uniform sampler2D tex;
uniform float lightlevel;

#define DOOMLIGHTFACTOR 232.0

void main()
{
  vec4 color = gl_Color;

  // Doom lighting equation ripped from EDGE.
  float L = 63.0 * lightlevel;
  float min_L = clamp(36.0 - L, 0.0, 31.0);
  float dist = max(0.0, gl_FragCoord.z);
  float index = 59.0 + DOOMLIGHTFACTOR - L - DOOMLIGHTFACTOR / dist;
  color.rgb *= (1.0 - clamp(index, min_L, 31.0) / 31.0);

  gl_FragColor = color * texture2D(tex, gl_TexCoord[0].st);
}