fishEyeShader.glsl

float PI = 3.1415926535;
 
uniform float aperture = 178.0;
float apertureHalf = 0.5 * aperture * (PI / 180.0);
float maxFactor = sin(apertureHalf);

vec2 uv;
vec2 xy;
xy.x = 2.0 * SCREEN_UV.x - 1.0;
xy.y = 2.0 * SCREEN_UV.y - 1.0;

float d = length(xy);

if (d < (2.0-maxFactor))
{

  d = length(xy * maxFactor);
  float z = sqrt(1.0 - d * d);
  float r = atan2(d,z) / PI;
  float phi = atan2(xy.y, xy.x);
  
  uv.x = r * cos(phi) + 0.5;
  uv.y = r * sin(phi) + 0.5;

}
else
{
  uv = SCREEN_UV;
}


COLOR.rgb = texscreen(uv);