From a66056217fe548d7a84d90d95c59d14a02c41fd3 Mon Sep 17 00:00:00 2001
From: "Unknown W. Brackets" <checkins@unknownbrackets.org>
Date: Sun, 13 Nov 2022 09:23:06 -0800
Subject: [PATCH] softgpu: Avoid splitting rectangles for fog.

If the fog factor would result in the same amount of fog, we're just
adding more work for no reason.
---
 GPU/Software/Clipper.cpp | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/GPU/Software/Clipper.cpp b/GPU/Software/Clipper.cpp
index 11db812e1a..bae2be1881 100644
--- a/GPU/Software/Clipper.cpp
+++ b/GPU/Software/Clipper.cpp
@@ -203,7 +203,15 @@ void ProcessRect(const ClipVertexData &v0, const ClipVertexData &v1, BinManager
 		else if (outsidePos >= 2 || outsideNeg >= 2)
 			return;
 
-		if (v0.v.fogdepth != v1.v.fogdepth) {
+		bool splitFog = v0.v.fogdepth != v1.v.fogdepth;
+		if (splitFog) {
+			// If they match the same 1/255, we can consider the fog flat.  Seen in Resistance.
+			// More efficient if we can avoid splitting.
+			static constexpr float foghalfstep = 0.5f / 255.0f;
+			if (v1.v.fogdepth - foghalfstep <= v0.v.fogdepth && v1.v.fogdepth + foghalfstep >= v0.v.fogdepth)
+				splitFog = false;
+		}
+		if (splitFog) {
 			// Rectangles seem to always use nearest along X for fog depth, but reversed.
 			// TODO: Check exactness of middle.
 			VertexData vhalf0 = v1.v;