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2007.01.17 20:52 "Re: Elevation Data", by Joris Van Damme
Ed, Joris wrote: > > > and, conversely, any partial use of the > > > range specified in SMin/SMax is to be taken as partial use of the > > > colorspace or range, and does not imply histogram stretching or > > > equalising is required. > > > > I'm not sure that I understand this "partial use" and what you are > > saying it does or does not imply. I just need more explanation. > > > > What bothers me is that I think I *do* want to stretch or equalize > > this data to fit the 0..255 range of my monitor when I display it. > > So how do I communicate to you a photograph of a very bright seen, > with no actual black in it? Thinking on this further, I realize we might have misunderstood each other. Let me rephrase the proposed reader recommendation to be more clear. ** ...and, conversely, any partial use of the range specified in SMin/SMax is to be taken as partial use of the colorspace or range, and does not imply histogram stretching or equalising is required based on that detected partial usage rather then the SMin/SMax range. ** Of course you may want to stretch SMin-SMax range to 0-255 range for display. If that's what you meant, that's OK with the proposed recommendation. What I meant in the recommendation is the following... Suppose you have SMin = -0.5, and SMax = 2.0. If you were examine the image data, you'd find out that the actual used range in the image data is -0.3 to 2.7. In that case, a normal rendering process would follow the writer recommendation, map SMin = -0.5 to 0 and SMax = 2.0 to 255 for display. This means a normal rendering process doesn't need to have actual usage (histogram) information before starting. It also means applications can encode ranges different from usage. For example, you can encode the bright scene without no actual black patches, and needn't fear a normal image rendering is going to turn your darkest gray into black because your SMax can be higher then actual maximum used value. You can encode the (grayscale) elevation map that the writer intends to be rendered by normal image viewers with sea all black and still he wants to encode sea depth levels in there, by using values for beneath sea level that are lower the specified SMin. Was I indeed not clear in my first attempt? 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