CODEWITHSUNDEEP
iosmetal
sevensevens
sevensevens

Reputation: 1750

Best way to send an array of textures to Metal

I've got the below shader (pieces removed for length and clarity), and would like to find a bettter way to do this. I would like to send an array of textures, of which the size is variable, to my metal shader. I'll do some calculations on the vertex positions, and then figure out which texture to use.

Currently I have just hard coded things and used several if statements, but this is ugly (and I'm guessing not fast). Is there any way I can compute i and then use i as a texture subscript (like tex[i].sample)?

// Current code - its ugly
fragment half4 SimpleTextureFragment(VertextOut inFrag [[stage_in]],
                                   texture2d<half>  tex0   [[ texture(0) ]]
                                   texture2d<half>  tex1   [[ texture(1) ]]
                                   texture2d<half>  tex2   [[ texture(2) ]]
                                   ...
                                   texture2d<half>  texN   [[ texture(N) ]]
                                     )
{
   constexpr sampler quad_sampler;
   int i = (Compute_Correct_Texture_to_Use);
   if(i==0)
   {
      half4 color = tex0.sample(quad_sampler, inFrag.tex_coord);
   }
   else if(i==1)
   {
      half4 color = tex1.sample(quad_sampler, inFrag.tex_coord);
   }
   ...
   else if(i==n)
   {
      half4 color = texN.sample(quad_sampler, inFrag.tex_coord);
   }
   return color;
}

Upvotes: 2

Views: 3845

Answers (1)

Muzza
Muzza

Reputation: 1256

You are right that your method will not be fast. Best case, the shader will have lots of branching (which is not good), worse case, the shader will actually sample from ALL your textures and then discard the results it does not use (this will be even slower).

This is not a case that GPUs handle particularly well, so my advice would be to slightly refactor your approach to be more GPU friendly. Without knowing more about what you are doing at a higher level, my first suggestion would be to consider using 2d array textures.

2d array textures essentially merge X 2D textures in to a single texture with X slices to it. You only have to pass a single texture to Metal and you can calculate which slice to sample from in the shader exactly as you are already doing, but with this approach you will get rid of all the 'if' branches and will only need to call sample once like this: tex.sample( my_sampler, inFrag.tex_coord, i );

If your textures are all the same size and format, then this will work very easily. You just have to copy each of your 2D textures in to a slice of the array texture. If your textures are different in size or format, you may have to work around that possibly by stretching some so that they all end up the same dimensions.

See here for docs: https://developer.apple.com/library/ios/documentation/Miscellaneous/Conceptual/MetalProgrammingGuide/Mem-Obj/Mem-Obj.html#//apple_ref/doc/uid/TP40014221-CH4-SW10 (look for 'Texture Slices')

Also here: https://developer.apple.com/library/prerelease/ios/documentation/Metal/Reference/MTLTexture_Ref/index.html#//apple_ref/c/econst/MTLTextureType2DArray

Metal shader languages docs here: https://developer.apple.com/library/ios/documentation/Metal/Reference/MetalShadingLanguageGuide/std-lib/std-lib.html#//apple_ref/doc/uid/TP40014364-CH5-SW17 (look for '2D Texture Array')

Upvotes: 6

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