Message-Id: <mailto:199503202152.PAA16622@library.wustl.edu> Date: Mon, 20 Mar 1995 15:06:51 +0400 From: Terry Lund <mailto:lund@PCD.KODAK.COM> Subject: Re: 24-bit color - quality?! To: Multiple recipients of list IMAGELIB
At 11:18 AM 3/20/95, Susan Grupe x???? wrote: >Now that our WWW server is up for the public to see, we've come up
>with a frustrating situation. I scan in images (mostly color and
>black and white photographs) in "millions of colors" (24-bit) using
>Ulead Systems' ImagePals 2 and an HP ScanJet IIcx. Then, using
>ImagePals, I convert the images to 256 (8-bit) color, and save them
>in .GIF format. They look great on our monitor (which is set up in
>"millions of colors" 24-bit) in either format (24-bit or 8-bit) when
>viewed individually. When others view them on monitors set for 256
>colors, however, the images turn grainy, and areas which were solid
>color fills become blotchy.
>
>Could someone explain why this happens? Would converting to JPEG, or
>another file format, help solve this problem, or will we have to
>rescan images in a different format?
>
It's probably not the scanning itself, but rather the way the WWW browsers render the 8-bit color images. You are better off limiting the colors below the full 256 colors because some platforms reserve some colors from the color pallette. NCSA mosaic on Unix (x-windows) actually limits images to 50 colors, and the browser has an algorithm for choosing the 50 colors. I believe that the software actually places a limit of 50 colors total on a page (that can contain multiple images). I don't actually know the details of how the other browsers handle color, but we ran into this 50 color limit with NCSA mosaic when we were designing the graphic images for Kodak's web server (http://www.kodak.com/) and we ended up designing the graphic images to use no more than 50 colors, to get acceptable quality across platforms (assuming at least an 8-bit display capability).The grainy and blotchy artifacts are a result of the system color pallette making color choices that are not compatible with the colors in the saved images.
Converting to JPEG will only help if the full display path properly supports full 24 bit color imaging.
Terry Lund Phone: 716-726-0533 Digital and Applied Imaging FAX: 716-726-0500 Eastman Kodak Company 901 Elmgrove Road KNET/KMX: 236-0533 Rochester, NY 14653-5218 Internal Mail: 3/1//EP/35218