Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Path: utzoo!utgpu!water!watnot!watmath!clyde!rutgers!seismo!mcvax!ukc!warwick!rolf From: rolf@warwick.UUCP Newsgroups: comp.graphics Subject: Colour vision Message-ID: <505@ubu.warwick.UUCP> Date: Tue, 10-Mar-87 08:10:36 EST Article-I.D.: ubu.505 Posted: Tue Mar 10 08:10:36 1987 Date-Received: Thu, 12-Mar-87 23:12:08 EST Reply-To: rolf@ubu.UUCP (Rolf Howarth) Organization: Computer Science, Warwick University, UK Lines: 35 With the recent discussion about how to map colour images to black-and-white, I thought I'd ask if anyone could explain to me how the human brain perceives colour. One thing that always puzzled me at school was being told, for example, that red + green light "gives you yellow", where "yellow" is also what you see at a particular position in the spectrum (when you shine white light through a prism). As far as I understand it, these two "yellows" are different spectroscopically , yet the eye perceives them to be the same colour. I think I was confused because the distinction between what a colour *is* and how it *appears* to the eye was never made very clear. Obviously the effect of being able to combine three primary colours to give a complete (?) spectrum is vital for things like colour television (reducing the information needed to describe a colour from potentially infinite to about 3x8 bits). Few people seem to be willing to admit that this is only an illusion though, and is highly dependent on the properties of the human eye and brain. Is it not conceivable that someone with a certain type of colour defect might recognise a yellow banana in sunlight, but when shown a picture of one on TV say "I can only see superimposed red and green images" ? Certainly machines aren't fooled the way we are, for example, one can't do a spectral analysis of the television image of a sodium street light and expect to get anything like the original. Sorry if I have rambled on a bit, but I would be very grateful for any help out of my present confused state. -- Rolf Dept. of Computer Science, Tel: +44 203 523523 ext 2485 Warwick University, JANET: rolf@uk.ac.warwick.ubu Coventry, UUCP: {seismo,mcvax}!ukc!warwick!rolf CV4 7AL England. "Three pints? At lunchtime?!"