Path: utzoo!utgpu!watmath!att!tut.cis.ohio-state.edu!cs.utexas.edu!sun-barr!newstop!sun!vector!poynton From: poynton@vector.Sun.COM (Charles A. Poynton) Newsgroups: comp.graphics Subject: Re: Color desktop scanners Summary: Colour differences cannot be extracted using a real optical filter. Message-ID: <119920@sun.Eng.Sun.COM> Date: 8 Aug 89 04:00:48 GMT References: <1869@ucsd.EDU> <421@celit.UUCP> Sender: news@sun.Eng.Sun.COM Reply-To: poynton@sun.com (Charles A. Poynton) Organization: Sun Microsystems, Mountain View Lines: 33 Jim Hutchison says: > How about using a different color scheme, such as YIQ. N passes, based > on the response curves of your lights (3 in the case of YIQ). I and Q, and the related pairs (U and V) and (B-Y and R-Y), are signals that are ubiquitous in colour television, because the bandwidth of these signals can be dramatically reduced compared to that of luminance without visual effect. These signals are all colour-difference signals, which means that they are formed by the subtraction of luminance from something (for example, blue minus luminance). Each is bipolar, that is, goes negative for some substantial portion of the spectrum. No optical filter can be used to extract such a signal from a scene, because all optical filters are based on absorbing, reflecting, or transmitting photons. For practical purposes, there are only positive contributions from photons, and negative lobes are out. You can synthesize negative lobes electronically, but you can't build an optical filter to do it. Unless you know really a lot about scanners, stick to RGB of some sort. C. ----- Charles A. Poynton Sun Microsystems Inc. 2550 Garcia Avenue, MS 8-04 415-336-7846 Mountain View, CA 94043 "Japan has no laws against damage to its flag, but it has strict laws forbidding the burning of foreign flags lest this give offense to the country in question." -- The Economist, July 1, 1989, p. 19. -----