Added invert + tests + docs

src/bitmap/image.rs

@@ -437,6 +437,15 @@ impl BitMap {
         }
     }
 
+    ///
+    /// Invert the colors of the image
+    ///
+    pub fn invert(&mut self) {
+        for c in &mut self.pixels {
+            c.invert();
+        }
+    }
+
     ///
     /// Find all the pixels that are the same as the from color and convert them
     /// all to the "to" color.
@@ -1147,4 +1156,28 @@ mod test {
             assert!(pixel.is_black())
         }
     }
+
+    #[test]
+    fn invert_test() {
+        //create a some images of B/W
+        let originally_white = BitMap::new(10, 10);
+        let mut originally_black = BitMap::new(10, 10);
+        originally_black.clear_image(Rgba::black());
+
+        //clone them
+        let mut make_white = originally_black.clone();
+        let mut make_black = originally_white.clone();
+
+        //sanity
+        assert_eq!(make_black, originally_white);
+        assert_eq!(make_white, originally_black);
+
+        //invert them
+        make_white.invert();
+        make_black.invert();
+
+        //test
+        assert_eq!(make_black, originally_black);
+        assert_eq!(make_white, originally_white);
+    }
 }

src/bitmap/rgba.rs

@@ -158,6 +158,15 @@ impl Rgba {
         self.set_gray_scale_pixel(pixel_gray);
     }
 
+    ///
+    /// Invert the pixel color
+    ///
+    pub fn invert(&mut self) {
+        self.red = 255 - self.red.clamp(0, 255);
+        self.blue = 255 - self.blue.clamp(0, 255);
+        self.green = 255 - self.green.clamp(0, 255);
+    }
+
     ///
     /// set all the colors to the same color
     ///