Sample one texture channel only instead of four [Optimization?]

Unity Engine
1

Hello !

Can you optimize a shader when you only need one channel of a texture (instead of 4) ?

_MainTex(“Albedo (RGB)”, 2D) = “white” {}
sampler2D _MainTex;
float4 tex = tex2D(_MainTex, IN.uv_MainTex);
// do something with tex.a and waste rgb

You usually have this. Can we optimize it, when we only need one texture channel?
In Unity you have the texture option “Single Channel”, but does this reduce the workload of the tex2D function?

Happy to hear any ideas :slight_smile:

2

Graphics pipelines are built to push vectors through at once, not single values. So there wouldn’t really be any speedup to gain there.
But for sorting your shader out there, you could simply do:

float alpha = tex2D(_MainTex, IN.uv_MainTex).a;

This won’t be faster in any way, but it can help make your code more concise.

What you can do though, on modern hardware, is save some memory space, which is always a nice thing. If you only need the Alpha of a texture, then you can change the Texture’s Format to a GPU compressed format like R Compressed BC4 and store your alpha in your red channel instead. This format should take up less GPU memory since only 1 channel will actually be taking up space in memory and the other channels will simply be inferred by the format (returning black if accessed, and white for alpha channel). Which also means that transferring the texture from RAM to VRAM will be faster as well.

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3

The answer is “sort of”.

Like @Invertex said the main savings is from memory usage. When you sample a texture in a shader, it’s using dedicated hardware on the GPU to do that. That hardware, the texture sampler unit, does the work of sampling a texture at essentially a fixed rate, that is to say sampling a single channel texture like BC4 or Alpha8, or a full color DXT5, it takes the same amount of time. However those formats each have different bits per pixel, so while sampling the texture takes the same amount of time, there’s time required to move the texture data from the GPU main memory to the sampler unit’s cache.

So if you have an existing RGB texture that you only want one channel of, no, there’s no way to optimize sampling. However you can use a single channel texture format which may be slightly more efficient. Also a BC4 is the same size as a DXT1, and an Alpha8 or R8 is the same size as a DXT5, so unless you need the higher quality of the single channel formats you’re giving up a lot of potential optimization in the form of texture reuse.

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