ian@media.UUCP (Ian Poynter) (02/04/89)
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?)
(old) ian%media@hqda-ai.arpa (going away real soon now)john@cooper.cooper.EDU (John Barkaus) (03/28/89)
Hi,
I would like information on the format of .GIF files.
If there is any interest, I will post what I find out.
Thanks in advance.
John
John M. Barkaus at the Cooper Union, NY, NY.
INTERNET: john%cooper.cooper.edu@cmcl2.nyu.edu
UUCP: cmcl2!cooper!johndavidsen@steinmetz.ge.com (Wm. E. Davidsen Jr) (03/30/89)
In article <1479@cooper.cooper.EDU> john@cooper.cooper.EDU (John Barkaus) writes: | I would like information on the format of .GIF files. | If there is any interest, I will post what I find out. The GIF standard is in the GIF section of *IX BBS System sixhub (*IX BBS) Phone 518-346-8033 login bbs board mbs area GIF -- bill davidsen (wedu@crd.GE.COM) {uunet | philabs}!steinmetz!crdos1!davidsen "Stupidity, like virtue, is its own reward" -me
macs3440@rocky.oswego.edu (Craig Siegelson) (03/30/89)
Can anyone supply me with a list of systems connected to this
NETwork?
I know the list may be very extensive, but if anyone can mail me a copy
or post me a copy.
Thanx.....
Craig R. Siegelson
macs3440@rocky.Oswego.EDUporges@inmet.UUCP (03/31/89)
/* Written 8:42 pm Mar 27, 1989 by john@cooper.UUCP in comp.sys.ibm.pc */ >Hi, >I would like information on the format of .GIF files. >If there is any interest, I will post what I find out. My understanding is that GIF is proprietary information of Compuserve, and that they don't let just anyone know the format. In order to know it, you have to become a "GIF developer" and sign a non-disclosure agreement. -- Don Porges porges@inmet.com {...mirror,ima}!inmet!porges
sekoppenhoef@rose.waterloo.edu (04/01/89)
In article <149000030@inmet> porges@inmet.UUCP writes:
:>I would like information on the format of .GIF files.
:>If there is any interest, I will post what I find out.
:have to become a "GIF developer" and sign a non-disclosure agreement.
Not true! The GIF format specs can be ftp'd anonymously from
uscd.uscd.edu (128.54.16.1) in the /pub/graphics directory along with a bunch
of other interesting stuff... the TIFF format can be found there too!
______________________________________________________________________________
Shawn Koppenhoefer CS C&O, University of Waterloo, Canada N2L 3G1 _ _
sekoppenhoef@waterloo { .CSNET or .CDN or .EDU } <
Klein bottle for sale... enquire within -usenet@agate.BERKELEY.EDU (USENET Administrator) (04/02/89)
From: air@anableps.berkeley.edu (Arthur Ernest Wright) Path: anableps.berkeley.edu!air People's Technology | Participating in the war on apathy, ingnorance, and Arthur Ernest Wright| Stagnation, while simultaneusly ___ ___ ___ 1272 Willamette #404| working in the retail computer X@ @X (@ @) (X X) Eugene, Oregon 97401| equiptment nightmare. \o/ \X/ \o/ (503) 344-7969 | air@mica.berkeley.edu Deaf Dumb Blind
johnm@trsvax.UUCP (04/04/89)
>>Hi, >> >>I would like information on the format of .GIF files. >>If there is any interest, I will post what I find out. > > My understanding is that GIF is proprietary information of Compuserve, and >that they don't let just anyone know the format. In order to know it, you >have to become a "GIF developer" and sign a non-disclosure agreement. > > -- Don Porges > porges@inmet.com > {...mirror,ima}!inmet!porges Wrong. The only thing you get by becoming a GIF developer is access to another message area and data library in the PICS section on Compu$erve. This gives you access to source code for a variety of decoders/encoders and whatnot. The specification for the file format itself is freely distributable. John Munsch
pthiesse@jarthur.Claremont.EDU (Paul Thiessen) (04/06/89)
In article <149000030@inmet> you write: > > My understanding is that GIF is proprietary information of Compuserve, and >that they don't let just anyone know the format. In order to know it, you >have to become a "GIF developer" and sign a non-disclosure agreement. > > -- Don Porges > porges@inmet.com > {...mirror,ima}!inmet!porges Actually, getting information is free and easy! For example, on Simtel20, there is <msdos.gif>gifdoc.arc which has a big text document on the GIF format, and several example programs, source and executable, that deal with GIF pictures. I'd say it's better for CompuServe to have released their format, so other people can create GIF pictures. It wouldn't be much of a "Graphics Interchange Format" if no one else could use it! :-) - Paul -- ---------------------------------------------------------------------------- PAUL THIESSEN, Harvey Mudd College ...!uunet!jarthur!pthiesse pthiesse@jarthur.claremont.edu pthiessen@hmcvax.bitnet ----------------------------------------------------------------------------
cs__sjh@umt.UUCP (Jeffrey Heng) (05/02/90)
Does anyone out there have the specifications of GIF file formats? How many colors is a GIF file able to store? I've used a program to convert GIF files to TARGA-16 files and the converted TARGA-16 files are much , much bigger than GIF files. A friend suggested looking into CompuServe for the GIF file format, but I do not have access to CompuServe. If anyone could post the GIF specifications, it would be greatly appreciated. Thanks.
rosen@polar.bu.edu (David B. Rosen) (05/04/90)
I don't have the GIF Format, but I know why it's smaller: it's
compressed! As I understand it, the GIF file format has LZW (Lempel
Ziv Welch) compression (like unix compress) built in. If you use a
standard compression utility on your TARGA-16 files, they may end up
about the same size as the GIF file (unless TARGA-16 has an internal
structure that would make it relatively incompressible on a
byte-string basis). LZ algorithms are somewhat subtle, but they rely
on the encoder and the decoder each building up the same dictionary of
frequently-occuring strings of arbitrary length, where each string is
represented as a fixed-size codeword (typically 16 bits or less) in
the compressed data stream.
--
David B Rosen, Cognitive & Neural Systems rosen@bucasb.bu.edu
Center for Adaptive Systems Bitnet: rosen%thalamus@buacca
Boston University UUCP: {harvard,uunet}!bu.edu!bucasb!rosenraveling@isi.edu (Paul Raveling) (05/08/90)
In article <ROSEN.90May3192224@polar.bu.edu>, rosen@polar.bu.edu (David B. Rosen) writes: > I don't have the GIF Format, but I know why it's smaller: it's > compressed! As I understand it, the GIF file format has LZW (Lempel > Ziv Welch) compression (like unix compress) built in. If you use a > standard compression utility on your TARGA-16 files, they may end up > about the same size as the GIF file... That's true. When the Img Software Set writes an image file by default it pipes it through compress; the resulting file is about the same size as a corresponding GIF file. Most are very slightly smaller, probably because the entire file (header included) is compressed. I believe Jef Poskanzer has reported similar results for the PBM software. I like the idea of compressing the entire file rather than having compression/decompression logic embedded in software that reads or writes the files for 2 reasons: 1. When the entire file is compressed, it's possible to use existing tools to manipulate it (for example, "zcat file | favorite_dump_utility), rather than having to write specialized tools. 2. Software to read and write the files is simpler. The ideal would probably be to have the OS disk i/o functions automatically do compression/decompression. Best would be to add a hardware assist -- it shouldn't take much silicon to do this EXTREMELY fast and to make that process invisible to even OS software. ---------------- Paul Raveling Raveling@isi.edu