Compiler error when trying to add constant float3x3 to shader file - ios

Question

I am trying to add this code to my Metal language file:
constant float3x3 rgb2xyz(
float3(0.412453f, 0.212671f, 0.019334f),
float3(0.357580f, 0.715160f, 0.119193f),
float3(0.180423f, 0.072169f, 0.950227f)
);
or this
constant float3x3 rgb2xyz = float3x3(
float3(0.412453f, 0.212671f, 0.019334f),
float3(0.357580f, 0.715160f, 0.119193f),
float3(0.180423f, 0.072169f, 0.950227f)
);
The metal compiler gives me the following error:
No matching constructor for initialization of 'const constant float3x3' (aka 'const constant matrix<float, 3, 3>')
However if I do
typedef struct {
float3x3 matrix;
float3 offset;
float zoom;
} Conversion;
constant Conversion colorConversion = {
.matrix = float3x3(
float3 ( 1.164f, 1.164f, 1.164f ),
float3 ( 0.000f, -0.392f, 2.017f ),
float3 ( 1.596f, -0.813f, 0.000f )
),
.offset = float3 ( -(16.0f/255.0f), -0.5f, -0.5f )
};
I don't get any compile error.
Any ideas what is going wrong? It also works without problems with vector types:
constant float3 bgr2xyzCol1(0.357580f, 0.715160f, 0.119193f);
How would be a good way to define a constant matrix directly in the code?

Answer 1

You should pass it in as a constant reference, see WWDC session 604.
e.g. see matrices here, TransformMatrices is a custom data structure in this case
vertex VertexOutput my_vertex(const global float3* position_data [[ buffer(0) ]], const global
 float3* normal_data [[ buffer(1) ]], constant TransformMatrices& matrices [[ buffer(2) ]], uint vid [[ vertex_id ]])
{
VertexOutput out;
float3 n_d = normal_data[vid];
float3 transformed_normal = matrices.normal_matrix * n_d;
float4 p_d = float4(position_data[vid], 1.0f);
 out.position = * p_d;
float4 eye_vector = * p_d;
...
return out;
}