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polar2rect.frag
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#version 120

uniform sampler2D tex0;
const float PI = 3.14159265358979323846264;

void main(void) {
   vec2 texCoord = gl_TexCoord[0].st;
   
   //polar 2 rect filter
   vec2 norm = texCoord * 2.0 - 1.0;
   float theta = PI + norm.x * PI;
   float r = (1.0 + norm.y) * 0.5 ;
   vec2 cart = vec2(-r * sin(theta), -r * cos(theta));
   cart = ( (cart/2.0) + 0.5 );
   
   //sample from polarized coords
   vec4 color = texture2D(tex0, cart);
   
   //if under 30% transparency, we'll use "texCoord.t" as our distance
   //otherwise use white (1.0) as a max value
   float distColor = color.a>0.3f ? texCoord.t : 1.0f;
   gl_FragColor = vec4(distColor, distColor, distColor, 1.0);
}





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lightmap.frag
----------------------------------------------------------

#version 120

uniform sampler2D tex0;
uniform float pixelBias; //px / renderTargetSize => e.g. 4/256
const float PI = 3.14159265358979323846264;

void main(void) {
   vec2 texCoord = gl_TexCoord[0].st;
   vec4 color = texture2D(tex0, texCoord);
   
   //distance from top
   float dist = texCoord.t;
   
   //give it a slight bias of a couple pixels, for a nicer effect
   dist -= pixelBias;
   
   //if we haven't yet reached our shadow caster, make it white
   float lit = dist < color.r ? 1.0 : 0.0;
   
   gl_FragColor = vec4(1, 1, 1, lit);
} 






------------------------------------------------------------
rect2polar.frag
------------------------------------------------------------

#version 120

uniform sampler2D tex0;
const float PI = 3.14159265358979323846264;

void main(void) {
   vec2 texCoord = gl_TexCoord[0].st;
   
   //rectangular to polar
   vec2 norm = texCoord * 2.0 - 1.0;
   float theta = PI + atan(norm.x, norm.y);
   float r = length(norm);
   vec2 polar = vec2(theta/(2.0*PI), r);
   
   //sample like usual
   vec4 color = texture2D(tex0, polar);
   
   //just make it all white (for tinting)
   //anything not lit will be transparent
   gl_FragColor = vec4(1, 1, 1, color.a);
}