LÖVE

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--
-- The original Light Scattering GLSL shader by Fabien Sanglard
-- http://fabiensanglard.net/lightScattering/index.php
--
-- Adapted to the LOVE PixelEffect by ioxu
-- http://blog.ioxu.com/?p=909
--
-- 'Luminosity' wrapper by mz1
-- www.mz1net.com
--  

-- =================================================================================

-- EXAMPLE USAGE:
-- ---

--[[

function love.load()

	windowSizeX, windowSizeY = 800, 600
	l = Luminosity.new(windowSizeX, windowSizeY)

end

function love.draw()

	-- Draw BG, ...
	-- ...
	
	-- When you want to draw with luminosity:
	
	-- 1] Call 'receive'
	l:receive()
	
	-- 2] Now the luminosity instance will receive all your renders up until you 
	-- call 'process', so draw whatever you want to draw illuminated:
	love.graphics.draw(sprite1, 0, 0, ...)
	love.graphics.draw(sprite2, 0, 0, ...)
	
	-- 3] Call 'process'
	l:process()
	
	-- 4] Now the luminosity instance contains everything that you have drawn in the 
	-- step #2, and it is illuminated. Draw it to the screen when convenient.
	l:draw()

	-- Done. Continue to render whatever else you want to render.
	-- ...

end

-- Remember that you can set the light's position with 'setLightPosition', and also
-- tweak the shader values in the constructor.

]]

-- =================================================================================
-- =================================================================================

Luminosity = {}
Luminosity.__index = Luminosity

-- =================================================================================

function Luminosity.new (canvasSizeX, canvasSizeY)

	local new = {}
	setmetatable(new, Luminosity)
	
	new.canvasSizeX = canvasSizeX
	new.canvasSizeY = canvasSizeY
	
	new.backCanvas = love.graphics.newCanvas( canvasSizeX, canvasSizeY )
	new.raysCanvas = love.graphics.newCanvas( canvasSizeX, canvasSizeY )
		
	new.FX = love.graphics.newPixelEffect[[

		extern number exposure = 1.0;
		extern number decay = 1.0;
		extern number density = 1.0;
		extern number weight = 1.0;
		extern vec2 lightPositionOnScreen= vec2(0.0,0.0);
		extern number NUM_SAMPLES = 100.0 ;

		vec4 effect(vec4 color, Image texture, vec2 texture_coords, vec2 pixel_coords)
		{
			vec2 deltaTextCoord = vec2( texture_coords - lightPositionOnScreen.xy );
			vec2 textCoo = texture_coords.xy;
			deltaTextCoord *= 1.0 / float(NUM_SAMPLES) * density;
			float illuminationDecay = 1.0;
			vec4 cc = vec4(0.0, 0.0, 0.0, 1.0);

			for(int i=0; i < NUM_SAMPLES ; i++)
			{
				textCoo -= deltaTextCoord;
				vec4 sample = Texel( texture, textCoo );
				sample *= illuminationDecay * weight;
				cc += sample;
				illuminationDecay *= decay;
			}
			cc *= exposure;
			return cc;
		}
		]]
	
	new:setLightSource( 0.5, 0.5 )
		
	new.FX:send( 'exposure', 0.2 )
	new.FX:send( 'decay', 0.95 )
	new.FX:send( 'density', 0.4 )
	new.FX:send( 'weight', 0.3 )
	new.FX:send( 'NUM_SAMPLES', 70 )
		
	return new

end

-- =================================================================================

-- 
-- x, y = [0, 1], relative to the canvas size
-- 

function Luminosity:setLightSource (x, y)

	self.FX:send( 'lightPositionOnScreen', { x, 1-y } )

end

-- =================================================================================

function Luminosity:receive()

	-- 
	self.backCanvas:clear()
	self.raysCanvas:clear()
	
	--
	love.graphics.setCanvas( self.backCanvas )
	
end

-- =================================================================================

function Luminosity:process()

	-- 
	love.graphics.setColor( 255,255,255 )
	love.graphics.setCanvas( self.raysCanvas )
	love.graphics.setPixelEffect( self.FX )
			
	--
	love.graphics.draw( self.backCanvas, 0, 0, 0, 1, 1 )
	
	-- 
	love.graphics.setCanvas()
	love.graphics.setPixelEffect()
	
end

-- =================================================================================

function Luminosity:draw (x, y)

	-- 
	love.graphics.setColor( 255,255,255 )
	love.graphics.draw( self.backCanvas, x or 0, y or 0 )

	--
	local bm = love.graphics.getBlendMode()
	love.graphics.setBlendMode( 'additive' )
	love.graphics.draw( self.raysCanvas, x, y, 0, 1, 1 )
	love.graphics.draw( self.raysCanvas, x, y, 0, 1, 1 )
	
	-- Restore the previous blend mode
	love.graphics.setBlendMode( bm )

end