Palette shader

main.lua

function love.load()

	--[[Include this to initialise the shader]]
	pltshader = love.graphics.newShader("palettise.fs")
	
	-- Sending the colours from http://sharecart1000.com/img/SHARECART1000guide.png
	pltshader:send("colors",
		{148/256,146/256,76/256,1},
		{73/256,84/256,65/256,1},
		{4/256,174/256,204/256,1},
		{148/256,74/256,28/256,1},
		{60/256,136/256,38/256,1},
		{252/256,254/256,252/256,1},
		{95/256,57/256,31/256,1},
		{108/256,250/256,220/256,1},
		{144/256,144/256,158/256,1},
		{50/256,45/256,35/256,1},
		{28/256,62/256,68/256,1},
		{220/256,198/256,124/256,1},
		{28/256,70/256,36/256,1},
		{164/256,218/256,196/256,1},
		{4/256,92/256,156/256,1}
	)


end

palletise.fs

extern vec4 colors[15];
extern float ditherAmt;

// Magic number function
// Returns a value between 0 and 1
float rand(vec2 co){
    return fract(sin(dot(co.xy ,vec2(12.9898,78.233))) * 43758.5453);
}

// Returns the square of the rgb distance
float dist32(vec4 a, vec4 b)
{
    vec4 d = b-a;
    return (d.r*d.r + d.g*d.g + d.b*d.b);
}

// Returns the rgb distance
//! sqrt is a hungry baby, do not over-use
float dist3(vec4 a, vec4 b)
{
    return sqrt(dist32(a,b));
}

// Change this function for extra juicy
// Should return a value between -1 and 1
float jitterFunc(vec2 coord)
{
    return sin((coord.x-coord.y)*1000);
}


// Gets the index of the closest colour to tested colour.
int getClosest(vec4 c, vec2 coord){
    int res; // Result
    float mindist = 10000; // rbg values have a maximum distance of sqrt(3) so this should be big enough
    int i;
    for (i = 0; i < 15; ++i)
    {
        float dist = dist3(c, colors[i]);       // Use dist32() for better performance, but dodgy jittering
        if (dist<mindist+ditherAmt*jitterFunc(coord)) // dither and jitter work together to break nasty lines
        {
            mindist = dist;
            res = i;
        }
    }
    return res;
}

vec4 effect( vec4 color, Image texture, vec2 texture_coords, vec2 screen_coords )
{
    vec4 c = Texel(texture, texture_coords); // Get the ACTUAL colour
    int i = getClosest(c, screen_coords); // Find the index of the closest colour
    return colors[i]; // Return said closest colour
}