ShaderEffect on layer create weird artifacts on text

ebatsin

Hi,

I am trying to apply an effect on a Item and its children. To do so I'm using a layer with a ShaderEffect but even when I'm just passing the source texture through I get a different result than if I had not used an effect.

Here is the code I use:

import QtQuick 2.15
import QtGraphicalEffects 1.15

Item {
    id: root

    layer.enabled: true
    layer.samplerName: 'source'
    layer.effect: ShaderEffect {
        fragmentShader: "
            #version 330 core // My whole application is using core 3.3 so that matches the global profile

            uniform highp sampler2D source;
            uniform lowp float qt_Opacity;

            in highp vec2 qt_TexCoord0;
            out highp vec4 out_color;

            void main() {
                // Just pass the source texture through and apply opacity (does not change anything if I do not multiply by the opacity)
                out_color = texture(source, qt_TexCoord0) * qt_Opacity;
            }
        "
    }
}

And then at the point of usage (MyItem is centered in an orange window)

MyItem {
    width: 350
    height: 200

    Rectangle {
        width: 50
        height: 50
        x: 3
        y: 3
        gradient: Gradient {
            GradientStop { position: 0.0; color: "red" }
            GradientStop { position: 1.0; color: Qt.hsla(1.0, 0, 0, 0) }
        }
    }

    Text {
        text: 'This is a test text. See if the text looks "normal"'
        color: 'red'
    }
}

Then if I disable the layer I get the following image that looks correct and how I would expect it to look. (This is my ground truth):

But when I enable the layer I get the following:

The gradient is preserved correctly but I get "dark" pixels on the text and it is more visible over the gradient then it should be.

Am I wrong to assume that just passing the source texture through should result in the exact same result as if there was no shader effect? What have I done wrong and how can I fix that? This effect will be used in positions where there will be a lot of text so it needs to preserve its look.

Thanks

edit:
Adding my configuration in case it is important:
Qt Version: 5.15.2
OS: Ubuntu 21.04 on X11

ebatsin

I tested it on both Intel & Nvidia GPUs and I can see the issue on both hardware.

What's weird is it that it looks like the kind of issues that would happen when something is wrong in the premultiplied alpha but the documentation says that the input texture is already premultiplied and the output FBO should be premultiplied. So by directly passing the input to the output it should work out okay.