PICO-8 Palette shader in Love2D

main.lua

palette_swap_shader = [[
	extern vec4 colours_lookup[16];
	extern vec4 screen_colours[16];
	extern int transparent_colour = 0;
	vec4 transparent = vec4(0,0,0,0);
	
	vec4 effect(vec4 color, Image tex, vec2 texture_coords, vec2 screen_coords) {
		vec4 texcolour = Texel(tex, texture_coords);

		for (int i = 0; i < 16; i++) {
			if (texcolour == colours_lookup[i]) {
				if (i == transparent_colour) {
					return transparent * color;
				} else {
					return screen_colours[i] * color;
				}
			}
		}
		
		return texcolour * color;
	}
]]

BLACK = {0,0,0, 1.0}
DARK_BLUE = {29/255.0, 43/255.0, 83/255.0, 1.0}
DARK_PURPLE = {126/255.0, 37/255.0, 83/255.0, 1.0}
DARK_GREEN = {0/255.0, 135/255.0, 81/255.0, 1.0}
BROWN = {171/255.0, 82/255.0, 54/255.0, 1.0}
DARK_GRAY = {95/255.0, 87/255.0, 79/255.0, 1.0}
LIGHT_GRAY = {194/255.0, 195/255.0, 199/255.0, 1.0}
WHITE = {255/255.0, 241/255.0, 232/255.0, 1.0}
RED = {255/255.0, 0/255.0, 77/255.0, 1.0}
ORANGE = {255/255.0, 163/255.0, 0/255.0, 1.0}
YELLOW = {255/255.0, 236/255.0, 39/255.0, 1.0}
GREEN = {0/255.0, 228/255.0, 54/255.0, 1.0}
BLUE = {41/255.0, 173/255.0, 255/255.0, 1.0}
GRAY_BLUE = {131/255.0, 118/255.0, 156/255.0, 1.0}
PINK = {255/255.0, 119/255.0, 168/255.0, 1.0}
LIGHT_PEACH = {255/255.0, 204/255.0, 170/255.0, 1.0}
BROWN_BLACK = {41/255.0,24/255.0,20/255.0, 1.0}
DARKER_BLUE = {17/255.0, 29/255.0, 53/255.0, 1.0}
DARKER_PURPLE = {66/255.0, 33/255.0, 54/255.0, 1.0}
BLUE_GREEN = {18/255.0, 83/255.0, 89/255.0, 1.0}
DARK_BROWN = {116/255.0, 47/255.0, 41/255.0, 1.0}
DARKER_GRAY = {73/255.0, 51/255.0, 59/255.0, 1.0}
MEDIUM_GRAY = {162/255.0, 136/255.0, 121/255.0, 1.0}
LIGHT_YELLOW = {243/255.0, 239/255.0, 125/255.0, 1.0}
DARK_RED = {190/255.0, 18/255.0, 80/255.0, 1.0}
DARK_ORANGE = {255/255.0, 108/255.0, 36/255.0, 1.0}
LIME_GREEN = {168/255.0, 231/255.0, 46/255.0, 1.0}
MEDIUM_GREEN = {0/255.0, 181/255.0, 67/255.0, 1.0}
MEDIUM_BLUE = {6/255.0, 90/255.0, 181/255.0, 1.0}
MAUVE = {117/255.0, 70/255.0, 101/255.0, 1.0}
DARK_LIGHT_PEACH = {255/255.0, 110/255.0, 89/255.0, 1.0}
PEACH = {255/255.0, 157/255.0, 129/255.0, 1.0}

all_colours = {
	BLACK, DARK_BLUE, DARK_PURPLE, DARK_GREEN, 
	BROWN, DARK_GRAY, LIGHT_GRAY, WHITE, 
	RED, ORANGE, YELLOW, GREEN,
	BLUE, GRAY_BLUE, PINK, LIGHT_PEACH,
	BROWN_BLACK, DARKER_BLUE, DARKER_PURPLE, BLUE_GREEN,
	DARK_BROWN, DARKER_GRAY, MEDIUM_GRAY, LIGHT_YELLOW,
	DARK_RED, DARK_ORANGE, LIME_GREEN, MEDIUM_GREEN,
	MEDIUM_BLUE, MAUVE, DARK_LIGHT_PEACH, PEACH
}

draw_palette = {
	BLACK, DARK_BLUE, DARK_PURPLE, DARK_GREEN,
	BROWN, DARK_GRAY, LIGHT_GRAY, WHITE,
	RED, ORANGE, YELLOW, GREEN, 
	BLUE, GRAY_BLUE, PINK, LIGHT_PEACH
}

current_palette = {
	BLACK, DARK_BLUE, DARK_PURPLE, DARK_GREEN,
	BROWN, DARK_GRAY, LIGHT_GRAY, WHITE,
	RED, ORANGE, YELLOW, GREEN,
	BLUE, GRAY_BLUE, PINK, LIGHT_PEACH
}

function pal(i,c,draw_palette_flag)
  if draw_palette_flag == 1 then
		changed_draw_palette = true
		draw_palette[i+1] = all_colours[c+1]
  else
		changed_screen_palette = true
		if not i then
			for f=1,16 do
				current_palette[f] = draw_palette[f]
			end
		else
			current_palette[i+1] = draw_palette[c+1]
		end
	end
end

function palt(i,transparent)
	shader:send("transparent_colour", i)	
end

function love.load()
  love.graphics.setDefaultFilter("nearest", "nearest", 1)
	-- just a png with a GREEN background (see colours above)
	spritesheet = love.graphics.newImage("ball-3.png")
	shader = love.graphics.newShader(palette_swap_shader)
	changed_screen_palette = true
	update_palette_shader()
	palt(11,true)
	shader:send("colours_lookup", 
			draw_palette[1],
			draw_palette[2],
			draw_palette[3],
			draw_palette[4],
			draw_palette[5],
			draw_palette[6],
			draw_palette[7],
			draw_palette[8],
			draw_palette[9],
			draw_palette[10],
			draw_palette[11],
			draw_palette[12],
			draw_palette[13],
			draw_palette[14],
			draw_palette[15],
			draw_palette[16]
		)

	balls = {}
	
	pal(0,1)
	pal(2,3)
	pal(4,11)
	pal(9,10)
	pal(10,7)
	spr(0,100,50)

	palettes = {
		{{0,1},{2,2},{4,4},{9,9}},
		{{4,8},{2,2},{9,14},{10,15}},
		{{0,1},{2,3},{4,11},{9,10},{10,7}},
		{{2,1},{4,13},{9,12}}
	}
	for i = 1, 80 do
		balls[i] = {
			x = love.math.random(400),
			y = love.math.random(300),
			vx = love.math.random(4) - 2 + love.math.random(),
			vy = -0.3 - love.math.random(2) + love.math.random(),
			palette = palettes[math.floor(love.math.random(#palettes))]
		}
	end
end

function update_palette_shader()
	if changed_screen_palette then
		shader:send("screen_colours", 
			current_palette[1],
			current_palette[2],
			current_palette[3],
			current_palette[4],
			current_palette[5],
			current_palette[6],
			current_palette[7],
			current_palette[8],
			current_palette[9],
			current_palette[10],
			current_palette[11],
			current_palette[12],
			current_palette[13],
			current_palette[14],
			current_palette[15],
			current_palette[16]
		)
		changed_screen_palette = false
	end
end

function spr(s,x,y)
	update_palette_shader()
	love.graphics.setShader(shader)
	love.graphics.draw(spritesheet, x, y)
end

function love.update()
	for _,b in ipairs(balls) do
		update_ball(b)
	end
end

function love.draw()
	changed_screen_palette = false
	
  love.graphics.print("FPS: "..tostring(love.timer.getFPS( )), 2, 2)
	love.graphics.scale(2,2)

	for _,b in ipairs(balls) do
		draw_ball(b)
	end
end

function update_ball(b)
	if b.x > 400 then
		b.x = 400
		b.vx = -b.vx
	end
	if b.x < 0 then
		b.x = 0
		b.vx = -b.vx
	end
	if b.y > 270 then
		b.y = 270
		b.vy = -b.vy
	end
	if b.y < 0 then
		b.y = 0
		b.vy = -b.vy
	end
	b.vy = b.vy + 0.3
	b.x = b.x + b.vx
	b.y = b.y + b.vy
end

function draw_ball(b)
	for _,p in ipairs(b.palette) do
		pal(p[1], p[2])
	end
	spr(0, b.x, b.y)
end