From 5b83e0b9a3b2fbed8cc92397b8f874ab4375fc6d Mon Sep 17 00:00:00 2001
From: fkaelberer <l_l@gmx-topmail.de>
Date: Fri, 1 Aug 2014 15:29:03 +0200
Subject: [PATCH] Faster JBIG2 bitmap decoding
---
src/core/jbig2.js | 43 +++++++++++++++++++++++++++++++++++++++++++
1 file changed, 43 insertions(+)
diff --git a/src/core/jbig2.js b/src/core/jbig2.js
index e4ae7c751..7e6e3fd6e 100755
--- a/src/core/jbig2.js
+++ b/src/core/jbig2.js
@@ -152,6 +152,42 @@ var Jbig2Image = (function Jbig2ImageClosure() {
0x0008 // '0000' + '001000'
];
+ function decodeBitmapTemplate0(width, height, decodingContext) {
+ var decoder = decodingContext.decoder;
+ var contexts = decodingContext.contextCache.getContexts('GB');
+ var contextLabel, i, j, pixel, row, row1, row2, bitmap = [];
+
+ // ...ooooo....
+ // ..ooooooo... Context template for current pixel (X)
+ // .ooooX...... (concatenate values of 'o'-pixels to get contextLabel)
+ var OLD_PIXEL_MASK = 0x7BF7; // 01111 0111111 0111
+
+ for (i = 0; i < height; i++) {
+ row = bitmap[i] = new Uint8Array(width);
+ row1 = (i < 1) ? row : bitmap[i - 1];
+ row2 = (i < 2) ? row : bitmap[i - 2];
+
+ // At the beginning of each row:
+ // Fill contextLabel with pixels that are above/right of (X)
+ contextLabel = (row2[0] << 13) | (row2[1] << 12) | (row2[2] << 11) |
+ (row1[0] << 7) | (row1[1] << 6) | (row1[2] << 5) |
+ (row1[3] << 4);
+
+ for (j = 0; j < width; j++) {
+ row[j] = pixel = decoder.readBit(contexts, contextLabel);
+
+ // At each pixel: Clear contextLabel pixels that are shifted
+ // out of the context, then add new ones.
+ // If j + n is out of range at the right image border, then
+ // the undefined value of bitmap[i - 2][j + n] is shifted to 0
+ contextLabel = ((contextLabel & OLD_PIXEL_MASK) << 1) |
+ (row2[j + 3] << 11) | (row1[j + 4] << 4) | pixel;
+ }
+ }
+
+ return bitmap;
+ }
+
// 6.2 Generic Region Decoding Procedure
function decodeBitmap(mmr, width, height, templateIndex, prediction, skip, at,
decodingContext) {
@@ -159,6 +195,13 @@ var Jbig2Image = (function Jbig2ImageClosure() {
error('JBIG2 error: MMR encoding is not supported');
}
+ // Use optimized version for the most common case
+ if (templateIndex === 0 && !skip && !prediction && at.length === 4 &&
+ at[0].x === 3 && at[0].y === -1 && at[1].x === -3 && at[1].y === -1 &&
+ at[2].x === 2 && at[2].y === -2 && at[3].x === -2 && at[3].y === -2) {
+ return decodeBitmapTemplate0(width, height, decodingContext);
+ }
+
var useskip = !!skip;
var template = CodingTemplates[templateIndex].concat(at);