[BTW, the original postings were due to someone announcing some nude anime
backdrops which had some very bad thresholding problems, and so I, er,
used those as examples. They're long gone now it appears. The
instructions you posted are actually not the good ones -- later on in the
thread I corrected myself and added some new stuff. Here's the later
messages, with gif examples that actually still are there: ]
------------------------------
For everyone who uses GraphicConverter: if you don't set things up right
it will not do error-diffusion when converting to grayscale. It's an easy
mistake to make. Here's the steps you should do:
1. Scale your image *first*. This step should be _before_ you
convert to 16-bit grayscale.
2. Do any other preprocessing and cleanup in color.
_Don't_ do it in gray.
3. Turn on error-diffusion with Picture:Colors:Dither *important!*
4. Convert the image directly to 16-bit grayscale with
Picture:Colors:Grayscale:16 Grays
Ignore my previous directions, they also do error-diffusion but converting
straight to 16-grays does a better diffusion job. Also, if you do not
turn on Dither, then converting directly to 16 grays will not do
error-diffusion. If you convert first to 256 grays and then change to "16
colors" like I had said before, it does error-diffusion regardless of
whether or not Dither is on, but doesn't do quite as good a job.
------------------------------
For those of use who are saying "what the heck are you talking about,
Sean?", I've created three Newton-ready grayscale GIF images, each done
with a different GraphicConverter conversion process. The image is the
Apple G4 tower. View them in a web browser that provides at least 16-bit
color.
Good Error Diffusion:
http://scruffy.cs.umd.edu:8080/seanl/temp/G4good.gif
This one was done by turning on Picture:Colors:Dither, then selecting
Picture:Colors:Grayscale:16 Grays
Bad Error Diffusion:
http://scruffy.cs.umd.edu:8080/seanl/temp/G4bad.gif
This one was done by first selecting Picture:Colors:Grayscale:256 Grays,
then selecting Picture:Colors:Change to 16 Colors (4 Bit).
Thresholding (ick):
http://scruffy.cs.umd.edu:8080/seanl/temp/G4ugly.gif
This one was done by *not* turning on Picture:Colors:Dither, then
selecting Picture:Colors:Grayscale:16 Grays. This is also the effect you
achieve if you load the image through Newt's Cape or NetHopper on your
Newton, and save it to a book.
------------------------------
One last mention about thresholding vs. error diffusion. If you're
converting a pen-and-ink drawing or cartoon where there is no gradual
change from color to color, like a South Park picture say, then you may
want to use thresholding. But if you're converting nearly anything else,
especially photographs, oil or watercolor or airbrush art, etc., you
definitely want error diffusion.
------------------------------
Sean
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