From a4440a1c6bf96707caa2bcc5e45a1b49c40d794d Mon Sep 17 00:00:00 2001
From: Jonas Jenwald <jonas.jenwald@gmail.com>
Date: Sat, 1 Feb 2020 11:42:22 +0100
Subject: [PATCH] Avoid re-calculating the `xScaleBlockOffset` when not
necessary in `JpegImage._getLinearizedBlockData`
As can be seen in the code, the `xScaleBlockOffset` typed array doesn't depend on the actual image data but only on the width and x-scale. The width is obviously consistent for an image, and it turns out that in practice the `componentScaleX` is quite often identical between two (or more) adjacent image components.
All-in-all it's thus not necessary to *unconditionally* re-compute the `xScaleBlockOffset` when getting the JPEG image data.
While avoiding, in many cases, one or more loops can never be a bad thing these changes are unfortunately completely dominated by the rest of the JpegImage code and consequently doesn't really show up in benchmark results. *Hence I'd understand if this patch is ultimately deemed not necessary.*
---
src/core/jpg.js | 13 +++++++++----
1 file changed, 9 insertions(+), 4 deletions(-)
diff --git a/src/core/jpg.js b/src/core/jpg.js
index 5f973ba6e..9abc8de80 100644
--- a/src/core/jpg.js
+++ b/src/core/jpg.js
@@ -1105,6 +1105,7 @@ var JpegImage = (function JpegImageClosure() {
var data = new Uint8ClampedArray(dataLength);
var xScaleBlockOffset = new Uint32Array(width);
var mask3LSB = 0xfffffff8; // used to clear the 3 LSBs
+ let lastComponentScaleX;
for (i = 0; i < numComponents; i++) {
component = this.components[i];
@@ -1113,10 +1114,14 @@ var JpegImage = (function JpegImageClosure() {
offset = i;
output = component.output;
blocksPerScanline = (component.blocksPerLine + 1) << 3;
- // precalculate the xScaleBlockOffset
- for (x = 0; x < width; x++) {
- j = 0 | (x * componentScaleX);
- xScaleBlockOffset[x] = ((j & mask3LSB) << 3) | (j & 7);
+ // Precalculate the `xScaleBlockOffset`. Since it doesn't depend on the
+ // component data, that's only necessary when `componentScaleX` changes.
+ if (componentScaleX !== lastComponentScaleX) {
+ for (x = 0; x < width; x++) {
+ j = 0 | (x * componentScaleX);
+ xScaleBlockOffset[x] = ((j & mask3LSB) << 3) | (j & 7);
+ }
+ lastComponentScaleX = componentScaleX;
}
// linearize the blocks of the component
for (y = 0; y < height; y++) {