Use a shader to hide part of an image

[plaristote]

Greetings !
I'm using Qt 5.15, and I would like to hide part of an image. Basically, cutting out a circle at given coordinates. And I've seen some puzzling things while trying to achieve that.

Firstly, I thought that shaders were actually allowing me to modify an image before drawing it. But it seems that the shader actually draws on top o the existing image. Is that so ?

Here's my shader so far:

ShaderEffect {
  property variant source
  property point    position
  property real     radius: 40
  property real     centerX: position.x - parent.x
  property real     centerY: position.y - parent.y

  anchors.fill: parent
  vertexShader: "
      uniform highp mat4 qt_Matrix;
      attribute highp vec4 qt_Vertex;
      attribute highp vec2 qt_MultiTexCoord0;
      varying highp vec2 coord;
      void main() {
          coord = qt_MultiTexCoord0;
          gl_Position = qt_Matrix * qt_Vertex;
      }"
  fragmentShader: "
      varying highp vec2 coord;
      uniform sampler2D source;
      uniform lowp float qt_Opacity;
      uniform lowp float centerX;
      uniform lowp float centerY;
      uniform lowp float radius;
      uniform lowp float width;
      uniform lowp float height;
      void main() {
          lowp vec4 tex = texture2D(source, coord);
          if (coord.x * width  >= centerX - radius && coord.x * width  <= centerX + radius
           && coord.y * height >= centerY - radius && coord.y * height <= centerY + radius) {
            gl_FragColor = vec4(0.0, 0.0, 0.0, 1.0);
          }
          else {
            gl_FragColor = tex * qt_Opacity;
          }
      }"
}

Currently, it draws a black rectangle where I want it to.
If I set gl_FragColor to vec4(0.0, 0.0, 0.0, 0.0), then it just does nothing... while I was expecting it to hide the part of the source image where the black rectangle appears in my test case.

Is there any obvious reason why it's not working ? Am I wrong in assuming the shader changes the pixel of the source image ?

Thanks !

[plaristote]

Ok, I figured it out !

The shader does not change the Image, it just draws something else on the shader item's position.

The solution was pretty darn simple: make the Image invisible and let the shadereffect item handle all the rendering part.