more dither tweaks

src/colormap.rs

@@ -214,6 +214,22 @@ impl ColorMap {
         neighbor
     }
 
+    pub fn closest_neighbor(&self, index: usize, target: FloatColor) -> usize {
+        let color = self.colors[index];
+        let to_target = target - color;
+        let mut neighbor = index;
+        let mut best = ::std::f64::MAX;
+        for &i in &self.neighbors[index] {
+            let c = self.colors[i];
+            let d = (c - target).abs();
+            if d > 0.0 && d < best && (c - color).dot(&to_target) >= 0.0 {
+                neighbor = i;
+                best = d;
+            }
+        }
+        neighbor
+    }
+
     pub fn float_color(&self, index: usize) -> FloatColor {
         self.colors[index]
     }

src/remapper.rs

@@ -84,9 +84,10 @@ impl Remapper for RemapperOrdered2 {
                 }
                 let dir = diff * (1.0 / d);
                 let j = self.map.neighbor_in_dir(i, dir);
+                // let j = self.map.closest_neighbor(i, color);
                 let c2 = self.map.float_color(j);
-                let d2 = (c2 - color).abs();
-                let f = d / (d + d2);
+                let span = c2 - c;
+                let f = (color - c).dot(&span) / span.dot(&span);
                 let offset = if f > 0.375 {
                     2
                 } else if f > 0.125 {