1994.09.16 17:58 "Re: Question About Separated TIFF Images", by Sam Leffler
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:
- "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.
- "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.
- "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).