ShaderEffect low texture precision

  • I use QQuickFramebufferObject with GL_RGB32F Internal texture format. Each fragment of the result texture represents it's position in 3d space. Then I postprocess it with
    ShaderEffect, the texture, I get in the shader is a GL_RGB8 representation of the one, the FBO storages. It gives me poor color gradation and bad aliasing effect. How can I access the source 32bit texture?

                    CTile {  //GL_RGB32F
                        anchors.fill: parent
                        layer.enabled: true
                        layer.smooth: true
                        layer.effect: ShaderEffect {
                            fragmentShader: "
                                uniform highp sampler2D source; // this item
                                varying highp vec2 qt_TexCoord0;
                                uniform int mode;
                                highp out vec4 color;
                                void main() {
                                    highp vec4 srccolor = texture2D(source, qt_TexCoord0); //GL_RGB8

    It also cuts negative values.
    Here is one of the effects:
    alt text

  • I guess, I've found the reason of the problem, it's incorrect internal format in shadereffectsource.cpp:

    enum Format {
        Alpha = GL_ALPHA,
        RGB = GL_RGB,
        RGBA = GL_RGBA

    There should be

    enum Format {
        Alpha = GL_R32F,
        RGB = GL_RGB32F,
        RGBA = GL_RGBA32F

    I tried to fix it though qml this way:

    ShaderEffectSource {
        anchors.fill: parent
        sourceItem: item
        format: 0x8814 // GL_RGBA32F
        recursive: false
        hideSource: true

    But I get this error: Invalid property assignment: unknown enumeration
    Why there is a buffer in the middle, wouldn't it be faster to use source fbo directly?
    Can I fix it without recompiling Qt ?
    Is there a way to bind texture from the source fbo right before ShaderEffectItem::renderEffect, so I would be able to use source texture instead of it's bad analog?

  • Hi, Dmitry.
    I believe ShaderEffectSource/layer.enabled will rerender a source item, despite it has a source texture.
    In your case, you can just pass the original texture to the ShaderEffect.

    CTile { 
        id: sourceItem
        visible: false
    ShaderEffect {
        anchors.fill: sourceItem
        property Item source: sourceItem
        fragmentShader: "..."

    Support of RGBA32F in ShaderEffectSource can be useful if the result of shader processing should remain in high precision format. The issue with the request for this feature was created.

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