//===========================================================================
//
// Doom lighting equation exactly as calculated by zdoom.
//
//===========================================================================
float R_DoomLightingEquation(float light)
{
   // Calculated from r_visibility. It differs between walls, floor and sprites.
   //
   // Wall: globVis = r_WallVisibility
   // Floor: r_FloorVisibility / abs(plane.Zat0 - ViewPos.Z)
   // Sprite: same as wall
   // All are calculated in R_SetVisibility and seem to be decided by the
   // aspect ratio amongst other things.
   //
   // 1706 is the value for walls on 1080p 16:9 displays.
   float globVis = 1706.0;

   /* L is the integer light level used in the game */
   float L = light * 255.0;

   /* z is the depth in view/eye space, positive going into the screen */
   float z = pixelpos.w;

   /* The zdoom light equation */
   float vis = globVis / z;
   float shade = 64.0 - (L + 12.0) * 32.0/128.0;
   float lightscale = clamp((shade - min(24.0, vis)) / 32.0, 0.0, 31.0/32.0);

   // Result is the normalized colormap index (0 bright .. 1 dark)
   return lightscale;
}