author Neil Rashbrook <>
Mon, 30 Mar 2015 00:42:42 +0100
changeset 21680 5576f057ebbd3e81298c72d4bf55e50a2cacb4b5
parent 8729 923b924c9422bddc54a3d8c08b765da58dddeedd
permissions -rw-r--r--
Bug 962910 Find bar should use a system key event listener r=Ratty a=IanN a=Ratty for checkin to a CLOSED TREE

#!/usr/bin/env python
# $URL: $
# $Rev: 150 $

# pipdither
# Error Diffusing image dithering.
# Now with serpentine scanning.

# See

from bisect import bisect_left

import png

def dither(out, inp,
  bitdepth=1, linear=False, defaultgamma=1.0, targetgamma=None,
    """Dither the input PNG `inp` into an image with a smaller bit depth
    and write the result image onto `out`.  `bitdepth` specifies the bit
    depth of the new image.
    Normally the source image gamma is honoured (the image is
    converted into a linear light space before being dithered), but
    if the `linear` argument is true then the image is treated as
    being linear already: no gamma conversion is done (this is
    quicker, and if you don't care much about accuracy, it won't
    matter much).
    Images with no gamma indication (no ``gAMA`` chunk) are normally
    treated as linear (gamma = 1.0), but often it can be better
    to assume a different gamma value: For example continuous tone
    photographs intended for presentation on the web often carry
    an implicit assumption of being encoded with a gamma of about
    0.45 (because that's what you get if you just "blat the pixels"
    onto a PC framebuffer), so ``defaultgamma=0.45`` might be a
    good idea.  `defaultgamma` does not override a gamma value
    specified in the file itself: It is only used when the file
    does not specify a gamma.

    If you (pointlessly) specify both `linear` and `defaultgamma`,
    `linear` wins.

    The gamma of the output image is, by default, the same as the input
    image.  The `targetgamma` argument can be used to specify a
    different gamma for the output image.  This effectively recodes the
    image to a different gamma, dithering as we go.  The gamma specified
    is the exponent used to encode the output file (and appears in the
    output PNG's ``gAMA`` chunk); it is usually less than 1.


    # Encoding is what happened when the PNG was made (and also what
    # happens when we output the PNG).  Decoding is what we do to the
    # source PNG in order to process it.

    # The dithering algorithm is not completely general; it
    # can only do bit depth reduction, not arbitrary palette changes.
    import operator
    maxval = 2**bitdepth - 1
    r = png.Reader(file=inp)
    # If image gamma is 1 or gamma is not present and we are assuming a
    # value of 1, then it is faster to pass a maxval parameter to
    # asFloat (the multiplications get combined).  With gamma, we have
    # to have the pixel values from 0.0 to 1.0 (as long as we are doing
    # gamma correction here).
    # Slightly annoyingly, we don't know the image gamma until we've
    # called asFloat().
    _,_,pixels,info = r.asDirect()
    planes = info['planes']
    assert planes == 1
    width = info['size'][0]
    sourcemaxval = 2**info['bitdepth'] - 1
    if linear:
        gamma = 1
        gamma = info.get('gamma') or defaultgamma
    # Convert gamma from encoding gamma to the required power for
    # decoding.
    decode = 1.0/gamma
    # Build a lookup table for decoding; convert from pixel values to linear
    # space:
    sourcef = 1.0/sourcemaxval
    incode = map(sourcef.__mul__, range(sourcemaxval+1))
    if decode != 1.0:
        incode = map(decode.__rpow__, incode)
    # Could be different, later on.  targetdecode is the assumed gamma
    # that is going to be used to decoding the target PNG.  It is the
    # reciprocal of the exponent that we use to encode the target PNG.
    # This is the value that we need to build our table that we use for
    # converting from linear to target colour space.
    if targetgamma is None:
        targetdecode = decode
        targetdecode = 1.0/targetgamma
    # The table we use for encoding (creating the target PNG), still
    # maps from pixel value to linear space, but we use it inverted, by
    # searching through it with bisect.
    targetf = 1.0/maxval
    outcode = map(targetf.__mul__, range(maxval+1))
    if targetdecode != 1.0:
        outcode = map(targetdecode.__rpow__, outcode)
    # The table used for choosing output codes.  These values represent
    # the cutoff points between two adjacent output codes.
    choosecode = zip(outcode[1:], outcode)
    p = cutoff
    choosecode = map(lambda x: x[0]*p+x[1]*(1.0-p), choosecode)
    def iterdither():
        # Errors diffused downwards (into next row)
        ed = [0.0]*width
        flipped = False
        for row in pixels:
            row = map(incode.__getitem__, row)
            row = map(operator.add, ed, row)
            if flipped:
                row = row[::-1]
            targetrow = [0] * width
            for i,v in enumerate(row):
                # Clamp.  Necessary because previously added errors may take
                # v out of range.
                v = max(0.0, min(v, 1.0))
                # `it` will be the index of the chosen target colour;
                it = bisect_left(choosecode, v)
                t = outcode[it]
                targetrow[i] = it
                # err is the error that needs distributing.
                err = v - t
                # Sierra "Filter Lite" distributes          * 2
                # as per this diagram.                    1 1
                ef = err/2.0
                # :todo: consider making rows one wider at each end and
                # removing "if"s
                if i+1 < width:
                    row[i+1] += ef
                ef /= 2.0
                ed[i] = ef
                if i:
                    ed[i-1] += ef
            if flipped:
                ed = ed[::-1]
                targetrow = targetrow[::-1]
            yield targetrow
            flipped = not flipped
    info['bitdepth'] = bitdepth
    info['gamma'] = 1.0/targetdecode
    w = png.Writer(**info)
    w.write(out, iterdither())

def main(argv=None):
    from getopt import getopt
    import sys
    if argv is None:
        argv = sys.argv
    opt,argv = getopt(argv[1:], 'b:c:g:lo:')
    k = {}
    for o,v in opt:
        if o == '-b':
            k['bitdepth'] = int(v)
        if o == '-c':
            k['cutoff'] = float(v)
        if o == '-g':
            k['defaultgamma'] = float(v)
        if o == '-l':
            k['linear'] = True
        if o == '-o':
            k['targetgamma'] = float(v)
        if o == '-?':
            print >>sys.stderr, "pipdither [-b bits] [-c cutoff] [-g assumed-gamma] [-l] [in.png]"

    if len(argv) > 0:
        f = open(argv[0], 'rb')
        f = sys.stdin

    return dither(sys.stdout, f, **k)

if __name__ == '__main__':