Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!tut.cis.ohio-state.edu!usenet.ins.cwru.edu!eagle!news From: fsfacca@ZoSo.lerc.nasa.gov (Tony Facca) Newsgroups: comp.graphics Subject: Re: GIF images lack gamma info too: (was pixel aspect ratio in GIF images) Message-ID: <1990Jul24.124203.10040@eagle.lerc.nasa.gov> Date: 24 Jul 90 12:42:03 GMT References: <9866@pt.cs.cmu.edu> <9894@pt.cs.cmu.edu> <5031@labtam.oz> <1990Jul23.154140.20291@pdn.paradyne.com> Reply-To: fsfacca@ZoSo.UUCP (Tony Facca) Organization: NASA/Lewis Research Center, Cleveland Lines: 37 In article <1990Jul23.154140.20291@pdn.paradyne.com> efwjr@bam-bam.paradyne.com (Ellsworth Warmouth x2882) writes: > >I have always wondered why my images looked different on a Macintosh and a >Sun. Does anyone have any idea how to change the display gamma? Does it >just involve converting the colormap of a gif that displays correctly on a >Mac using some scale factor, to get the image to display properly on a Sun? >What are typical compensation factors involved with a conversion? Does it >matter what type of monitor you have, or is it the monitor driver card that >determines the gamma compensation level. > There is a non-linear relationship between the voltage to the display and the brightness of the display surface. This has something to do with the acceler- ation voltage of the display's electron beam which I can't pretend to fully understand. The net result is that one must compensate for the non-linearity by raising the input voltage to some power to generate the brightness value. This value is call the "gamma" of a monitor and is said to be between 1.4 and 3.0 depending on the type of monitor. I am not sure what the original poster had in mind when he suggested adding a "gamma" value to the GIF description. For example if I told you that some image I am sending you was displayed on a monitor with a gamma of 2.4, you could use this value to "gamma correct" the display of your image, assuming you knew the gamma of your particular monitor. I believe Foley and Van Dam discuss gamma correction in the "old" book. I have never done much with gamma correction, other than use the handy "gamma" command courtesy of Paul Haeberli on the Iris's. I think this command gamma corrects the entire screen, it would be an interesting modification to allow gamma correction on individual windows. How would this work? Hmmm... -- ----------------------------------------------------------------------------- Tony Facca | phone: 216-433-8318 NASA Lewis Research Center | Cleveland, Ohio 44135 | email: fsfacca@avelon.lerc.nasa.gov -----------------------------------------------------------------------------