Auto merge of #9911 - magni-:dont_use_lookup_tables, r=michaelwu

Do not use lookup tables for put_image_data

Fixes #9599

"This is the first Rust code I have ever written" (and also my first time writing real browser code, it's been quite the learning experience).

Some questions:

> For really fast CPU results, use integer SIMD instructions to handle more than one pixel at a time.

This was out of the scope of #9599, right? I started looking into doing that, but it seems to be a lot more work than the `E-easy` label would suggest. [`std::simd`](https://doc.rust-lang.org/1.0.0/std/simd/index.html) is marked as "unstable", is that a blocker?

> 128 can be added before dividing to round more accurately.

@michaelwu, what did you mean by that?

Also, the #9599 is `Do not use lookup tables for {Get,Put}ImageData operations`, but we only use lookup tables for the `Put`, not the `Get`, right?

Sorry for all the noobish questions.

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components/canvas/canvas_paint_thread.rs

@@ -15,7 +15,6 @@ use gfx_traits::color;
 use ipc_channel::ipc::IpcSharedMemory;
 use ipc_channel::ipc::{self, IpcSender};
 use num::ToPrimitive;
-use premultiplytable::PREMULTIPLY_TABLE;
 use std::borrow::ToOwned;
 use std::mem;
 use util::opts;
@@ -538,10 +537,7 @@ impl<'a> CanvasPaintThread<'a> {
         })
     }
 
-    fn image_data(&self,
-                      dest_rect: Rect<i32>,
-                      canvas_size: Size2D<f64>,
-                      chan: IpcSender<Vec<u8>>) {
+    fn image_data(&self, dest_rect: Rect<i32>, canvas_size: Size2D<f64>, chan: IpcSender<Vec<u8>>) {
         let mut dest_data = self.read_pixels(dest_rect, canvas_size);
 
         // bgra -> rgba
@@ -615,11 +611,12 @@ impl<'a> CanvasPaintThread<'a> {
         for _ in 0 .. dest_rect.size.height {
             let mut src_offset = src_line;
             for _ in 0 .. dest_rect.size.width {
-                // Premultiply alpha and swap RGBA -> BGRA.
-                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]);
+                let alpha = imagedata[src_offset + 3] as u16;
+                // add 127 before dividing for more accurate rounding
+                let premultiply_channel = |channel: u8| (((channel as u16 * alpha) + 127) / 255) as u8;
+                dest.push(premultiply_channel(imagedata[src_offset + 2]));
+                dest.push(premultiply_channel(imagedata[src_offset + 1]));
+                dest.push(premultiply_channel(imagedata[src_offset + 0]));
                 dest.push(imagedata[src_offset + 3]);
                 src_offset += 4;
             }

components/canvas/lib.rs

@@ -23,5 +23,4 @@ extern crate util;
 extern crate webrender_traits;
 
 pub mod canvas_paint_thread;
-mod premultiplytable;
 pub mod webgl_paint_thread;