Improve fog vertex interpolation

The old var_TexCoords.t (fraction of distance between viewer and vertex
inside fog) was a ratio which does not give the correct
result when linearly interpolating between vertexes. Use instead the
previous numerator of that fraction, which gives a true result when
linearly interpolated.

Effects:
- Reduces the extent of artifacts at the intersection of the fog
  plane and a model (when viewed from inside the fog).
- Models that use larger triangles (more than a few qu) look better.
- Incorrectly set up fog volumes which can be entered from more than one
  plane can now be rendered when viewed from the opposite side of the
  designated fog plane. For example Pulse, 2609 -4097 -2109 -1 4
- The edges of BSP surfaces near the fog plane are slightly more fogged
  when viewed from outside the fog. (Not clearly good or bad.)

Author: slipher

src/engine/renderer/glsl_source/fogQuake3_fp.glsl

@@ -26,6 +26,8 @@ Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA
 
 uniform sampler2D u_FogMap;
 
+uniform float u_FogEyeT;
+
 IN(smooth) vec2		var_TexCoords;
 IN(smooth) vec4		var_Color;
 
@@ -35,7 +37,17 @@ void	main()
 {
 	#insert material_fp
 
-	vec4 color = texture2D(u_FogMap, var_TexCoords);
+	float t = step( 0, var_TexCoords.t );
+
+	if ( u_FogEyeT < 0 ) // eye outside fog
+	{
+		// fraction of the viewer-to-vertex ray which is inside fog
+		t *= var_TexCoords.t / ( max( 0, var_TexCoords.t ) - u_FogEyeT );
+	}
+
+	t = 1.0 / 32.0 + ( 30.0 / 32.0 ) * t;
+
+	vec4 color = texture2D(u_FogMap, vec2(var_TexCoords.s, t));
 
 	color *= var_Color;
 

src/engine/renderer/glsl_source/fogQuake3_vp.glsl

@@ -36,10 +36,9 @@ uniform mat4		u_ModelViewProjectionMatrix;
 
 uniform vec4		u_FogDistanceVector; // view axis, scaled by fog density
 uniform vec4		u_FogDepthVector; // fog plane
-uniform float		u_FogEyeT;
 
 // var_TexCoords.s is distance from viewer to vertex dotted with view axis
-// var_TexCoords.t is the fraction of the viewer-to-vertex ray which is inside fog
+// var_TexCoords.t is how far the vertex is under the fog plane
 OUT(smooth) vec2	var_TexCoords;
 OUT(smooth) vec4	var_Color;
 
@@ -79,29 +78,12 @@ void main()
 	float s = dot(position.xyz, u_FogDistanceVector.xyz) + u_FogDistanceVector.w;
 	float t = dot(position.xyz, u_FogDepthVector.xyz) + u_FogDepthVector.w;
 
-	// partially clipped fogs use the T axis
-	if(u_FogEyeT < 0.0)
-	{
-		if(t < 1.0)
-		{
-			t = 1.0 / 32.0;	// point is outside, so no fogging
-		}
-		else
-		{
-			t = 1.0 / 32.0 + 30.0 / 32.0 * t / (t - u_FogEyeT);	// cut the distance at the fog plane
-		}
-	}
-	else
-	{
-		if(t < 0.0)
-		{
-			t = 1.0 / 32.0;	// point is outside, so no fogging
-		}
-		else
-		{
-			t = 31.0 / 32.0;
-		}
-	}
+	// HACK: increase cutoff to avoid dark lines at the edge of BSP triangles that stop
+	// right at the fog plane (when viewed from under fog)
+	// But don't do it for models as this causes a doubly-fogged light band at the fog plane
+	#if !defined(USE_VERTEX_ANIMATION) && !defined(USE_VERTEX_SKINNING)
+		t += 1.5;
+	#endif
 
 	var_TexCoords = vec2(s, t);