LÖVE is a fun 2d graphics/game engine based around Lua.

I adapted a glsl shader for a light ray effect in a LÖVE pixeleffect. I used this effect in a hastily assembled demo of pixel shading in a sidescroller mockup:

Fabien Sanglard’s original code, and a brief but excellent description of the effect, is here.

This LÖVE wiki page lists the main differences between pixeleffects and glsl you should be aware of while adapting shaders to LÖVE.

adapting glsl to pixeleffect

To adapt Fabien’s glsl pixel shader to a LÖVE pixeleffect I only had to change a couple of things.
The biggest change was moving the void main() function to one that returns a vec4. LÖVE requires this function to be named effect, and to have a bunch of arguments supplied:

void main(){

became

vec4 effect(vec4 color, Image texture, vec2 texture_coords, vec2 pixel_coords){

which returns a vec4 value instead of setting a value to gl_FragColor.

The adapted shader, as a LÖVE pixeleffect looks like this:

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;
}

an actual example

Here is a .love with the pixeleffect in action: (you’ll need LÖVE to run this)

lightrays.love

Like Fabien’s example, there are keys to play with the effect in realtime:

exposure +-: w, s
decay +-: e, d
weight +-: r, f
density +-: t, g
vsync toggle: v
cycle through rendering buffers: b

With vsync off (v key) I get a capped 1000 fps on a Nvidia GTX 580 card.

Feel free to use the LÖVE code for whatever use you desire.