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The Go programming language
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This is not a complete JPEG implementation (e.g. it does not handle progressive JPEGs or restart markers), but I was able to take a photo with my phone, and view the resultant JPEG in pure Go. The decoder is simple, but slow. The Huffman decoder in particular should be easily improvable, but optimization is left to future changelists. Being able to inline functions in the inner loop should also help performance. The output is not pixel-for-pixel identical to libjpeg, although identical behavior isn't necessarily a goal, since JPEG is a lossy codec. There are at least two reasons for the discrepancy. First, the inverse DCT algorithm used is the same as Plan9's src/cmd/jpg, which has different rounding errors from libjpeg's default IDCT implementation. Note that libjpeg actually has three different IDCT implementations: one floating point, and two fixed point. Out of those four, Plan9's seemed the simplest to understand, partly because it has no #ifdef's or C macros. Second, for 4:2:2 or 4:2:0 chroma sampling, this implementation does nearest neighbor upsampling, compared to libjpeg's triangle filter (e.g. see h2v1_fancy_upsample in jdsample.c). The difference from the first reason is typically zero, but sometimes 1 (out of 256) in YCbCr space, or double that in RGB space. The difference from the second reason can be as large as 8/256 in YCbCr space, in regions of steep chroma gradients. Informal eyeballing suggests that the net difference is typically imperceptible, though. R=r CC=golang-dev, rsc https://golang.org/cl/164056 |
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This is the source code repository for the Go programming language. For documentation about how to install and use Go, visit http://golang.org/ or load doc/install.html in your web browser. After installing Go, you can view a nicely formatted doc/install.html by running godoc --http=:6060 and then visiting http://localhost:6060/doc/install.html. Unless otherwise noted, the Go source files are distributed under the BSD-style license found in the LICENSE file.