From: lmccullo@nyx.cs.du.edu (Michael McCulloch)
Subject: Re: Lack of color brilliance in GIF pictures?
Organization: University of Denver, Dept. of Math & Comp. Sci.
Date: Wed, 23 Dec 92 07:14:48 GMT

Scott Byer writes:
>Florian Gutzwiller writes
>
>> If I compare the color brillance of GIF prictures on a NeXT 16bit
>> system and a Sun 8 bit display the Sun is much more brillant. This
>> is not a monitor problem for sure. I'm using ImageViewer.
>
>There are entirely too many variables between the two.  Phosphor
>frequencies, electron gun emission curves, lighting, monitor type,
>driving circutry.  Unless both monitors are calibrated by the same
>equipment and corrected in software, they will never look the same.
>And who is to say which is more "correct"?
>
>Don't sweat it.
>

This is not _the_ reason the two look different. The dithering required of
an NS color for a paletted image surely is -- and I do sweat it.

The previous post by William Shipley <1992Dec18.084848.433@u.washington.edu>
was an excellent explanation of this. However, he wrote:

>PS: PostScript also gives you dithering, which allows you to simulate having  
>more colors by drawing two (or more) colors right beside each other and  
>trusting your eye to blur them together as if they were one pixel.  Thus, if  
>your GIFs look washed out, one solution is to magnify them and back away from  
>your monitor.  (Another would be to buy a 24-bit true color board, like the
>NeXTdimension, but the former seems cheaper.)

There's another option. Change the way the GIF is converted. I've noticed
this 'washed out' appearance of GIFs on my nice Sony monitor -- even with
images that look fine on other machines. My solution?

I use the gif2tiff converter in Sam Lefler's TIFF utilities library. It is
available on the sgi.com anonymous ftp site under graphics/tiff. But, before
you rush to get it, you must compile the package to use it. Although I
hesitate since this is not my software, I have considered posting just the
executable for gif2tiff on the NeXT -- since most people are probably only
interested in that conversion. This may make it to the Purdue ftp site by
the first of the year.

Sam applies what appears to be a gamma correction (I think -- I'm not an
expert in image processing) to the color palette of converted GIFs. The
result is a much more brilliant converted image in which the PS dithering is
_significantly_ subdued. I have been told that the same 'correction' degrades
image quality on an ND. This makes sense, since the ND doesn't have to dither
to display paletted images. I made a slight modification to Sam's gif2tiff
converter that allows the user to specify the value of the 'correction'
factor on the command line. Also, he uses the Class P type of TIFF which is
slightly different from the apparent preferred TIFF formats on the NeXT,
however, I've had no trouble opening converted files in Preview or Icon
Builder.

Please don't flood my mailbox with requests, I'm extremely busy with 'real'
projects at this time. If you can't wait, then get the full package from the
SGI sight. Some enterprising soul could possibly even modify one of the PD GIF
converters to do the same -- perhaps add a Preferences item.

Michael McCulloch
Huntsville, AL
Independent NeXT Developer
Newsgroups: comp.sys.next.misc