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- From: wjs@blorf.omnigroup.com (William Shipley)
- Subject: Explanation- Re: Lack of color brilliance in GIF pictures?
- Message-ID: <1992Dec18.084848.433@u.washington.edu>
- Sender: news@u.washington.edu (USENET News System)
- Organization: University of Washington
- References: <1992Dec16.154431.18394@bernina.ethz.ch>
- Date: Fri, 18 Dec 1992 08:48:48 GMT
- Lines: 54
-
- 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.
-
- Assuming that the Sun has 8-bit graphics that use a Color Look-Up Table (CLUT)
- to map to 24-bit color values, it's no surprise that GIF images look better on
- the Sun.
-
- Basically, the NeXTstation color has 12-bit color, where these twelve bits
- actually represent the Red, Green, and Blue components at each pixel (4 bits
- apiece). This is called true color. This gives your NSC the ability to
- simultaneously display 4096 different colors from a palette of 4096 possible
- colors.
-
- The Sun has 8-bit color, but those 8 bits are just used as an offset into a
- table of 256 colors, where each color in the table has 24 bits. This is called
- indexed color. Thus on the Sun you have the ability to simultaneously display
- 256 different colors from a palette of 16,777,216 possible.
-
- GIF files, as it happens, are stored the way the Sun displays images; ie each
- pixel is just an 8-bit index into a table of 256 24-bit colors.
-
- When you display a GIF on an NSC, the 24-bit colors in the file must be
- approximated with 4096 colors the NSC has available. So, obviously, when you
- have an image that can have a subset of 16 million distinct colors, and you try
- to display it on a monitor that can only show a subset of 4096 distinct colors,
- the image is going to look a little washed out.
-
- When you display the GIF on the Sun, the Sun just sets its Color Look-Up Table
- to be the 256 colors in the GIF file, and VOILA you're displaying the original
- 24-bit color values that are actually in the file.
-
- Well, great, you say, why does anyone bother with this true color junk when
- indexed color gives you so many more colors? Well, it doesn't, really.
- Indexed color gives you a nice large palette of colors, but you have to pick
- your 256 favorite of them.
-
- Thus, images without a lot of different colors in them look great in indexed
- formats (like GIF), since you can have a lot more shades of the same color.
- Images with a lot of different colors will probably look better in true color,
- since you have the ability to display more different colors, even if the
- palette you are choosing from is smaller.
-
- -William Shipley
- The Omni Group
- DBKit consulting, and now 3.0 image filtering
-
- 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.)
-