add premultiply table for PutImageData

components/canvas/canvas_paint_task.rs

@@ -17,6 +17,7 @@ use ipc_channel::ipc::{self, IpcSender};
 use ipc_channel::router::ROUTER;
 use layers::platform::surface::NativeSurface;
 use num::ToPrimitive;
+use premultiplytable::PREMULTIPLY_TABLE;
 use std::borrow::ToOwned;
 use std::mem;
 use std::sync::mpsc::{Sender, channel};
@@ -597,11 +598,10 @@ impl<'a> CanvasPaintTask<'a> {
             let mut src_offset = src_line;
             for _ in 0 .. dest_rect.size.width {
                 // Premultiply alpha and swap RGBA -> BGRA.
-                // TODO: may want a precomputed premultiply table to make this fast. (#6969)
-                let alpha = imagedata[src_offset + 3] as f32 / 255.;
-                dest.push((imagedata[src_offset + 2] as f32 * alpha) as u8);
-                dest.push((imagedata[src_offset + 1] as f32 * alpha) as u8);
-                dest.push((imagedata[src_offset + 0] as f32 * alpha) as u8);
+                let alpha = imagedata[src_offset + 3] as usize;
+                dest.push(PREMULTIPLY_TABLE[256 * alpha + imagedata[src_offset + 2] as usize]);
+                dest.push(PREMULTIPLY_TABLE[256 * alpha + imagedata[src_offset + 1] as usize]);
+                dest.push(PREMULTIPLY_TABLE[256 * alpha + imagedata[src_offset + 0] as usize]);
                 dest.push(imagedata[src_offset + 3]);
                 src_offset += 4;
             }

components/canvas/lib.rs

@@ -25,4 +25,5 @@ extern crate offscreen_gl_context;
 extern crate util;
 
 pub mod canvas_paint_task;
+mod premultiplytable;
 pub mod webgl_paint_task;