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Faster JBIG2 decoding

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This commit is contained in:
fkaelberer 2014-02-28 10:53:09 +01:00
parent cab5d7b96f
commit abcc72a173
1 changed files with 52 additions and 11 deletions
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63
src/core/jbig2.js
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@ -368,13 +368,39 @@ var Jbig2Image = (function Jbig2ImageClosure() {
var useskip = !!skip;
var template = CodingTemplates[templateIndex].concat(at);
// Sorting is non-standard, and it is not required. But sorting increases
// the number of template bits that can be reused from the previous
// contextLabel in the main loop.
template.sort(function (a, b) {
return (a.y - b.y) || (a.x - b.x);
});
var templateLength = template.length;
var templateX = new Int32Array(templateLength);
var templateY = new Int32Array(templateLength);
var templateX = new Int8Array(templateLength);
var templateY = new Int8Array(templateLength);
var changingTemplateEntries = [];
var reuseMask = 0, minX = 0, maxX = 0, minY = 0;
for (var k = 0; k < templateLength; k++) {
templateX[k] = template[k].x;
templateY[k] = template[k].y;
minX = Math.min(minX, template[k].x);
maxX = Math.max(maxX, template[k].x);
minY = Math.min(minY, template[k].y);
// Check if the template pixel appears in two consecutive context labels,
// so it can be reused. Otherwise, we add it to the list of changing
// template entries.
if (k < templateLength - 1 &&
template[k].y === template[k + 1].y &&
template[k].x === template[k + 1].x - 1) {
reuseMask |= 1 << (templateLength - 1 - k);
} else {
changingTemplateEntries.push(k);
}
}
changingTemplateEntries = new Uint8Array(changingTemplateEntries);
var changingEntriesLength = changingTemplateEntries.length;
var pseudoPixelContext = ReusedContexts[templateIndex];
var bitmap = [];
@ -382,7 +408,7 @@ var Jbig2Image = (function Jbig2ImageClosure() {
var decoder = decodingContext.decoder;
var contexts = decodingContext.contextCache.getContexts('GB');
var ltp = 0;
var ltp = 0, c, j, i0, j0, k, contextLabel = 0;
for (var i = 0; i < height; i++) {
if (prediction) {
var sltp = decoder.readBit(contexts, pseudoPixelContext);
@ -394,18 +420,33 @@ var Jbig2Image = (function Jbig2ImageClosure() {
}
var row = new Uint8Array(width);
bitmap.push(row);
for (var j = 0; j < width; j++) {
for (j = 0; j < width; j++) {
if (useskip && skip[i][j]) {
row[j] = 0;
continue;
}
var contextLabel = 0;
for (var k = 0; k < templateLength; k++) {
var i0 = i + templateY[k], j0 = j + templateX[k];
if (i0 < 0 || j0 < 0 || j0 >= width)
contextLabel <<= 1; // out of bound pixel
else
contextLabel = (contextLabel << 1) | bitmap[i0][j0];
// Are we in the middle of a scanline, so we can reuse contextLabel
// bits?
if (i + minY > 0 && j + minX >= 0 && j + maxX < width) {
// If yes, we can just shift the bits that are reusable and only
// fetch the remaining ones.
contextLabel = (contextLabel << 1) & reuseMask;
for (c = 0; c < changingEntriesLength; c++) {
k = changingTemplateEntries[c];
i0 = i + templateY[k];
j0 = j + templateX[k];
contextLabel |= bitmap[i0][j0] << (templateLength - 1 - k);
}
} else {
// compute the contextLabel from scratch
contextLabel = 0;
for (k = 0; k < templateLength; k++) {
i0 = i + templateY[k];
j0 = j + templateX[k];
if (i0 >= 0 && j0 >= 0 && j0 < width) {
contextLabel |= bitmap[i0][j0] << (templateLength - 1 - k);
}
}
}
var pixel = decoder.readBit(contexts, contextLabel);
row[j] = pixel;