ShaderEffect low texture precision

Dmitry87

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

Dmitry87

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?

t.artikov

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.
https://bugreports.qt.io/browse/QTBUG-38279