Detect single channel texture in pixel shader

Question

Is it possible to detect when a format has a single channel in HLSL or GLSL? Or just as good, is it possible to extract a greyscale color from such a texture without knowing if it has a single channel or 4?

When sampling from texture formats such as DXGI_FORMAT_R8_*/GL_R8 or DXGI_FORMAT_BC4_UNORM, I am getting pure red RGBA values (g,0,0,1). This would not be a problem if I knew (within the shader) that the texture only had the single channel, as I could then flood the other channels with that red value. But doing anything of this nature would break the logic for color textures, requiring a separate compiled version for the grey sampling (for every texture slot).

Is it not possible to make efficient use of grey textures in modern shaders without specializing the shader for them?

The only solution I can come up with at the moment would be to detect the grey texture on the CPU side and generate a macro on the GPU side that selects a different compiled version of the shader for every texture slot. Doing this with 8 texture slots would add up to 8x8=64 compiled versions every shader that wants to support grey inputs. That's not counting the other macro-like switches that actually make sense being there.

Just to be clear, I do know that I can load these textures into GPU memory as 4-channel greyscale textures, and go from there. But doing that uses 4X the memory, and I would rather load in 3 more textures.

Answer 1

In OpenGL there's two ways to achieve what you're looking for:

1. Legacy: The INTENSITY and LUMINANCE texture formats will when sampled result in vec4(I,I,I,I) or vec4(L,L,L,1) .
2. Modern: Use a swizzle mask to apply user defined channel swizzling per texture: glTexParameteriv(GL_TEXTURE_2D, GL_TEXTURE_SWIZZLE_RGBA, {GL_RED,GL_RED,GL_RED,GL_ONE});

In DirectX 12 you can use component mapping during the creation of a ShaderResourceView .