Check small triangle draws in through-mode for pixel mapping too

GPU/Common/SoftwareTransformCommon.cpp

@@ -585,6 +585,35 @@ void SoftwareTransform::BuildDrawingParams(int prim, int vertexCount, u32 vertTy
 
 				inds = newInds;
 			}
+		} else if (throughmode) {
+			// We check some common cases for pixel mapping.
+			// TODO: It's not really optimal that some previous step has removed the triangle strip.
+			if (vertexCount <= 6 && prim == GE_PRIM_TRIANGLES) {
+				// It's enough to check UV deltas vs pos deltas between vertex pairs:
+				// 0-1 1-3 3-2 2-0. Maybe can even skip the last one. Probably some simple math can get us that sequence.
+				// Unfortunately we need to reverse the previous UV scaling operation. Fortunately these are powers of two
+				// so the operations are exact.
+				bool pmapped = true;
+				const u16 *indsIn = (const u16 *)inds;
+				for (int t = 0; t < vertexCount; t += 3) {
+					float uscale = gstate_c.curTextureWidth;
+					float vscale = gstate_c.curTextureHeight;
+					struct { int a; int b; } pairs[] = { {0, 1}, {1, 2}, {2, 0} };
+
+					for (int i = 0; i < ARRAY_SIZE(pairs); i++) {
+						int a = indsIn[t + pairs[i].a];
+						int b = indsIn[t + pairs[i].b];
+						float du = fabsf((transformed[a].u - transformed[b].u) * uscale);
+						float dv = fabsf((transformed[a].v - transformed[b].v) * vscale);
+						float dx = fabsf(transformed[a].x - transformed[b].x);
+						float dy = fabsf(transformed[a].y - transformed[b].y);
+						if (du != dx || dv != dy) {
+							pmapped = false;
+						}
+					}
+				}
+				result->pixelMapped = pmapped;
+			}
 		}
 	}