fix: convert box sizes to radii

write test to compare gaussian and fast blur

examples/fast_blur/main.rs

@@ -6,7 +6,7 @@ fn main() {
         .decode()
         .unwrap();
 
-    let img2 = img.fast_blur(10.0);
+    let img2 = img.blur(10.0);
 
     img2.save("examples/fast_blur/mandril_color_blurred.tif")
         .unwrap();

src/imageops/fast_blur.rs

@@ -24,14 +24,14 @@ pub fn fast_blur<P: Pixel>(
             &samples,
             width as usize,
             height as usize,
-            *radius,
+            (*radius-1)/2,
             P::CHANNEL_COUNT as usize,
         );
         samples = horizontal_fast_blur_half::<P::Subpixel>(
             &horizontally_blurred_transposed,
             height as usize,
             width as usize,
-            *radius,
+            (*radius-1)/2,
             P::CHANNEL_COUNT as usize,
         );
     }

src/imageops/mod.rs

@@ -542,4 +542,27 @@ mod tests {
         let image = GrayImage::new(50, 50);
         let _ = fast_blur(&image, 1.0);
     }
+
+    #[test]
+    fn fast_blur_approximates_gaussian_blur_well() {
+        let path = concat!(
+            env!("CARGO_MANIFEST_DIR"),
+            "/tests/images/tiff/testsuite/rgb-3c-16b.tiff"
+        );
+        let image = crate::open(path).unwrap();
+        let image1 = image.blur(50.0).to_rgb8();
+        let binding = image1.as_flat_samples();
+        let bytes1 = binding.as_slice();
+        let image2 = image.fast_blur(50.0).to_rgb8();
+        let binding = image2.as_flat_samples();
+        let bytes2 = binding.as_slice();
+
+        let error = bytes1
+            .iter()
+            .zip(bytes2.iter())
+            .map(|(a, b)| ((*a as f32 - *b as f32) / (*a as f32)))
+            .sum::<f32>()
+            / (bytes1.len() as f32);
+        assert!(error<0.05);
+    }
 }