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From: marcos@netcom.COM (Marcos H. Woehrmann)
Newsgroups: comp.graphics,comp.compression
Subject: Re: gif encoding questions
Message-ID: <1991Apr10.034324.28971@netcom.COM>
Date: 10 Apr 91 03:43:24 GMT
References: <1991Apr9.142255.186@unhd.unh.edu>
Followup-To: comp.compression
Organization: Netcom - Online Communication Services  UNIX System {408 241-9760 guest}
Lines: 55

In article <1991Apr9.142255.186@unhd.unh.edu>, rg@msel.unh.edu (Roger Gonzalez) writes:
> 
> When encoding in GIF format, do you increase the bitsize when you
> _output_ code c, or when you _add_ an entry to the c-th entry in the 
> string table?
> 
> What about when c = 4095?  Do you output the clear code and reinitialize
> after 4095 is output, or after the 4095th entry in the table is used?

  if (hashLocation>maxHashTable)
    panic("Internal error while writing Gif (1)\n");
  if (lastUsed==maxUsable) {
    if (codeSize==maxBits) { /* can't increase bit size, clear table! */
      resetTable(clearCode,out);
      return;
    } else {
      codeSize++;
      maxUsable=1 << codeSize;
    }
  }

However, according to the GIF89A spec (availble from CompuServe and from
Simtel): "There is not a requirement to send a clear code when the string
table is full".  (And they go on to say you could actually clear the table
before it is full, though how you decide the optimal time to do that
is left as an exercise to the interested reader :-) ).

As an aside does anyone know if any of the GIF writers make use of this yet?
The document I have points out that most (all?) gif readers will break
unless you only issue a clear code when the table is full, and suggests that
gif writers don't implement a early or late clear code until after Jan. 1991,
by which time all the gif readers will have been fixed. 


> 
> Why have I seen gray scale (256) GIFs that don't have simple color maps like
> 00 00 00 01 01 01 02 02 02 03 03 03 ... FE FE FE FF FF FF
> 
> 

The GIF spec suggests that you sort the Global Color Table (aka palette)
by order of importance (which most people of interpreted to mean in order
of frequency).  "This assists a decoder, with fewer available colors, in
choosing the best subset of colors..."

marcos

-- 
Marcos H. Woehrmann    {claris|apple}!netcom!marcos  |  marcos@netcom.COM

 Oh, I'm sure you've heard it all before, but remember it's not what you
 say, it's how you say it, and how much you're paid to do so, and besides
 who's listening anyway.  No one, that's who, because it's all been said
 and done and done and done and done to death.  Let's talk about art,
 said the fool to the idiot.  --Lydia Lunch