Normalize quality levels to 0-100 range

rust/src/main.rs

@@ -29,12 +29,10 @@ fn main() -> Result<()> {
             output.put_pixel(x, y, *pixel);
         }
     }
-    quadtree_quant(
-        &mut output,
-        args.max_err.unwrap_or(2000.0) * 128.0 * 128.0,
-        [0, 0],
-        128,
-    );
+    let error_scale_factor = 0.2; // Adjusted by feel. Q=0 is unrecognizable but not blank.
+    let error_per_pixel = (1.0 - args.max_err.unwrap_or(80.0) / 100.0) * error_scale_factor;
+    let sq_error_per_pixel = error_per_pixel * error_per_pixel;
+    quadtree_quant(&mut output, sq_error_per_pixel * 128.0 * 128.0, [0, 0], 128);
     output.save(&args.output)?;
     println!("Wrote {}", &args.output);
     Ok(())
@@ -83,10 +81,12 @@ fn quadtree_quant(img: &mut RgbImage, max_err: f32, pos: [u32; 2], size: u32) {
 
 fn redmean_sq(x: [u8; 3], y: [u8; 3]) -> f32 {
     let mut ds: [f32; 3] = [0.0, 0.0, 0.0];
-    for i in 0..2 {
-        ds[i] = x[i] as f32 - y[i] as f32;
-        ds[i] *= ds[i];
+    for (i, d) in ds.iter_mut().enumerate() {
+        let delta = x[i] as f32 - y[i] as f32;
+        *d = delta * delta;
     }
     let rm = (x[0] as f32 + y[0] as f32) / 2.0;
-    (2.0 + rm / 256.0) * ds[0] + 4.0 * ds[1] + (2.0 + (255.0 - rm) / 256.0) * ds[2]
+    let result = (2.0 + rm / 256.0) * ds[0] + 4.0 * ds[1] + (2.0 + (255.0 - rm) / 256.0) * ds[2];
+    // normalize value to the range 0.0 - 1.0
+    result / 584971.0
 }