Path: csiph.com!x330-a1.tempe.blueboxinc.net!newsfeed.hal-mli.net!feeder1.hal-mli.net!weretis.net!feeder4.news.weretis.net!eternal-september.org!feeder.eternal-september.org!.POSTED!not-for-mail From: sfeam Newsgroups: comp.graphics.apps.gnuplot Subject: Re: set palette cubehelix Followup-To: comp.graphics.apps.gnuplot Date: Sat, 08 Oct 2011 14:30:39 -0700 Organization: gnuplot development team Lines: 46 Message-ID: References: <9brre9Fvf3U1@mid.individual.net> <9bulcrFl92U1@mid.individual.net> <73c4a8f3-cb45-4e03-b887-58466e760963@v18g2000yqj.googlegroups.com> <9fas1rF6r5U1@mid.individual.net> Reply-To: sfeam@users.sourceforge.net Mime-Version: 1.0 Content-Type: text/plain; charset="ISO-8859-1" Content-Transfer-Encoding: 7Bit Injection-Date: Sat, 8 Oct 2011 21:30:46 +0000 (UTC) Injection-Info: mx04.eternal-september.org; posting-host="a5sqwvDD9P994JK/mw/xtg"; logging-data="19218"; mail-complaints-to="abuse@eternal-september.org"; posting-account="U2FsdGVkX1+eZNqtBr5jFuwLtfNzzSax" User-Agent: KNode/4.4.9 Cancel-Lock: sha1:/Cbg29B1TjFBNI6pX6DKHrMT6dk= Xref: x330-a1.tempe.blueboxinc.net comp.graphics.apps.gnuplot:641 sfeam wrote: > >> According to http://en.wikipedia.org/wiki/Gamma_correction I would >> say that mine interpretation is correct (see the example photograph >> of the lady in the water). But the Wikipedia article might be wrong >> as well. > > The Wikipedia article is very confusing as it does not, that I can > see, define the basic quantity V used in the fundamental equation > given at the top of the article: > > Vout = Vin ** gamma > > For any quantity V that increases with brightness (e.g. amplitude, > intensity, brightness) it is clear that gamma > 1 increases > brightness. And yes, it seems to me that the images on the Wikipedia > page are mis-described. Upon re-reading, I now see that the Wikipedia article has lifted that equation from the section on CRT voltage response. So in that context V is a normalized voltage that runs from 0->1, and indeed gamma>1 means that the image displayed by the CRT is darker than would be the case for linear response. So the article is consistent with itself, but still it conflates the gamma of the CRT with that of the algorithm used in image processing to correct for it. A CRT with gamma > 1 is dark, so in order to correct for this you must brighten the image sent to it. I believe that the caption for the images on that page should read something like "This is the way the original image would appear if displayed on various CRT monitors with gamma = XXX". Ethan > > I am not certain, but I suspect the source of confusion is that CRT > performance is also described using a gamma term, and this description > is in some sense the inverse of what you do to the image. I.e., a > CRT with gamma = 2.0 looks dark. To make the displayed image look > "correct" you have to brighten it with a gamma correction of 2.0 > The Wikipedia article calls this an "inverse gamma correction", > which may well be the correct jargon in some fields. But that > correction is what all programs I am familiar with apply as a > "gamma correction". Certainly it is what gnuplot has always done. >