Path: utzoo!utgpu!watmath!iuvax!purdue!mentor.cc.purdue.edu!ahg
From: ahg@mentor.cc.purdue.edu (Allen Braunsdorf)
Newsgroups: comp.graphics
Subject: Re: Gif to IFF
Summary: TIFF aspect ratio tags present, but are they used?
Keywords: aspect ratio information required for maximum portability
Message-ID: <4632@mentor.cc.purdue.edu>
Date: 23 Oct 89 04:50:08 GMT
References: <1198@clinet.FI> <390039@hpfcdq.HP.COM> <4608@mentor.cc.purdue.edu> <1989Oct21.235312.416@tessera.uucp>
Reply-To: ahg@mentor.cc.purdue.edu (Allen Braunsdorf)
Organization: Photosurrealism
Lines: 51

In article <1989Oct21.235312.416@tessera.uucp> jtc@tessera.UUCP (J.T. Conklin) writes:
>In article <4608@mentor.cc.purdue.edu> ahg@mentor.cc.purdue.edu (Allen Braunsdorf) writes:
>>TIFF	A very flexible image storage format.  Files contain a small header
>>	and then a directory of tags.  These tags describe attributes
>>	of the image.  By adding tags, the format can be extended.
>>
>>	TIFF is used primarily for scanners and image processing programs
>>	on large microcomputers ("workstations", if you insist) and some
>>	personal computers.  With the addition (and strict adherence to)
>>	an aspect ratio tag, this format could be used most anywhere.  It
>>	just takes a lot of code to do right.
>
>TIFF doesn't have an aspect ratio tag because it doesn't need one.
>It stores XResolution, YResolution, and ResolutionUnit as separate
>tags.
>
>The aspect ratio is computed by dividing the XResolution by YResolution.

I ran off to grab my TIFF 5.0 spec to check the phrasing on this.  It goes
like this:

	X Resolution

	The number of pixels per Resolution Unit in the X direction,
	i.e., in the Image Width direction.  It is, of course, not
	mandatory that the image be actually printed at the size
	implied by this parameter.  It is up-to the application to use
	this information as it wishes.

					[ TIFF 5.0 8/8/88 Page 16 ]

OK, I'll buy that the tags are there.  I'm sorry that I missed them somehow.

I'd like this entry much better, however, if the last sentence assured me that
my aspect ratio would be preserved.  If any TIFF reader ever distorts one of
my images without warning me ("Sorry, I'm too dumb to do a simple aspect ratio
correction."), I'll blame the author instead of the format, even though the
loose wording of the specification gives the idiot some room to move.

A tag like this is fundamental to image fidelity.  Rules should be well
defined and strictly followed.  To me, this is as important as the Image Width
or Length.

Now I'm curious if the TIFF reading applications I use actually follow these
parameters.  I'll check the NeXT later this week.  Thanks again for pointing
this out to me.  I know I've seen it (I've read the whole nauseating
specification), but I forgot it somehow.

---
Allen Braunsdorf			 Purdue University Computing Center
staff.cc.purdue.edu!ahg		  UNIX Group Part Time, Consulting the Rest