.

The return value must be true whenever a change was made to any of the members in ps.

Note: The contents of ps is not persistent between invocations of this function.

The current rendering state is passed from the scene graph.

The subclass specific state can be extracted from newMaterial. When oldMaterial is null, this shader was just activated.

[virtual] void QSGMaterialShader::updateSampledImage(QSGMaterialShader::RenderState &state, int binding, QSGTexture **texture, QSGMaterial *newMaterial, QSGMaterial *oldMaterial)

This function is called by the scene graph to prepare use of sampled images in the shader, typically in the form of combined image samplers.

binding is the binding number of the sampler. The function is called for each combined image sampler variable in the shader code associated with the QSGMaterialShader.

texture is an array of QSGTexture pointers. The number of elements in the array matches the number of elements in the image sampler variable specified in the shader code. This variable may be an array, and may have more than one dimension. The number of elements in the array may be found via QSGMaterialShader::combinedImageSamplerCount

When an element in texture is null, it must be set to a valid QSGTexture pointer before returning. When non-null, it is up to the material to decide if a new QSGTexture * is stored to it, or if it updates some parameters on the already known QSGTexture. The ownership of the QSGTexture is not transferred.

The current rendering state is passed from the scene graph. Where relevant, it is up to the material to trigger enqueuing texture data uploads via QSGTexture::commitTextureOperations().

The subclass specific state can be extracted from newMaterial.

oldMaterial can be used to minimize changes. When oldMaterial is null, this shader was just activated.

See also QSGMaterialShader::combinedImageSamplerCount.

[virtual] bool QSGMaterialShader::updateUniformData(QSGMaterialShader::RenderState &state, QSGMaterial *newMaterial, QSGMaterial *oldMaterial)

This function is called by the scene graph to get the contents of the shader program's uniform buffer updated. The implementation is not expected to perform any real graphics operations, it is merely responsible for copying data to the QByteArray returned from RenderState::uniformData(). The scene graph takes care of making that buffer visible in the shaders.

The current rendering state is passed from the scene graph. If the state indicates that any relevant state is dirty, the implementation must update the appropriate region in the buffer data that is accessible via RenderState::uniformData(). When a state, such as, matrix or opacity, is not dirty, there is no need to touch the corresponding region since the data is persistent.

The return value must be true whenever any change was made to the uniform data.

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

oldMaterial can be used to minimize buffer changes (which are typically memcpy calls) when updating material states. When oldMaterial is null, this shader was just activated.

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