Sam Hocevar on wrote...
| On Sat, Aug 09, 2008, Anthony Thyssen wrote:
|
| > 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.
|
That is okay. Any change in the quantization or dithering causes me
this problem, I have been trying to develop a 'test' that will compare
two GIF images and inform me then they differ, but not due to dithering
effects. Not a lot of luck with this as yet, as it is a very fine line.
But this is becides that point. Which was that BOTH dithers should be
made available.
Actually as "-dither" is a extremely rare option, it could probably be
directly given an 'what dither' argument without upsetting too many people.
| > 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.
|
Was that test with BOTH a odd and even number of pixels wide image?
| 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);
|
It looks like Crisy has added this to your replacement...
ResetMagickMemory(scanlines,0,2*sizeof(*scanlines));
No indication of any move to give an argument to the rarely use option,
"-dither" so that BOTH forms of dither can be used.
| 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.
|
Ues I reported this before.
It is caused by Gray50 being defined as #7F7F7F
which expands to a non 50% 16 bit vaue of #7F7F7F7F7F7F
Instead of a true 50% gray of #7FFF7FFF7FFF
You may like to check your test code...
Hmmm... If you use the color gray(50%) which is a recently added
color function. you get... #800080008000
convert xc:'gray(50%)' -depth 16 txt:
# ImageMagick pixel enumeration: 1,1,65535,rgb
0,0: (32768,32768,32768) #800080008000 grey50
UPDATE: Quick test show gray50 is also defined as "rgb(50%,50%,50%)"
the equivelent to "gray(50%)", so your tests may work better now.
Hmmmm testing...
convert -size 100x100 xc:'#7FFF7FFF7FFF' \
-size 1x2 -map gradient: -scale 500% x:
and
convert -size 100x100 xc:'#800080008000' \
-size 1x2 -map gradient: -scale 500% x:
produces a tiling of 2 pixel high columns, rather than a checkerboard.
while
convert -size 101x101 xc:'#7FFF7FFF7FFF' \
-size 1x2 -map gradient: -scale 500% x:
and
convert -size 101x101 xc:'#800080008000' \
-size 1x2 -map gradient: -scale 500% x:
Produces a mix of 2 pixel columns with checker board rows interspersed.
All of them naturally had some inital distrubence for the first couple
of rows.
| > 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.
|
The image "alpha_gradient.png" is being seperated into -separate
channel images. The alpha channel image (a linear gradient) is then
being -monochrome dithered, be fore the channel images are being
-combine back into a original image.
That is IsAssociatedAlpha() IS always false.
The dither aspect should be equivelent to...
convert -size 100x100 gradient: -monochrome alpha_channel.png
And that works fine. But when it is part of the larger image processing
sequence, it stuffs up...
This should be the equivelent to the last command...
convert alpha_gradient.png -channel RGBA -separate \
\( -size 100x100 gradient: -negate -monochrome \) +swap +delete \
-combine alpha_dither_monochrome.gif
And that works correctly. Definatally some interaction when dithering
a 'channel' image is causing the dither to 'just stop' halfway through
without any indication of an error.
However i am also getting "Floating point exception" when I run "display"
of the resulting GIF images, or if I add "-montior", so something may
be wrong elsewhere.
Anthony Thyssen ( System Programmer ) <[EMAIL PROTECTED]>
-----------------------------------------------------------------------------
The nice thing about machines, Roger thought, was that they
couldn't be impressed by rank. They took you as you were.
--- Anne McCaffrey, "Damia's Children"
-----------------------------------------------------------------------------
Anthony's Home is his Castle http://www.cit.gu.edu.au/~anthony/
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