I'm experiencing an issue with passing a uniform value to my shader in a Flutter application using the flutter_gpu package. Despite setting the intended value in the Dart code, the vertex shader always receives a default matrix of mat4(0.0). A minimal reproducible repository is available here: https://github.com/vulpeep/gpu_image_example.

Steps to Reproduce

1. CustomPainter setup:

   • In image_painter.dart, a transform matrix is created and written to a uniform buffer.
   • The shader is expected to use this matrix to compute texture coordinates.

2. Vertex Shader Code:

   #version 460
   
   in vec2 inPosition;
   out vec2 fragTexCoord;
   
   uniform Transform {
       mat4 transform;
   };
   
   void main() {
       gl_Position = vec4(inPosition * 2.0 - 1.0, 0.0, 1.0);
       fragTexCoord = (transform * vec4(vec2(inPosition.x, 1.0 - inPosition.y), 0.0, 1.0)).xy;
   }

3. Fragment Shader Code:

   #version 460
   
   in vec2 fragTexCoord;
   out vec4 FragColor;
   
   uniform sampler2D uTexture;
   
   void main() {
       FragColor = texture(uTexture, fragTexCoord);
   }

4. Execution:

   • The Flutter app loads a texture (as shown in utils.dart and image_widget.dart) and renders it via a custom paint widget.
   • Although the uniform is bound with the intended matrix, the output behaves as if no uniform was passed, resulting in incorrect texture mapping.

Behavior

• Expected: The value set in the Dart code should affect the vertex shader, producing correct texture coordinates and a properly rendered image.
  Screenshot

  
• Actual: The shader always defaults to mat4(0.0), ignoring the provided uniform value and leading to unexpected render output.
  Screenshot

  

Additional Details

• Flutter version: I'm currently working with Flutter version 3.27.4. Due to an existing Flutter issue Impeller GPU calls fail to render on Windows (GLES) #160948, I am unable to test this problem on later versions.
• Platform specific behavior: Testing on macOS did not reveal this problem - the uniform is passed correctly on macOS for both Flutter version 3.27.4 and on the master branch.