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Aandi, Ranges aside, how would you explain that (0xA0, 0, 0) shows up as in cyan color, whearas (0xA0, 0x80, 0x80) is gray? About the Decode array: where would I get that? TIFFs I have been working are produced by Photoshop and don't seem to have that information embedded. Peter Persits Persits Software, Inc. http://www.persits.com ----- Original Message ----- From: "Aandi Inston" <[EMAIL PROTECTED]> To: <[EMAIL PROTECTED]> Sent: Monday, March 29, 2004 4:02 AM Subject: RE: [PDFdev] L*a*b Color Space problem -- an empirical solution found > > PDFdev is a service provided by PDFzone.com | http://www.pdfzone.com > _____________________________________________________________ > > > I wanted to share with everyone a solution to an interesting > > Lab color space problem that I discovered empirically, but cannot > > explain why it works. > > It doesn't work! (Except in this one case, by good luck). > > > The trio (0xA0, 0, 0) should produce a gray color > > I think so. > > > (Start of code) > 4 0 obj > <</Type /XObject > /Subtype /Image > /Width 2 > /Height 2 > /ColorSpace [/Lab <</WhitePoint [1 1 1] > /Range [-128 127 -128 127] > >>] > /BitsPerComponent 8 > >> > stream > 0xA0 0 0 > 0xA0 0 0 > 0xA0 0 0 > 0xA0 0 0 > endstream > endobj > > > Through trial and error, I discovered that the *a and *b values > > have to be XOR'ed with 0x80, and then it works fine. > > No, you are missing the vital step of using the Decode array. What, > you say, there isn't a decode array. Well, actually there always is: > the image samples (which may be 1,2,4,8 or now 16 bits) have to be > converted into the target range using the Decode array in every case, > and the Decode array has a different default for each Color Space. > > Just as RGB values, which might be in the range 0 to 255, have to > be decoded into the valid range 0.0 to 1.0, so Lab values have to > be decoded - you are lulled into a false sense of security because > the values seem to be in range. > > Aandi > > > To change your subscription: > http://www.pdfzone.com/discussions/lists-pdfdev.html > > To change your subscription: http://www.pdfzone.com/discussions/lists-pdfdev.html
