What is an Argument Buffer?

📚 Official Books Metal by Tutorials
1

Hello -

This question may sound rather basic, but I am having a difficult time distinguishing an argument buffer from a “normal” buffer. I thought if I tagged a member of a struct with an id[[]] attribute the struct was effectively an argument buffer. But that does not seem to be the case.

For a point of reference, this is the struct:

typedef struct Mesh {
constant float * bufferA [[ id(0) ]];
constant uint * bufferB [[ id(1) ]];
} Mesh;

The two buffers are just an array of floats/uints. If I try to get an argument encoder to a function that has this struct, the API throws an error and says there is no argument buffer at this index.

So is a struct an argument buffer only if it mixes and matches types (for example a pointer to a buffer type and a texture)? Section 2.11.2 of the MSL Language Specification made this much more confusing for me.

2

If the error is something like this:

failed assertion `Function fragment_main does not have a buffer argument with buffer index 21'

then that’s because you don’t have a buffer argument at that number.

In Chapter 15 sample code, you can get this message by changing in Model’s initializeTextures():

let textureEncoder =
  fragmentFunction.makeArgumentEncoder(
    bufferIndex: Int(BufferIndexTextures.rawValue))

to:

let textureEncoder =
  fragmentFunction.makeArgumentEncoder(
    bufferIndex: 21))

The matching fragment code for this buffer is:

struct Textures {
  texture2d<float> baseColorTexture;
  texture2d<float> normalTexture;
};

fragment float4 fragment_main(VertexOut in [[stage_in]],
        constant Textures &textures [[buffer(BufferIndexTextures)]]

So the fragmentFunction.makeArgumentEncoder buffer index has to match the buffer index of the parameter into the fragment function, not the buffer index inside the struct.

Does that answer your question?

3

Yes! Thank you. That was the error I received and the change you recommended solved it.

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