On Sat, Aug 09, 2008, Anthony Thyssen wrote:
> 1/ Your patch just completely replaces the Hilbert Curve.
>
> 2/ Cristy has applied your patch directly, and has now started the
> release of IM v6.4.2-7. So that will have FS dither, rather than
> a Hilbret Curve Dither.
>
> This has removed the old dither completely (I have saved a copy)
>
> It will also mean that every GIF image in IM Examples will on the
> next update flag as having changed!! Arrrggghhhh....
Sorry, I really didn't mean it to happen like that. I hope things can
still be sorted out.
> 3/ It is doing direct ratios of the error, with no 'remainer' to
> add to the last color being updated. As such its error distribution
> and rounding errors.
>
> See notes in probably one of the best (and oldest) guides..
> Dithering and Halftoning "DHALF.TXT"
>
> 4/ From that text...
> Floyd and Steinberg carefully chose this filter so that it
> would produce a checkerboard pattern in areas with intensity
> of 1/2.
> Your Serpentine implementation does NOT provide that effect, but
> produces columns of pixels.
I disagree here. Since the code is handling RealPixelPacket variables
there is no practical benefit in handling the rounding error. I once
simulated Floyd-Steinberg on a 50% gray 1000000x1000000 image and even
with floats instead of doubles or long doubles the result was a perfect
checkerboard.
However, there are indeed several problems right now that cause this
issue. First, my patch was modified and the code that was checked in is
now incorrect:
[...]
pixel.red+=previous[u-v].red-15*previous[u-v].red/16;
pixel.green+=previous[u-v].red-15*previous[u-v].green/16;
^^^ should be "green"
pixel.blue+=previous[u-v].red-15*previous[u-v].blue/16;
^^^ should be "blue"
if (IsAssociatedAlpha(image,cube_info) != MagickFalse)
pixel.opacity+=previous[u-v].red-15*previous[u-v].opacity/16;
^^^ should be "opacity"
[...]
Second, I mistakenly assumed that AcquireQuantumMemory would
zero the memory, which it apparently doesn't do. The following code
is therefore required after the AcquireQuantumMemory() call in
FloydSteinbergDither():
memset(scanlines, 0, 2*sizeof(*scanlines) * image->columns);
Third, and I am afraid you are never going to see a perfect
checkerboard until this is dealt with, the IM RealPixelPacket values for
a 50% gray pixel are 32639 instead of 32767. Not sure where to fix that.
> Hmmm it has a fault however...
>
> Compare the results for the IM examples alpha_dither_monochrome.gif
> (from IM Examples, Quantization and Dithering,
> Color Quantization and Transparency
> http://www.imagemagick.org/Usage/quantize/#color_trans
>
> convert xc:red xc:yellow xc:green1 xc:cyan xc:blue \
> +append -filter Cubic -resize 100x100\! -size 100x100 \
> gradient: +matte -compose CopyOpacity -composite alpha_gradient.png
>
> convert alpha_gradient.png -channel RGBA -separate \
> \( +clone -monochrome \) +swap +delete \
> -combine alpha_dither_monochrome.gif
>
> The dither starts, but then just seems to die about half way through!
> I have attempted to simplify the above example as yet.
>
> If you can't see the problem, let me know and I'll post you the images.
> and try to simplify the problem.
I can reproduce the problem, but I don't really know where to look.
It looks like a problem somewhere else: IsAssociatedAlpha() always
returns false here, so the opacity error is never computed.
Cheers,
--
Sam.
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