Apple-2-SW
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ii-pix
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With recent optimizations converter became ~2.1x faster, so 

--lookahead=8 is a reasonable new default.
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kris 2021-03-15 17:55:21 +00:00
parent 5487b4aa7e
commit 7a7923503f
2 changed files with 3 additions and 3 deletions
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2
README.md
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@ -162,7 +162,7 @@ Rendering the same .dhr image with 4-pixel colour shows the reason for the diffe
In the particular case of DHGR this algorithm runs into difficulties, because each pixel only has two possible colour choices (from a total of 16+). If we only consider the two possibilities for the immediate next pixel then neither may be a particularly good match. However it may be more beneficial to make a suboptimal choice now (deliberately introduce more error), if it allows us access to a better colour for a subsequent pixel. "Classical" dithering algorithms do not account for these palette constraints, and produce suboptimal image quality for DHGR conversions.
We can deal with this by looking ahead N pixels (6 by default) for each image position (x,y), and computing the effect of choosing all 2^N combinations of these N-pixel states on the dithered source image.
We can deal with this by looking ahead N pixels (8 by default) for each image position (x,y), and computing the effect of choosing all 2^N combinations of these N-pixel states on the dithered source image.
Specifically, for a fixed choice of one of these N pixel sequences, we tentatively perform the error diffusion as normal on a copy of the image, and compute the total mean squared distance from the (fixed) N-pixel sequence to the error-diffused source image. For the perceptual colour distance metric we use [CIE2000 delta-E](https://en.wikipedia.org/wiki/Color_difference#CIEDE2000).

4
convert.py
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@ -25,9 +25,9 @@ def main():
parser.add_argument("output", type=str, help="Output file for converted "
"Apple II image.")
parser.add_argument(
"--lookahead", type=int, default=6,
"--lookahead", type=int, default=8,
help=("How many pixels to look ahead to compensate for NTSC colour "
"artifacts (default: 6)"))
"artifacts (default: 8)"))
parser.add_argument(
'--dither', type=str, choices=list(dither_pattern.PATTERNS.keys()),
default=dither_pattern.DEFAULT_PATTERN,