ImageData:mapPixel

Transform an image by applying a function to every pixel.

This function is a higher-order function. It takes another function as a parameter, and calls it once for each pixel in the ImageData.

The passed function is called with six parameters for each pixel in turn. The parameters are numbers that represent the x and y coordinates of the pixel and its red, green, blue and alpha values. The function should return the new red, green, blue, and alpha values for that pixel.

function pixelFunction(x, y, r, g, b, a)
    -- template for defining your own pixel mapping function
    -- perform computations giving the new values for r, g, b and a
    -- ...
    return r, g, b, a
end

In versions prior to 11.0, color component values were within the range of 0 to 255 instead of 0 to 1.

This function locks the ImageData until it is done, making it safe to use from multiple Threads, albeit without any performance gains.

Function

Synopsis

ImageData:mapPixel( pixelFunction, x, y, width, height )

Arguments

function pixelFunction

Function to apply to every pixel.

Available since LÖVE 0.9.0

number x (0)

The x-axis of the top-left corner of the area within the ImageData to apply the function to.

number y (0)

The y-axis of the top-left corner of the area within the ImageData to apply the function to.

number width (ImageData:getWidth())

The width of the area within the ImageData to apply the function to.

number height (ImageData:getHeight())

The height of the area within the ImageData to apply the function to.

Returns

Nothing.

Examples

Brighten an image

local function brighten(x,y, r,g,b,a)
   r = math.min(r*3, 1)
   g = math.min(g*3, 1)
   b = math.min(b*3, 1)
   return r,g,b,a
end
imageData:mapPixel(brighten)

Add colored stripes to an image

local function stripey(x,y, r,g,b,a)
   r = math.min(r * math.sin(x*100)*2, 1)
   g = math.min(g * math.cos(x*150)*2, 1)
   b = math.min(b * math.sin(x* 50)*2, 1)
   return r,g,b,a
end
imageData:mapPixel(stripey)

(Source: https://web.archive.org/web/20150515111551/http://khason.net/blog/hlsl-pixel-shader-effects-tutorial/)

Tint grayscaled image

local tintColor = {r=0.6, g=0.5, b=0.3} -- Sepia tone.

local function tint(x,y, r,g,b,a)
	local gray = (r+g+b) / 3
	if gray < 0.5 then
		-- Between black and tintColor.
		r = 2 * gray*tintColor.r
		g = 2 * gray*tintColor.g
		b = 2 * gray*tintColor.b
	else
		-- Between tintColor and white.
		r = 2 * (gray + tintColor.r - gray*tintColor.r) - 1
		g = 2 * (gray + tintColor.g - gray*tintColor.g) - 1
		b = 2 * (gray + tintColor.b - gray*tintColor.b) - 1
	end
	return r,g,b,a
end

imageData:mapPixel(tint)

A more efficient method using FFI

Here we invert the colors for two images - one using mapPixel and one using FFI functionality. What we should see is that the FFI method is a lot faster (albeit a bit less convenient).

Note: We're reading and writing directly from/to memory. Not being careful will likely lead to crashes, or worse!

function love.load()
	collectgarbage("stop")

	-- Use mapPixel.
	local imageData = love.image.newImageData("bigTexture.png")
	local startTime = love.timer.getTime()

	imageData:mapPixel(function(x,y, r,g,b,a)
		return 1-r, 1-g, 1-b, a
	end)

	local time1 = love.timer.getTime() - startTime
	_G.image1   = love.graphics.newImage(imageData)

	-- Use FFI.
	local imageData  = love.image.newImageData("bigTexture.png")
	local pointer    = require("ffi").cast("uint8_t*", imageData:getFFIPointer()) -- imageData has one byte per channel per pixel.
	local pixelCount = imageData:getWidth() * imageData:getHeight()
	local startTime  = love.timer.getTime()

	for i = 0, 4*pixelCount-1, 4 do -- Loop through the pixels, four values at a time (RGBA).
		pointer[i  ] = 255 - pointer[i  ] -- r  (Remember that we're working with bytes and not normalized color values.)
		pointer[i+1] = 255 - pointer[i+1] -- g
		pointer[i+2] = 255 - pointer[i+2] -- b
	end

	local time2 = love.timer.getTime() - startTime
	_G.image2   = love.graphics.newImage(imageData)

	-- Results. (FFI can be more than ten times faster!)
	print(string.format("Times: mapPixel=%.4f FFI=%.4f", time1, time2))
	print(string.format("Difference: FFI is %.1f times faster", time1/time2))
end

function love.draw()
	-- Show that the resulting images look the same.
	local windowWidth = love.graphics.getWidth()
	local x1          = 0
	local x2          = windowWidth / 2
	love.graphics.draw(image1, x1,0, 0, windowWidth/image1:getWidth())
	love.graphics.draw(image2, x2,0, 0, windowWidth/image2:getWidth())
end

See Also

• ImageData
• ImageData:setPixel

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