updates

Backend/venv/bin/priditherpng

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+#!/home/gnx/Desktop/Hotel-Booking/Backend/venv/bin/python3
+
+# pipdither
+# Error Diffusing image dithering.
+# Now with serpentine scanning.
+
+# See http://www.efg2.com/Lab/Library/ImageProcessing/DHALF.TXT
+
+# http://www.python.org/doc/2.4.4/lib/module-bisect.html
+from bisect import bisect_left
+
+
+import png
+
+
+def dither(
+    out,
+    input,
+    bitdepth=1,
+    linear=False,
+    defaultgamma=1.0,
+    targetgamma=None,
+    cutoff=0.5,  # see :cutoff:default
+):
+    """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=input)
+
+    _, _, pixels, info = r.asDirect()
+    planes = info["planes"]
+    # :todo: make an Exception
+    assert planes == 1
+    width = info["size"][0]
+    sourcemaxval = 2 ** info["bitdepth"] - 1
+
+    if linear:
+        gamma = 1
+    else:
+        gamma = info.get("gamma") or defaultgamma
+
+    # Calculate an effective gamma for input and output;
+    # then build tables using those.
+
+    # `gamma` (whether it was obtained from the input file or an
+    # assumed value) is the encoding gamma.
+    # We need the decoding gamma, which is the reciprocal.
+    decode = 1.0 / gamma
+
+    # `targetdecode` is the assumed gamma that is going to be used
+    # to decoding the target PNG.
+    # Note that even though we will _encode_ the target PNG we
+    # still need the decoding gamma, because
+    # the table we use maps from PNG pixel value to linear light level.
+    if targetgamma is None:
+        targetdecode = decode
+    else:
+        targetdecode = 1.0 / targetgamma
+
+    incode = build_decode_table(sourcemaxval, decode)
+
+    # For encoding, we still build a decode table, because we
+    # use it inverted (searching with bisect).
+    outcode = build_decode_table(maxval, targetdecode)
+
+    # The table used for choosing output codes.  These values represent
+    # the cutoff points between two adjacent output codes.
+    # The cutoff parameter can be varied between 0 and 1 to
+    # preferentially choose lighter (when cutoff > 0.5) or
+    # darker (when cutoff < 0.5) values.
+    # :cutoff:default: The default for this used to be 0.75, but
+    # testing by drj on 2021-07-30 showed that this produces
+    # banding when dithering left-to-right gradients;
+    # test with:
+    # priforgepng grl | priditherpng | kitty icat
+    choosecode = list(zip(outcode[1:], outcode))
+    p = cutoff
+    choosecode = [x[0] * p + x[1] * (1.0 - p) for x in choosecode]
+
+    rows = repeat_header(pixels)
+    dithered_rows = run_dither(incode, choosecode, outcode, width, rows)
+    dithered_rows = remove_header(dithered_rows)
+
+    info["bitdepth"] = bitdepth
+    info["gamma"] = 1.0 / targetdecode
+    w = png.Writer(**info)
+    w.write(out, dithered_rows)
+
+
+def build_decode_table(maxval, gamma):
+    """Build a lookup table for decoding;
+    table converts from pixel values to linear space.
+    """
+
+    assert maxval == int(maxval)
+    assert maxval > 0
+
+    f = 1.0 / maxval
+    table = [f * v for v in range(maxval + 1)]
+    if gamma != 1.0:
+        table = [v ** gamma for v in table]
+    return table
+
+
+def run_dither(incode, choosecode, outcode, width, rows):
+    """
+    Run an serpentine dither.
+    Using the incode and choosecode tables.
+    """
+
+    # Errors diffused downwards (into next row)
+    ed = [0.0] * width
+    flipped = False
+    for row in rows:
+        # Convert to linear...
+        row = [incode[v] for v in row]
+        # Add errors...
+        row = [e + v for e, v in zip(ed, row)]
+
+        if flipped:
+            row = row[::-1]
+        targetrow = [0] * width
+
+        for i, v in enumerate(row):
+            # `it` will be the index of the chosen target colour;
+            it = bisect_left(choosecode, v)
+            targetrow[i] = it
+            t = outcode[it]
+            # err is the error that needs distributing.
+            err = v - t
+
+            # Sierra "Filter Lite" distributes          * 2
+            # as per this diagram.                    1 1
+            ef = err * 0.5
+            # :todo: consider making rows one wider at each end and
+            # removing "if"s
+            if i + 1 < width:
+                row[i + 1] += ef
+            ef *= 0.5
+            ed[i] = ef
+            if i:
+                ed[i - 1] += ef
+
+        if flipped:
+            ed = ed[::-1]
+            targetrow = targetrow[::-1]
+        yield targetrow
+        flipped = not flipped
+
+
+WARMUP_ROWS = 32
+
+
+def repeat_header(rows):
+    """Repeat the first row, to "warm up" the error register."""
+    for row in rows:
+        yield row
+        for _ in range(WARMUP_ROWS):
+            yield row
+        break
+    yield from rows
+
+
+def remove_header(rows):
+    """Remove the same number of rows that repeat_header added."""
+
+    for _ in range(WARMUP_ROWS):
+        next(rows)
+    yield from rows
+
+
+def main(argv=None):
+    import sys
+
+    # https://docs.python.org/3.5/library/argparse.html
+    import argparse
+
+    parser = argparse.ArgumentParser()
+
+    if argv is None:
+        argv = sys.argv
+
+    progname, *args = argv
+
+    parser.add_argument("--bitdepth", type=int, default=1, help="bitdepth of output")
+    parser.add_argument(
+        "--cutoff",
+        type=float,
+        default=0.5,
+        help="cutoff to select adjacent output values",
+    )
+    parser.add_argument(
+        "--defaultgamma",
+        type=float,
+        default=1.0,
+        help="gamma value to use when no gamma in input",
+    )
+    parser.add_argument("--linear", action="store_true", help="force linear input")
+    parser.add_argument(
+        "--targetgamma",
+        type=float,
+        help="gamma to use in output (target), defaults to input gamma",
+    )
+    parser.add_argument(
+        "input", nargs="?", default="-", type=png.cli_open, metavar="PNG"
+    )
+
+    ns = parser.parse_args(args)
+
+    return dither(png.binary_stdout(), **vars(ns))
+
+
+if __name__ == "__main__":
+    main()