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Indexing argument buffers

Assign resource indices within an argument buffer.

Overview

You can index an argument buffer similarly to buffers, textures, and samplers. However, you index individual argument buffer resources with a generic [[id(n)]] attribute instead of the specific type [[buffer(n)]], [[texture(n)]], and [[sampler(n)]] attributes.

Manually assigned argument buffer resource indices don’t need to be contiguous, but they need to be unique and arranged in an increasing order. The following example shows manual and automatic index assignment:

struct My_Indexed_AB {
    texture2d<float> texA [[id(1)]];
    texture2d<float> texB [[id(3)]];
};
struct My_Aggregate_AB {
    My_Indexed_AB abX; // abX = id(0); texA = id(1); texB = id(3)
    My_Indexed_AB abY; // abY = id(4); texA = id(5); texB = id(7)
};

Automatically assigned index IDs

If the [[id(n)]] attribute is omitted for any argument buffer resource, an index ID is automatically assigned according to preset rules:

Structure Members

IDs are assigned to structure members in order, starting at 0, by adding 1 to the highest ID used by the previous structure member. The following example shows automatically assigned index IDs for structure members:

struct MaterialTexture {
    texture2d<float> tex; // Assigned to index 0
    float4 uvScaleOffset; // Assigned to index 1
};

Array Elements

IDs are assigned to array elements in order, starting at 0, by adding 1 to the highest ID used by the previous array elements. The following example shows automatically assigned index IDs for array elements:

struct Material {
    float4 diffuse;                     // Assigned to index 0
    array<texture2d<float>, 3> texSet1; // Assigned to indices 1-3
    texture2d<float> texSet2[3];        // Assigned to indices 4-6
    MaterialTexture materials[3];       // Assigned to indices 7-12
    int constants[4] [[id(20)]];        // Assigned to indices 20-23
};

Nested Structs and Arrays

If a structure member or array element is itself a structure or array, its own structure members or array elements are assigned indices according to the previous rules. If an ID is provided for a top-level structure or array, this ID becomes the starting index for nested structure members or array elements. The following example shows automatically assigned index IDs for nested structures and arrays:

struct Material {
    MaterialTexture diffuse;          // Assigned to indices 0-1
    MaterialTexture normal [[id(4)]]; // Assigned to indices 4-5
    MaterialTexture specular;         // Assigned to indices 6-7
}

Combined Argument Buffer Resources and Regular Resources

Argument buffer resources are assigned generic indices according to the previous rules. Regular resources are assigned type indices in their respective resource argument tables. The following example shows automatically assigned index IDs for combined argument buffer resources and regular resources:

fragment float4 my_fragment(
    constant texture2d<float> & texturesAB1 [[buffer(0)]],     // Assigned to generic index 0 and buffer index 0
    constant texture2d<float> & texturesAB2[10] [[buffer(1)]], // Assigned to generic indices 0-9 and buffer index 1
    array<texture2d<float>, 10> texturesArray [[texture(0)]]    // Assigned to texture indices 0-9
)
{...}

See Also

Argument buffers