glsl/lightMapping: increase lower bound in computation to restore original light color, fix #299

Author: illwieckz

src/engine/renderer/glsl_source/lightMapping_fp.glsl

@@ -88,8 +88,25 @@ void	main()
 
 	vec3 lightDir = normalize(2.0 * deluxe.xyz - 1.0);
 
+	// Lightmaps generated by q3map2 don't store the raw light value, but
+	// they store light premultiplied with the dot product of the light
+	// direction and surface normal. The line is just an attempt to reverse
+	// that and get the original light values.
+	// The lightmap stores the light in this way because for the diffuse
+	// lighting formula the outgoing light is equal to the incoming light
+	// multiplied by the above dot product multiplied by the surface albedo.
+	// So this premultiplication means that the diffuse lighting value can
+	// be calculated with a single multiply operation.
+	// But specular lighting and/or normal mapping formulas are more complex,
+	// and so you need the true light value to get correct lighting.
+	// Obviously the data is not good enough to recover the original color
+	// in all cases. The lower bound was an arbitrary chose factor to
+	// prevent too small divisors resulting in too bright lights. Increasing
+	// the value should reduce these artifacts.
+	// -- gimhael https://github.com/DaemonEngine/Daemon/issues/299#issuecomment-606186347
+
 	// divide by cosine term to restore original light color
-	lightColor /= clamp(dot(normalize(var_Normal), lightDir), 0.004, 1.0);
+	lightColor /= clamp(dot(normalize(var_Normal), lightDir), 0.3, 1.0);
 
 	computeLight(lightDir, normal, viewDir, lightColor, diffuse, material, color);
 #else // !USE_DELUXE_MAPPING