Auto merge of #8086 - froydnj:premultiply-tables, r=jdm

add (un)premultiply tables for canvas {Get,Put}ImageData operations

Pretty straightforward use of lookup tables to replace a bunch of expensive float operations.

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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;

components/script/dom/canvasrenderingcontext2d.rs

@@ -41,6 +41,7 @@ use std::cell::RefCell;
 use std::str::FromStr;
 use std::sync::mpsc::channel;
 use std::{cmp, fmt};
+use unpremultiplytable::UNPREMULTIPLY_TABLE;
 use url::Url;
 use util::str::DOMString;
 use util::vec::byte_swap;
@@ -887,13 +888,11 @@ impl CanvasRenderingContext2DMethods for CanvasRenderingContext2D {
         let mut data = receiver.recv().unwrap();
 
         // Un-premultiply alpha
-        // TODO: may want a precomputed un-premultiply table to make this fast.
-        // https://github.com/servo/servo/issues/6969
         for chunk in data.chunks_mut(4) {
-             let alpha = chunk[3] as f32 / 255.;
-             chunk[0] = (chunk[0] as f32 / alpha) as u8;
-             chunk[1] = (chunk[1] as f32 / alpha) as u8;
-             chunk[2] = (chunk[2] as f32 / alpha) as u8;
+            let alpha = chunk[3] as usize;
+            chunk[0] = UNPREMULTIPLY_TABLE[256 * alpha + chunk[0] as usize];
+            chunk[1] = UNPREMULTIPLY_TABLE[256 * alpha + chunk[1] as usize];
+            chunk[2] = UNPREMULTIPLY_TABLE[256 * alpha + chunk[2] as usize];
         }
 
         Ok(ImageData::new(self.global.root().r(), sw, sh, Some(data)))

components/script/lib.rs

@@ -95,6 +95,7 @@ pub mod parse;
 pub mod script_task;
 pub mod textinput;
 mod timers;
+mod unpremultiplytable;
 mod webdriver_handlers;
 
 use dom::bindings::codegen::RegisterBindings;

tests/wpt/metadata/2dcontext/pixel-manipulation/2d.imageData.put.alpha.html.ini

@@ -1,5 +0,0 @@
-[2d.imageData.put.alpha.html]
-  type: testharness
-  [putImageData() puts non-solid image data correctly]
-    expected: FAIL
-

tests/wpt/metadata/2dcontext/pixel-manipulation/2d.imageData.put.unchanged.html.ini

@@ -1,5 +0,0 @@
-[2d.imageData.put.unchanged.html]
-  type: testharness
-  [putImageData(getImageData(...), ...) has no effect]
-    expected: FAIL
-