Improvement for the Paeth transformation

paeth.diff

diff --git a/image.c b/image.c
index f22fc6f..36fbe46 100644
--- a/image.c
+++ b/image.c
@@ -257,9 +257,9 @@ void image_transform_fast_rev( struct image *image )
 void image_transform( struct image *image )
 {
 	int x, y, i, width, height, aerr, berr, cerr, ia, ib, ic;
-	struct pixel p;
+	struct pixel a, b, c, p;
 	struct pixel *pixels;
-	int pr, pg, pb;
+	int px, py, pz, qx, qy, qz;
 
 	image->transform = 2;
 
@@ -276,35 +276,28 @@ void image_transform( struct image *image )
 			ib = i-width;
 			ic = i-1-width;
 
-			pr = pixels[ ia ].x + pixels[ ib ].x - pixels[ ic ].x;
-			pg = pixels[ ia ].y + pixels[ ib ].y - pixels[ ic ].y;
-			pb = pixels[ ia ].z + pixels[ ib ].z - pixels[ ic ].z;
+			a = pixels[ ia ];
+			b = pixels[ ib ];
+			c = pixels[ ic ];
 
-			p = pixels[ ia ];
-			aerr = abs(p.x - pr);
-			aerr += abs(p.y - pg);
-			aerr += abs(p.z - pb);
+			px = b.x - c.x;
+			py = b.y - c.y;
+			pz = b.z - c.z;
 
-			p = pixels[ ib ];
-			berr = abs(p.x - pr);
-			berr += abs(p.y - pg);
-			berr += abs(p.z - pb);
-		
-			p = pixels[ ic ];
-			cerr = abs(p.x - pr);
-			cerr += abs(p.y - pg);
-			cerr += abs(p.z - pb);
-
-			if( ( aerr < berr ) && ( aerr < cerr ) )
-				p = pixels[ ia ];
-			else if( berr < cerr )
-				p = pixels[ ib ];
-			else
-				p = pixels[ ic ];
-
-			pixels[ i ].x -= p.x;
-			pixels[ i ].y -= p.y;
-			pixels[ i ].z -= p.z;
+			qx = a.x - c.x;
+			qy = a.y - c.y;
+			qz = a.z - c.z;
+
+			aerr = abs(px) + abs(py) + abs(pz);
+			berr = abs(qx) + abs(qy) + abs(qz);
+			cerr = abs(px + qx) + abs(py + qy) + abs(pz + qz);
+
+			if (berr < aerr) aerr = berr, a = b;
+			if (cerr < aerr) a = c;
+
+			pixels[ i ].x -= a.x;
+			pixels[ i ].y -= a.y;
+			pixels[ i ].z -= a.z;
 		}
 		
 		i = y*width;
@@ -338,9 +331,9 @@ void image_transform( struct image *image )
 void image_transform_rev( struct image *image )
 {
 	int x, y, i, width, height, aerr, berr, cerr, ia, ib, ic;
-	struct pixel p;
+	struct pixel p, a, b, c;
 	struct pixel *pixels;
-	int pr, pg, pb;
+	int px, py, pz, qx, qy, qz;
 
 	image->transform = 0;
 
@@ -376,35 +369,28 @@ void image_transform_rev( struct image *image )
 			ib = i-width;
 			ic = i-1-width;
 
-			pr = pixels[ ia ].x + pixels[ ib ].x - pixels[ ic ].x;
-			pg = pixels[ ia ].y + pixels[ ib ].y - pixels[ ic ].y;
-			pb = pixels[ ia ].z + pixels[ ib ].z - pixels[ ic ].z;
-
-			p = pixels[ ia ];
-			aerr = abs(p.x - pr);
-			aerr += abs(p.y - pg);
-			aerr += abs(p.z - pb);
-
-			p = pixels[ ib ];
-			berr = abs(p.x - pr);
-			berr += abs(p.y - pg);
-			berr += abs(p.z - pb);
-
-			p = pixels[ ic ];
-			cerr = abs(p.x - pr);
-			cerr += abs(p.y - pg);
-			cerr += abs(p.z - pb);
-
-			if( ( aerr < berr ) && ( aerr < cerr ) )
-				p = pixels[ ia ];
-			else if( berr < cerr )
-				p = pixels[ ib ];
-			else
-				p = pixels[ ic ];
-
-			pixels[ i ].x += p.x;
-			pixels[ i ].y += p.y;
-			pixels[ i ].z += p.z;
+			a = pixels[ ia ];
+			b = pixels[ ib ];
+			c = pixels[ ic ];
+
+			px = b.x - c.x;
+			py = b.y - c.y;
+			pz = b.z - c.z;
+
+			qx = a.x - c.x;
+			qy = a.y - c.y;
+			qz = a.z - c.z;
+
+			aerr = abs(px) + abs(py) + abs(pz);
+			berr = abs(qx) + abs(qy) + abs(qz);
+			cerr = abs(px + qx) + abs(py + qy) + abs(pz + qz);
+
+			if (berr < aerr) aerr = berr, a = b;
+			if (cerr < aerr) a = c;
+
+			pixels[ i ].x += a.x;
+			pixels[ i ].y += a.y;
+			pixels[ i ].z += a.z;
 		}
 	}
 }