sksl/compute/DesaturateFunction.compute

layout(local_size_x = 16, local_size_y = 16) in;

layout(binding=0, rgba32f) readonly texture2D src;
layout(binding=1, rgba32f) writeonly texture2D dest;

noinline void desaturate(layout(rgba32f) readonly texture2D src,
                         layout(rgba32f) writeonly texture2D dest) {
    half4 color = textureRead(src, sk_GlobalInvocationID.xy);
    color.rgb = half3(dot(color.rgb, half3(0.22, 0.67, 0.11)));
    textureWrite(dest, sk_GlobalInvocationID.xy, color);
}

void main() {
    if (sk_GlobalInvocationID.x < textureWidth(src) &&
        sk_GlobalInvocationID.y < textureHeight(src)) {
        desaturate(src, dest);
    }
}