Using normalized sampler coordinates in CIFilter kernel - metal

Question

I was walking through this tutorial on custom CIFilter:
https://medium.com/#m_tuzer/using-metal-shading-language-for-custom-cikernels-metal-swift-7bc8e7e913e6
Everything works perfectly except that the coordinates of the sampler are not normalized. So, e.g a condition like this pos.y < 0.33 doesn’t work, and the kernel uses actual image coordinates.
Since the tutorial is old there probably has been changes in CIFilter that “broke” this code. I looked through the manual for CI kernels but could not find a way to get normalized coordinates of a sampler inside the kernel.
Here is the code of the kernel:
#include <metal_stdlib>
using namespace metal;
#include <CoreImage/CoreImage.h> // (1)
float3 multiplyColors(float3, float3);
float3 multiplyColors(float3 mainColor, float3 colorMultiplier) { // (2)
float3 color = float3(0,0,0);
color.r = mainColor.r * colorMultiplier.r;
color.g = mainColor.g * colorMultiplier.g;
color.b = mainColor.b * colorMultiplier.b;
return color;
};
extern "C" { namespace coreimage { // (3)
float4 dyeInThree(sampler src, float3 redVector, float3 greenVector, float3 blueVector) {
float2 pos = src.coord();
float4 pixelColor = src.sample(pos); // (4)
float3 pixelRGB = pixelColor.rgb;
float3 color = float3(0,0,0);
if (pos.y < 0.33) { // (5)
color = multiplyColor(pixelRGB, redVector);
} else if (pos.y >= 0.33 && pos.y < 0.66) {
color = multiplyColor(pixelRGB, greenVector);
} else {
color = multiplyColor(pixelRGB, blueVector);
}
return float4(color, 1.0);
}
}}

Answer 1

You can translate the source coordinates into relative values using the extent of the source like this:
#include <metal_stdlib>
using namespace metal;
#include <CoreImage/CoreImage.h>
extern "C" { namespace coreimage {
float4 dyeInThree(sampler src, float3 redVector, float3 greenVector, float3 blueVector) {
float2 pos = src.coord();
float4 pixelColor = src.sample(pos);
// transform to relative coordinates
pos -= src.origin();
pos /= src.size();
float3 color = float3(0,0,0);
if (pos.y < 0.33) {
color = pixelColor.rgb * redVector;
} else if (pos.y < 0.66) {
color = pixelColor.rgb * greenVector;
} else {
color = pixelColor.rgb * blueVector;
}
return float4(color, pixelColor.a);
}
}}