Xamarin OpenGL片段着色器使用制服时的行为异常

Question

The fragment shader I'm currently working on has some quite weird behaviour which is driving me nuts. I have a frame buffer object which I'm rendering. In the fragment shader I also read from the same texture using my own written 'texelFetch' (which is not implemented by default as I'm using OpenGL ES):

vec4 texelFetch(sampler2D tex, ivec2 size, ivec2 coord)
{
    vec2 texCoord= vec2((2.0*float(coord.x) + 1.0)/(2.0*float(size.x)), (2.0*float(coord.y) + 1.0)/(2.0*float(size.y)));
    return texture2D(tex, texCoord);
}

This means I also need to know the size of the texture I am rendering when I called this function. The size of the texture is 1440x900 which is represented by an uniform in my shader:

uniform vec2 screenImageSize;

I set this value in my program as follows:

if (imageProgram != null)
            imageProgram.Use();

screenImageSizeUniform = imageProgram.GetUniformIndex("screenImageSize");
GL.Uniform2(screenImageSizeUniform, 1440.0f, 900.0f);


// Draw triangles that represents frame buffer rectangle
GL.DrawArrays(BeginMode.Triangles, 0, 6);

The weird thing is that I don't receive the correct outputs when I'm using this uniform. I do receive the correct output when I hardcode the values however. In addition, if I perform a check to see what the values are, I see that the uniform is set correctly:

//Following line does not give me the correct result as it seems to shift pixel values
//ivec2 test=ivec2(int(screenImageSize.x),int(screenImageSize.y));

//Following line does give me the correct result
ivec2 test=ivec2(1440,900);

if(test.x==1440 && test.y==900)
    gl_FragColor=texelFetch(previousFullImage,test,ivec2(gl_FragCoord.x,gl_FragCoord.y));
else
    gl_FragColor=vec4(1.0,0.0,0.0,1.0);

Can anyone see what I'm doing wrong? Any help is greatly appreciated

Note: the centre image is slightly rotated but that is due to me making the screenshot incorrectly.

Answer 1

Based on the information that @Isogen74 provided, I concluded that it might indeed be a bug in the GPU stack somewhere. I therefore decided to look for a workaround and update to opengl es 3.0. texelFetch has been implemented there, without requiring the dimensions of the texture. Now it works like a charm! Thanks for all the help=)! Its really appreciated!