HLSL Set pixel position in pixel shader to control where the pixel will end up in the texture

Question

How can I manually set where the pixel ends up in the texture in PixelShaderFunction HLSL? Ideally I want the GPU to follow the next logic:

1. Write pixels one by one in no particular order. Meaning whenever first pixel comes out, write it into the top left corner of the texture. Write second one to the right of the first one and the third one to the right of the first one, and so on.
2. When you reach the end of the line - go to the next line.
3. When you reach the end of the texture - drop all the remaining pixels.

Thanks.

I feel like I can do it by manually computing the needed position for my pixel at the vertex shader level. If I could understand better how the pixel positioning works I might be able to pull it out. If I have a render target 2000*4. How can I ensure at the vertex shader level that my pixel will end up in the second row?

What if my RenderTarget is a texture with height = 1 can I not bother computing the positions? Or do I risk loosing data via pixel merging? I am planning to draw nothing but long lines through the screen, one by one and clear the target in between.

Answer 1

Basically you can't do what you're describing.

After the vertex shader, the GPU has a collection of triangles to draw. It fills them, pixel-by-pixel, on the render target (possibly the backbuffer). As part of this filling process - to determine the colour of each pixel - your pixel shader gets called (like a function) for that specific pixel being filled. There is no capacity at this point for "moving" the output pixel.

What you can do is modulate the texture coordinate parameter to tex2D (MSDN) when sampling from a texture in your pixel shader. You can apply whatever functions make sense to achieve your desired result.

Or, if the transform is simple, you can simply set the texture coordinates appropriately either in the vertex data, or using a vertex shader.