Issues with binding image texture to compute shader #2613

speediplayz · 2026-08-26T23:49:21Z

speediplayz
Aug 26, 2026

I have been trying to get compute shaders working with Silk.NET OpenGL, and i have got it to a point where they compile and link fine with no errors, however the issues start with trying to bind an image to the compute shader for reading and writing. if i use a 1x1 texture it works just fine, and then i can read the data back and print a pixel (the test shader simply sets pixel 0, 0 to red), but when i change the texture size to 2x2 or anything else, it suddenly stops working, pixel 0,0 suddenly reads 0,0,0,0 and I cannot figure out why.

Compute is just an object to store the program id etc

            Compute comp = new Compute("testCompute.comp");
            uint testTex = rend.gl.GenTexture();
            rend.gl.BindTexture(TextureTarget.Texture2D, testTex);

            uint sx = 2, sy = 2;
            rend.gl.TexImage2D(
                GLEnum.Texture2D,
                0,
                (int)GLEnum.Rgba8,
                sx,
                sy,
                0,
                GLEnum.Rgba,
                GLEnum.UnsignedByte,
                null
            );

            rend.gl.UseProgram(comp.program);

            rend.gl.BindImageTexture(
                0,
                testTex,
                0,
                false,
                0,
                BufferAccessARB.WriteOnly,
                InternalFormat.Rgba8
            );

            comp.dispatch(rend.gl, width, height, 1);

            byte[] data = new byte[sx * sy * 4];
            rend.gl.BindTexture(TextureTarget.Texture2D, testTex);

            fixed (byte* ptr = data)
            {
                rend.gl.GetTexImage(
                    TextureTarget.Texture2D,
                    0,
                    GLEnum.Bgra,
                    GLEnum.UnsignedByte,
                    ptr
                );
            }

            Console.WriteLine($"Pixel 0,0: B={data[0]}, G={data[1]}, R={data[2]}, A={data[3]}");`

the above code is my test code for the shader, I tried to change dispatch sizes to 1, 1, 1, or width, height, 1, or a few different combinations. it works perfectly fine with sx=sy=1 which tells me there is no issues with the compilation, linking or dispatching of the shader. For those curious the shader is:

#version 450
layout(local_size_x = 1, local_size_y = 1, local_size_z = 1) in;
layout(rgba8, binding = 0) uniform image2D imgTex;

void main() {
    imageStore(imgTex, ivec2(0, 0), vec4(1, 0, 0, 1));
}

I have verified as well that there are no errors after pretty much every line of code, gl.GetError prints NoError every time. I am curious if anyone else has had this issue/has a solution.

Maxaubert · 2026-09-28T19:53:19Z

Maxaubert
Sep 28, 2026

Your texture is mipmap-incomplete, so the shader's imageStore is being silently dropped by the driver. That's exactly why the 1x1 case "just works" and everything else doesn't.

Why

You never touch GL_TEXTURE_MIN_FILTER, so it stays at its default, GL_NEAREST_MIPMAP_LINEAR – a filter that requires mipmaps. You only ever upload level 0 via TexImage2D. For any texture bigger than 1x1, that means the mip chain is incomplete (a 2x2 texture needs levels 0 and 1 to be "mipmap complete"; you only have 0).

Per the GL_ARB_shader_image_load_store spec (core since GL 4.2), an image access is invalid if "the texture bound to the selected image unit is incomplete", and an incomplete image unit is treated as if nothing were bound: loads return 0, and stores have no effect. That's your 0,0,0,0 readback.

A 1x1 texture is the one size where this never bites you: level 0 is the only mip level that can possibly exist (floor(log2(1)) = 0), so the texture is always complete no matter what MIN_FILTER is set to. That's why only the 1x1 case works.

Fix

Either set a non-mipmapping min filter:

rend.gl.TexParameterI(TextureTarget.Texture2D, TextureParameterName.TextureMinFilter, (int)TextureMinFilter.Nearest);

or, better, use immutable storage (TexStorage2D), which is always considered mipmap complete regardless of the filter:

rend.gl.TexStorage2D(TextureTarget.Texture2D, 1, GLEnum.Rgba8, sx, sy);

(TexStorage2D needs the ARB_texture_storage/GL 4.2 entry point, which Silk.NET's GL class exposes directly.)

One more thing worth fixing while you're in there: you're calling GetTexImage right after dispatch with no glMemoryBarrier. Image stores are incoherent by default, so add this before the readback:

rend.gl.MemoryBarrier(MemoryBarrierMask.TextureUpdateBarrierBit);

It happens to work without it on most drivers for a trivial single-invocation shader like yours, but it's undefined per spec, and you don't want it becoming your next "works on my machine" bug.

Sources: GL_ARB_shader_image_load_store spec, "Texture Image Loads and Stores", OpenGL wiki, Texture completeness, Silk.NET's own TexParameterI usage pattern: Texture.cs#L81-L88.

1 reply

speediplayz Sep 28, 2026
Author

Thanks for the explanation, I could never figure out why I was having issues with the texture image. I did end up arriving at using a texture storage in place of it and having it work, just completely forgot about this thread. As to the memory barrier, my comp.dispatch method has a memory barrier in it.

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