metalHelpers.metal

//
//  MetalHelpers.metal
//  LargeFormatSimulator
//
//  Created by Anthony Forsythe on 1/22/20.
//  Copyright © 2020 Anthony Forsythe. All rights reserved.
//

#include <metal_stdlib>
using namespace metal;
#include "MetalHelpers.h"

constant half3 kRec709Luma = half3(0.2126, 0.7152, 0.0722);

half invert(half c) {
    return 1.0 - c;
}

half4 convertToGreyscale(half4 inputColor) {
    
    //  `inputColor.rgb` will give you some value like [ .5, .5, .5 ]
    //  and the `dot` function will give you the dot matrix multiplication
    //  product of its inputs
    //  In this case that would be [0.2126, 0.7152, 0.0722] * [ .5, .5, .5 ]
    //  Or when decomposed...
    //  result = 0.2126 * .5 + 0.7152 * .5 + 0.0722 * .5
    //
    //  You can test it here -> https://bit.ly/2RonWPm
    //
    //  To see where the luma values came from read this -> https://en.wikipedia.org/wiki/Grayscale 
    half gray = dot(inputColor.rgb, kRec709Luma);
    
    return half4(grey, grey, grey, 1.0);
}

half4 invertColor(half4 inputColor) {
    return half4(invert(inputColor.r), invert(inputColor.g), invert(inputColor.b), 1.0);
}

negativeEffect.metal

#include <metal_stdlib>
using namespace metal;
#include "MetalHelpers.h"

// Compute kernel
kernel void negativeEffect(texture2d<half, access::read>  inputTexture  [[ texture(0) ]],
                       texture2d<half, access::write> outputTexture [[ texture(1) ]],
                       uint2 gid [[thread_position_in_grid]])
{
    // Don't read or write outside of the texture.
    if ((gid.x >= inputTexture.get_width()) || (gid.y >= inputTexture.get_height())) {
        return;
    }
    
    half4 inputColor = inputTexture.read(gid);
    
    half4 negativeColor = invertColor(inputColor);
    
    outputTexture.write(negativeColor, gid);
}