Is there a iOS Metal value for bt601?

Question

I have sample metal code that I'm trying to convert to iOS. Is there an iOS compatible value that I can use for bt601?

#include <metal_stdlib>
#include "utilities.h" // error not found
using namespace metal;

kernel void laplace(texture2d<half, access::read> inTexture [[ texture(0) ]],
                    texture2d<half, access::read_write> outTexture [[ texture(1) ]],
                    uint2 gid [[ thread_position_in_grid ]]) {
    constexpr int kernel_size = 3;
    constexpr int radius = kernel_size / 2;

    half3x3 laplace_kernel = half3x3(0, 1, 0,
                                     1, -4, 1,
                                     0, 1, 0);

    half4 acc_color(0, 0, 0, 0);
    for (int j = 0; j <= kernel_size - 1; j++) {
        for (int i = 0; i <= kernel_size - 1; i++) {
            uint2 textureIndex(gid.x + (i - radius), gid.y + (j - radius));
            acc_color += laplace_kernel[i][j] * inTexture.read(textureIndex).rgba;
        }
    }

    half value = dot(acc_color.rgb, bt601); //bt601 not defined
    half4 gray_color(value, value, value, 1.0);

    outTexture.write(gray_color, gid);
}

Answer 1

It seems that the intention here is simply to derive a single "luminance" value from the RGB output of the kernel. In that case, bt601 would be a three-element vector whose components are the desired weights of the respective channels, summing to 1.0.

Borrowing values from Rec. 601, we might define it like this:

float3 bt601(0.299f, 0.587f, 0.114f);

This is certainly a common choice. Another popular choice uses coefficients found in the Rec. 709 standard. That would look like this:

float3 bt709(0.212671f, 0.715160f, 0.072169f);

Both of these vectors will give you a single gray value that approximates the brightness of a linear sRGB color. Whether either of them is "correct" depends on the provenance of your data and how you process it further down the pipeline.

For whatever it's worth, the MetalPerformanceShaders MPSImageThresholdBinary kernel seems to favor the BT.601 values.

I'd recommend taking a look at this answer for more detail on the issues, and conditions under which the use of these values is appropriate.