Compute Shader RenderTexture problem

Graphics Shaders & Materials
1

I am currently trying to get a compute shader to work (using this as a guide). At the moment id doesn’t do anything interesting, it is only supposed to copy one texture into another. I will later do some modification to the image, but I can’t even get the copying to work correctly. It only ever uses the first row of pixels, but repeats them over the whole image.

Shader:

#pragma kernel CSMain

Texture2D<float4> cTex;
RWTexture2D<float4> rTex;

[numthreads(32,32,1)]
void CSMain (uint2 id : SV_DispatchThreadID) {
    rTex[id] = cTex[id];
}

C# to run it:

ComputeShader _cShader;
Texture2D _tex;
RenderTexture _rTex;

//Start
_cShader = Resources.Load<ComputeShader>("TestShader");
_tex = new Texture2D(1920, 1080, TextureFormat.RGBA32, false);
_rTex = new RenderTexture(_tex.width, _tex.height, 0);
_rTex.enableRandomWrite = true;
_rTex.Create();

//Update
_tex.LoadRawTextureData(SomeFunctionThatReturnsByteArray());

int kernel = _cShader.FindKernel("CSMain");

_cShader.SetTexture(kernel, "rTex", _rTex);
_cShader.SetTexture(kernel, "cTex", _tex);
_cShader.Dispatch(kernel, _data.color.width/32, _data.color.height/32, 1);

I also tried an even simpler Shader that should only create an UV gradient, but this also didn’t create what I expected. The Y-Component was always 0.

[numthreads(32,32,1)]
void CSMain(uint2 id : SV_DispatchThreadID) {
    rTex[id] = float4(id.x/1920.0f, id.y/1080.0f, 0, 1);
}

Same behaviour with square pot-textures, with different combinations of threads per group and groups per dispatch*, switching x and y in the DispatchID (or even making it uint3 and try z) or when I calculate the index myself using ‘SV_GroupThreadID’ and ‘SV_GroupID’.
(Using Unity 5.2.0f3)

Clearly I am doing something wrong, but I don’t know what, where or how…

*: only difference: if number_of_groups_x * number_of_threads_per_group_x doesn’t add up to the texture’s width, the right side remains blank, but this is to be expected. No equivalent behaviour in Y direction, though

2

That is quite strange… id.y cannot be always zero, otherwise rTex[id] = stuff would only be writing to the first row and the rest of the image would be black or garbage… but you’re saying that all other rows are the same as the first row.
Are you sure you’re displaying the resulting texture correctly? What happens when you deliberately write only to the first row, like this:

void CSMain (uint2 id : SV_DispatchThreadID) {
    if (id.y == 0) {
        rTex[id] = float4(id.x/1920.0f, id.y/1080.0f, 0, 1);
    }
}
3

I just tested this, and it shows the same behaviour.

But I have found the Problem. It lies not with the shader, but in how I use the image. I use it in a RawImage UI Component, but because the source image was flipped along the y axis, I set the Rect’s height to -1 (this worked perfectly when I was using the source image directly). Setting this back to 1 shows the complete texture, but upside down. But since I already read each pixel from a source image and copy it modified into a destination texture, I can flip the image in the shader instead.

THank you for your suggestion. It put me on the right path.