Merge some byte swap/premultiply functions in their own crate

components/pixels/lib.rs

@@ -0,0 +1,40 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+// TODO(pcwalton): Speed up with SIMD, or better yet, find some way to not do this.
+pub fn byte_swap_colors_inplace(pixels: &mut [u8]) {
+    assert!(pixels.len() % 4 == 0);
+    for rgba in pixels.chunks_mut(4) {
+        let b = rgba[0];
+        rgba[0] = rgba[2];
+        rgba[2] = b;
+    }
+}
+
+pub fn byte_swap_and_premultiply_inplace(pixels: &mut [u8]) {
+    assert!(pixels.len() % 4 == 0);
+    for rgba in pixels.chunks_mut(4) {
+        let b = rgba[0];
+        rgba[0] = multiply_u8_color(rgba[2], rgba[3]);
+        rgba[1] = multiply_u8_color(rgba[1], rgba[3]);
+        rgba[2] = multiply_u8_color(b, rgba[3]);
+    }
+}
+
+/// Returns true if the pixels were found to be completely opaque.
+pub fn premultiply_inplace(pixels: &mut [u8]) -> bool {
+    assert!(pixels.len() % 4 == 0);
+    let mut is_opaque = true;
+    for rgba in pixels.chunks_mut(4) {
+        rgba[0] = multiply_u8_color(rgba[0], rgba[3]);
+        rgba[1] = multiply_u8_color(rgba[1], rgba[3]);
+        rgba[2] = multiply_u8_color(rgba[2], rgba[3]);
+        is_opaque = is_opaque && rgba[3] == 255;
+    }
+    is_opaque
+}
+
+pub fn multiply_u8_color(a: u8, b: u8) -> u8 {
+    return (a as u32 * b as u32 / 255) as u8;
+}