Xref: utzoo comp.graphics:12425 alt.graphics.pixutils:160
Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!usc!apple!well!jef
From: jef@well.sf.ca.us (Jef Poskanzer)
Newsgroups: comp.graphics,alt.graphics.pixutils
Subject: Re: solution: pixel aspect ratio in GIF images
Message-ID: <19081@well.sf.ca.us>
Date: 18 Jul 90 01:05:34 GMT
References: <10454@odin.corp.sgi.com>
Reply-To: Jef Poskanzer <jef@well.sf.ca.us>
Organization: Paratheo-Anametamystikhood Of Eris Esoteric, Ada Lovelace Cabal
Lines: 107

In the referenced message, jindak@surfside.esd.sgi.com (Chris Schoeneman) wrote:
}GIF allows for "extension blocks" which all readers must accept.  So I
}propose the following extension:
}
}   7 6 5 4 3 2 1 0   Byte #
}  +---------------+
}  |0 0 1 0 0 0 0 1|    1        '!' - GIF extension block introducer
}  +---------------+
}  |0 1 0 1 0 0 1 0|    2        'R' - For 'aspect Ratio'
}  +---------------+
}  |0 0 0 0 0 0 1 0|    3         2  - Two bytes in block
}  +---------------+
}  |  pixel width  |    4            - First part of ratio (numerator)
}  +---------------+
}  | pixel height  |    5            - Second part of ratio (denominator)
}  +---------------+
}  |0 0 0 0 0 0 0 0|    6         0  - extension block end code
}  +---------------+
}
}Let byte four equal 'x' and byte five equal 'y'  Then x:y is the _pixel_
}aspect ratio.

I like this.  I have appended a quick patch for giftoppm.c that should
(a) make it read and ignore extension blocks, and (b) make it handle
this new aspect ratio extension block by putting out a message advising
the user to do a ppmscale.  This is the same way that ilbmtoppm handles
non-square pixels.

I haven't tested this patch beyond compiling it, but it's pretty simple.
A very similar change should work for any other GIF readers derived from
Patrick Naughton's gif2ras.

If anyone modifies a GIF writer to generate these extension blocks,
drop me a line so we can check interoperability.
---
Jef

  Jef Poskanzer  jef@well.sf.ca.us  {ucbvax, apple, hplabs}!well!jef
  "I'll tell you right out, I'm a man who likes talking to a man who
         likes to talk." -- Kaspar Gutman [The Maltese Falcon]

*** giftoppm.c.old	Tue Jul 17 15:50:09 1990
--- giftoppm.c	Tue Jul 17 16:10:11 1990
***************
*** 65,70 ****
--- 65,72 ----
  #define False (0)
  
  #define NEXTBYTE (*ptr++)
+ #define EXTENSION 0x21
+ #define EXTENSION_ASPECTRATIO 0x52
  #define IMAGESEP 0x2c
  #define INTERLACEMASK 0x40
  #define COLORMAPMASK 0x80
***************
*** 245,253 ****
  	Red[1] = Green[1] = Blue[1] = 255;
      }
    
  /* Check for image seperator */
  
!     if (NEXTBYTE != IMAGESEP)
  	pm_error( "%s is a corrupt GIF file (nosep)", inf, 0,0,0,0 );
  
  /* Now read in values from the image descriptor */
--- 247,287 ----
  	Red[1] = Green[1] = Blue[1] = 255;
      }
    
+ /* Check for extension blocks */
+ 
+     for (;;) {
+ 	ch = NEXTBYTE;
+ 	if ( ch != EXTENSION )
+ 	    break;
+ 	ch = NEXTBYTE;
+ 	if (ch == EXTENSION_ASPECTRATIO) {
+ 	    int pwid, phei;
+ 	    if (NEXTBYTE != 2)
+ 		pm_error( "%s is a corrupt GIF file (bad aspectratio 1)", inf, 0,0,0,0 );
+ 	    pwid = NEXTBYTE;
+ 	    phei = NEXTBYTE;
+ 	    if (NEXTBYTE != 0)
+ 		pm_error( "%s is a corrupt GIF file (bad aspectratio 2)", inf, 0,0,0,0 );
+ 	    if (pwid != phei)
+ 		pm_message(
+ 		    "(Warning: non-square pixels; to fix do a 'ppmscale -%cscale %g'.)\n",
+ 		    pwid > phei ? 'x' : 'y',
+ 		    pwid > phei ? (float) pwid / phei : (float) phei / pwid, 0,0,0 );
+ 	} else {
+ 	    /* Unknown type of extension block - skip past it */
+ 	    for (;;) {
+ 		ch = NEXTBYTE;
+ 		if (ch == 0)
+ 		    break;
+ 		while (--ch >= 0)
+ 		    (void) NEXTBYTE;
+ 	    }
+ 	}
+     }
+ 
  /* Check for image seperator */
  
!     if (ch != IMAGESEP)
  	pm_error( "%s is a corrupt GIF file (nosep)", inf, 0,0,0,0 );
  
  /* Now read in values from the image descriptor */