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From: ian@media.UUCP (Ian Poynter)
Newsgroups: comp.graphics,comp.sys.ibm.pc
Subject: GIF file format
Keywords: GIF
Message-ID: <1125@media.UUCP>
Date: 3 Feb 89 20:59:24 GMT
Reply-To: ian@media (Ian Poynter)
Followup-To: comp.graphics
Distribution: na
Organization: Media Cybernetics, Inc.
Lines: 47

I have been asked by someone in our company who does not have access to news 
to post this question.  Please direct all responses either to me by e-mail
or to comp.graphics, since I don't read comp.sys.ibm.pc.

Thanks.

The following is an excerpt from the GIF image format specification:

 SCREEN DESCRIPTOR

        The Screen Descriptor describes the overall parameters for all  GIF
   images  following.  It defines the overall dimensions of the image space
   or logical screen required, the existence of color mapping  information,
   background  screen color, and color depth information.  This information
   is stored in a series of 8-bit bytes as described below.

              bits
         7 6 5 4 3 2 1 0  Byte #
        +---------------+
        |               |  1
        +-Screen Width -+      Raster width in pixels (LSB first)
        |               |  2
        +---------------+
        |               |  3
        +-Screen Height-+      Raster height in pixels (LSB first)
        |               |  4
        +-+-----+-+-----+      M = 1, Global color map follows Descriptor
        |M|  cr |0|pixel|  5   cr+1 = # bits of color resolution
        +-+-----+-+-----+      pixel+1 = # bits/pixel in image
        |   background  |  6   background=Color index of screen background
        +---------------+          (color is defined from the Global color
        |0 0 0 0 0 0 0 0|  7        map or default map if none specified)
        +---------------+

Unfortunately, the document does not specify anywhere what is meant by
"Color Resolution."  It does not appear to mean what comes to mind, i. e.
color resolution of 0 implying 2 ** 1 colors, 1 meaning 2 ** 2 ...
This poses no problem in reading a GIF file, as all necessary information is
provided elsewhere but it remains unclear what value to enter into this
field when writing out an image.

I would appreciate any clarification I can get.

-- 
Ian (I answer *all* mail) Poynter		Phone: +1 (301) 495-3305
UUCP: ..!{mimsy,sundc}!{prometheus,hqda-ai}!media!ian
Internet: (new) ian%media@pentagon-ai.army.mil (but too new to work?)
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