LÖVE

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col = { 255, 0, 0 }
love.graphics.setColor( col )

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love.graphics.setColor( unpack(col) )

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o = {} -- table with our buttons

-- fragment shader, only non-transparent pixels are allowed in the stencil
mask_effect = love.graphics.newShader [[ vec4 effect (vec4 color, Image texture, vec2 texture_coords, vec2 screen_coords) {if(Texel(texture, texture_coords).a == vec3(0.0)) discard; return vec4(1.0);} ]]

function love.load()

	button = love.graphics.newImage("button.png") -- button image with transparency

	-- creating several buttons, random coords, width & height equal to button image size
	for i = 1, 5 do
		o[i] = {}
		o[i].x = love.math.random(100, 800)
		o[i].y = love.math.random(100, 600)
		o[i].image = button
		o[i].w = o[i].image:getWidth()
		o[i].h = o[i].image:getHeight()
	end

	-- preparing canvas (to getPixel(x,y) later)
	offw, offh = love.window.getDesktopDimensions()
	pickbuf = love.graphics.newCanvas(offw, offh)

end

-- stencil function, we must draw all our buttons in the loop
function stencil()
	love.graphics.setShader(mask_effect)
	love.graphics.setColor(255, 255, 255, 255)
	love.graphics.setScissor(mx - 4, my - 4, 8, 8)
	for k, v in ipairs(o) do
		love.graphics.draw(v.image, v.x, v.y)
	end
	love.graphics.setScissor(0, 0, offw, offh)
	love.graphics.setShader()
end

-- now we draw our buttons as a simple rectangles, setting next color in each iteration
-- from color(0,0,1) to (255,255,255)
function pickrender()
	love.graphics.clear()
	love.graphics.setStencil(stencil)
	love.graphics.setScissor(mx - 4, my - 4, 8, 8)
	for k, v in ipairs(o) do
		love.graphics.setColor(math.floor(k / 2 ^ 16), math.floor(k / 2 ^ 8) % 2 ^ 8, k % 2 ^ 8, 255)
		love.graphics.rectangle("fill", v.x, v.y, v.w, v.h)
	end
	love.graphics.setScissor(0, 0, offw, offh)
end

-- here we get pixel color under cursor and convert it into object's id
function pick()
	mx, my = love.mouse.getPosition()
	love.graphics.setScissor(mx - 4, my - 4, 8, 8)
	pickbuf:renderTo(pickrender)
	mr, mg, mb = pickbuf:getPixel(mx, my)
	curr = mb + mg * 255 + mr * 65535
	love.graphics.setScissor(0, 0, offw, offh)
end

function love.draw()

	pick() -- do we have button under cursor?

	love.graphics.setColor(255, 255, 255, 255)

	-- draw buttons normally
	for _, v in ipairs(o) do
		love.graphics.draw(v.image, v.x, v.y)
	end

	-- here goes something if we have button under cursor
	if curr ~= 0 then
		love.graphics.setColor(128, 0, 0, 128)
		love.graphics.draw(button, o[curr].x, o[curr].y)
	end
end

function love.keypressed(key, isRepeat)
	if key == "escape" then
		love.event.quit()
	end
end

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for k, v in ipairs(o) do
      love.graphics.setColor(math.floor(k / 2 ^ 16), math.floor(k / 2 ^ 8) % 2 ^ 8, k % 2 ^ 8, 255)
...

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for k, v in ipairs(o) do
      love.graphics.setColor(k)
...

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local o = { {255,0,0}, {255,255,0} }
for i,c in ipairs(o) do
  love.graphics.setColor( c )
  love.graphics.circle( "fill", i*100, 100, 40 )
end