potential +dither bug in IM 6.6.2-4 Q16

[fmw42]

IM 6.6.2-4 Q16 Mac OSX Tiger

I have written a new script that appears to be having problems disabling dithering. Rather than repeat the script, I have simplified an example to show this.

If I start with JPG or PNG with lots of colors and end with a gif, then I get dithering even with +dither in command line. This does not happen if I start with 256 color or less GIF.

Here are the test input images:

JPG

PNG
http://www.fmwconcepts.com/misc_tests/d ... radise.png
GIF
http://www.fmwconcepts.com/misc_tests/d ... radise.gif

Processing:

JPG:
convert birdofparadise.jpg -sample 5% -scale 2000% +dither birdofparadise_pixelize_jpg.gif


Notice above that several squares on the left side show a speckling that I presume is due to dither. Other squares show similar speckling if the image is made brighter


PNG:
convert birdofparadise.png -sample 5% -scale 2000% +dither birdofparadise_pixelize_png.gif
http://www.fmwconcepts.com/misc_tests/d ... ze_jpg.gif

GIF:
convert birdofparadise.gif -sample 5% -scale 2000% +dither birdofparadise_pixelize_gif.gif
http://www.fmwconcepts.com/misc_tests/d ... ze_gif.gif

[magick]

Look for a patch to the problem you reported in ImageMagick 6.6.2-5 Beta in about an hour. Thanks.

[fmw42]

Thanks. The beta seems to have fixed the bug. My test case has no speckle and my script does not produce any speckle either.

Fred