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1994.09.16 17:58 "Re: Question About Separated TIFF Images", by Sam Leffler
To: tiff@sgi.sgi.com
Subject: Question About Separated TIFF Images
Date: Fri, 16 Sep 1994 02:17:07 CDT
From: John M Davison <davisonj@ecn.purdue.edu>
Page 69 of the TIFF 6.0 Specification says that "In a separated image,
each pixel consists of N components. Each component represents the amount of a
particular ink that is to be used to represent the image at that location..."
Does this mean that there can be no extra samples in a separated image?
CMYK support can be logically be combined with the ExtraSamples tag, but
it's not clear that CMYK TIFF readers will also grok the ExtraSamples
tag (ExtraSamples was added quite late and my guess is that folks like
Crossfield may not have any support for it). The wording you refer to
discusses pixel "color components"; do not lump any extra samples in also.
Once again, this is a confusion caused by the spec being a loose amalgamation
of independently written sections that have been thrown together with little
regard for the consistency of the document as a whole.
Further down on the page, the Specification says "BitsPerSample =
8,8,8,8 (for CMYK). SHORT. For now, only 8-bit components are recommended.
The value "8" is repeated SamplesPerPixel times." This raises the following
questions:
1. "BitsPerSample = 8,8,8,8 (for CMYK)". Does this mean
that CMYK images cannot have extra samples? What about
non-CMYK separated images?
A good TIFF 6.0-compliant reader will ignore extra samples. This however
means little when you've got a valid TIFF and the reader you're working
with barfs on the file. My recommendation is that if you're shipping a
file off to a printing house then don't send them extraneous data (such as
might be included with extra samples); only send them the data they really
need to print your images.
2. "For now, only 8-bit components are recommended." Does
this mean that extra samples must be confined to 8 bits
too?
It means nothing with regard to extra samples.
3. "For now, only 8-bit components are recommended." Are
there any cases in which 8-bit components are
recommended but not required? Must extra samples
(assuming one can have them) be confined to 8 bits too?
Forget about extra samples. Think about interoperability. If you're trying
to send a TIFF image to a company, ask them for detailed requirements.
This is a basic issue with TIFF and the inability of vendors to write proper
TIFF readers+writers. Since there is no mechanism for testing conformance
you must obtain specific descriptions of the set of TIFF tags that a reader
requires and/or expects. The 6.0 specification improves this situation
but really does nothing to solve the basic problem because it is far too
wishy washy in areas (something "may" be done but it never "must" be done).
Sam