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January 2007
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2007.01.17 08:00 "Re: Elevation Data", by Joris Van Damme
Craig, Craig Bruce wrote: > I would like to write signed 16-bit elevation data to a TIFF image > file, but according to the TIFF 6.0 specification, there doesn't appear to > be a correct way to do this. The specification allows the use of the > SampleFormat to specify that the samples are signed 16-bit integers, > but this range of values is incompatible with the > PhotometricInterpretation tag, which must be present. The obvious > PhotometricInterpretation to > use is BlackIsZero, but its range is defined as 0 to 2**nbits-1 (i.e., > unsigned integers). What is done, mostly, in color data, is adding an offset to the data. This is done in various encoding of CIE L*a*b*, in YCbCr, etc. In actual meaning, take your lowest level beneath the see to be 0. Encode positive offsets from that. Ideally, scale such that your highest mountain is 2**nbits-1 above that lowest beneath see level. The advantage of that is that readers that don't know your intent, are still able to render your (extra?) channel quite nicely. > Also, is there a semantic difference between SMinSampleValue and > MinSampleValue? The spec explicitly states that MinSampleValue is > just statistical and isn't part of a valid-sample-range definition, > but it's much more coy about SMinSampleValue. If it isn't part of a > hard range definition, then how can one make an interoperable image > with floating-point samples? It isn't part of a hard range definition. Floating point data is interoperable by having ranges defined as is logical to the colorspace. For example, the range of the L* channel, in CIE L*a*b*, encoded in floating point, is 0.0 to 100.0, as that is its intrinsic nature. The range of R, G, or B channels is 0.0 to 1.0. The range of floating point MinIsBlack or MinIsWhite channels is 0.0 to 1.0. Best regards, 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