Correctly invert signed integers

src/color.rs

@@ -969,48 +969,28 @@ pub(crate) trait Invert {
     /// Inverts a color in-place.
     fn invert(&mut self);
 }
-
 impl<T: Primitive> Invert for LumaA<T> {
     fn invert(&mut self) {
-        let l = self.0;
-        let max = T::DEFAULT_MAX_VALUE;
-
-        *self = LumaA([max - l[0], l[1]]);
+        let [l, a] = self.0;
+        *self = LumaA([l.invert_range(), a]);
     }
 }
-
 impl<T: Primitive> Invert for Luma<T> {
     fn invert(&mut self) {
-        let l = self.0;
-
-        let max = T::DEFAULT_MAX_VALUE;
-        let l1 = max - l[0];
-
-        *self = Luma([l1]);
+        let [l] = self.0;
+        *self = Luma([l.invert_range()]);
     }
 }
-
 impl<T: Primitive> Invert for Rgba<T> {
     fn invert(&mut self) {
-        let rgba = self.0;
-
-        let max = T::DEFAULT_MAX_VALUE;
-
-        *self = Rgba([max - rgba[0], max - rgba[1], max - rgba[2], rgba[3]]);
+        let [r, g, b, a] = self.0;
+        *self = Rgba([r.invert_range(), g.invert_range(), b.invert_range(), a]);
     }
 }
-
 impl<T: Primitive> Invert for Rgb<T> {
     fn invert(&mut self) {
-        let rgb = self.0;
-
-        let max = T::DEFAULT_MAX_VALUE;
-
-        let r1 = max - rgb[0];
-        let g1 = max - rgb[1];
-        let b1 = max - rgb[2];
-
-        *self = Rgb([r1, g1, b1]);
+        let [r, g, b] = self.0;
+        *self = Rgb([r.invert_range(), g.invert_range(), b.invert_range()]);
     }
 }
 

src/primitive_sealed.rs

@@ -8,7 +8,7 @@ use crate::imageops::fast_blur::BlurAccumulator;
 /// this crate.
 #[allow(private_bounds)]
 pub trait PrimitiveSealed:
-    Sized + NearestFrom<f32> + WithBlurAcc + BgraSwizzle + RgbToLuma
+    Sized + NearestFrom<f32> + WithBlurAcc + BgraSwizzle + RgbToLuma + InvertRange
 {
 }
 
@@ -229,3 +229,35 @@ impl RgbToLuma for f32 {
     }
 }
 impl RgbToLuma for f64 {}
+
+pub(crate) trait InvertRange {
+    /// Inverts the default range of this value.
+    ///
+    /// This has the following properties:
+    /// - `self.invert_range().invert_range() == self`
+    /// - `T::DEFAULT_MIN_VALUE.invert_range() == T::DEFAULT_MAX_VALUE` and
+    ///   `T::DEFAULT_MAX_VALUE.invert_range() == T::DEFAULT_MIN_VALUE`
+    ///
+    /// For floats, this is `1.0 - self`. For integers, this is `!self`.
+    fn invert_range(self) -> Self;
+}
+impl InvertRange for f32 {
+    fn invert_range(self) -> Self {
+        1.0 - self
+    }
+}
+impl InvertRange for f64 {
+    fn invert_range(self) -> Self {
+        1.0 - self
+    }
+}
+macro_rules! impl_invert_range_for_ints {
+    ($($t:ty),+) => { $(
+        impl InvertRange for $t {
+            fn invert_range(self) -> Self {
+                !self
+            }
+        }
+    )+ };
+}
+impl_invert_range_for_ints!(u8, u16, u32, u64, usize, i8, i16, i32, i64, isize);

src/traits.rs

@@ -560,7 +560,7 @@ pub trait Pixel: Copy + Clone {
     where
         F: FnMut(Self::Subpixel, Self::Subpixel) -> Self::Subpixel;
 
-    /// Invert this pixel
+    /// Invert the color channels of this pixel. Alpha channels are *not* inverted.
     fn invert(&mut self);
 
     /// Blend the color of a given pixel into ourself, taking into account alpha channels