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What do the double square brackets around "position" mean in Metal?

What does float4 position [[position]];

the next snippet do?

#include <metal_stdlib>
using namespace metal;

struct Vertex
{
    float4 position [[position]];
    float4 color;
};

vertex Vertex vertex_main(device Vertex *vertices [[buffer(0)]], uint vid [[vertex_id]])
{
    return vertices[vid];
}

I am confused by the part [[position]]

and similar usage in the function definition.

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3 answers


The metal shading language is documented at https://developer.apple.com/metal/metal-shading-language-specification.pdf



In particular, take a look at “Table 9” on page 68 of this document. There [[position]] is identified as an attribute attribute for the return type of the vertex function. I assume this means that when the vertex shader returns, the caller will use the values ​​in that part of the structure to determine the vertex positions that the shader would like to change.

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I don't have enough reputation to answer your comment regarding the name of the brackets, but the brackets [[]] are attribute syntax taken from C ++ 11.



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Metal is based on C ++ and this is just the attribute syntax in C ++ 11. For details on the grammar, see

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