• Nigel Tao's avatar
    Basic image/jpeg decoder. · 8bf58725
    Nigel Tao authored
    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
    8bf58725
huffman.go 4.54 KB