Auto merge of #13114 - glennw:webrender-canvas, r=emilio

Fix canvas image tests when using webrender.

When webrender is enabled, image decoding doesn't pre-multiply by
alpha, but the canvas code expects the image data to be
pre-multiplied form.

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1 files changed, 18 insertions, 0 deletions

diff --git a/components/canvas_traits/lib.rs b/components/canvas_traits/lib.rs
index db945750a0c..d148baf0550 100644
--- a/components/canvas_traits/lib.rs
+++ b/components/canvas_traits/lib.rs
@@ -552,3 +552,21 @@ pub fn byte_swap(data: &mut [u8]) {
         i += 4;
     }
 }
+
+pub fn byte_swap_and_premultiply(data: &mut [u8]) {
+    let length = data.len();
+
+    let mut i = 0;
+    while i < length {
+        let r = data[i + 2];
+        let g = data[i + 1];
+        let b = data[i + 0];
+        let a = data[i + 3];
+
+        data[i + 0] = ((r as u32) * (a as u32) / 255) as u8;
+        data[i + 1] = ((g as u32) * (a as u32) / 255) as u8;
+        data[i + 2] = ((b as u32) * (a as u32) / 255) as u8;
+
+        i += 4;
+    }
+}