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July 2005
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2005.07.25 12:27 "Re: XResolution/YResolution Tags", by Joris Van Damme
Gérald, > My question is quite simple : are XResolution, YResolution and ResolutionUnit > tags manfatory in a TIFF image ? In my opinion, the spec is often interpreted incorrectly. I think it says extremely little about what is required or mandatory. Instead, it does have a big section on what is required in *baseline* TIFF. The rationale being that TIFF is a very flexible format, and pretty much anything that makes sense is allowed. But only a limited subset is to be called 'baseline' and guaranteed to be suitable for mainstream interchange. In this case too, I don't think there's any section of the spec that says resolution tags are mandatory, but they do seem to be specified as mandatory for baseline TIFF. In the old days, that talk about 'baseline' made a lot of sense... but today, you should realize most readers are reading a lot more then only baseline. Flate compression, new-style JPEG compression, 16bits per channel grayscale or RGB, etc, all of these are not baseline, but most of these are necessities and very well supported by most readers. So the distinction between 'baseline' and not-baseline is somewhat less useful then it used to be. Perhaps, today, it might be more useful to replace the notion 'baseline' with 'widely supported'. > It seems to be masked as "required" in the specification and no default values > are indicated for the XResolution/YResolution tags. I've seen plenty of TIFFs without resolution tags. I've not seen any mainstream reader that gives up on these images because of the lack of resolution tags. If 'widely supported' is indeed a useful notion, I would say that images without resolution tags are widely supported. They certainly make sense, too. If you need defaults, I suggest XResolution = 1 YResolution = 1 ResolutionUnit = 1 (No absolute unit of measurement) This means that all that is known (or assumed) about the image is that pixels are square as opposed to rectangular. > Where is the truth ? ... It's out there, for sure. ;-) Joris Van Damme info@awaresystems.be http://www.awaresystems.be/ Download your free TIFF tag viewer for windows here: http://www.awaresystems.be/imaging/tiff/astifftagviewer.html