RE: [Leica] OFF-TOPIC: gamma setting for webpages

["Jean-Claude Berger" <jcberger@imaginet.fr>]

Hi Pascal,

Here is how I unserstand gamma. I may be wrong and I will be happy if
someone could correct me. Sorry for the bad english.

Monitors react to a signal sent by the video card. Theorically, it should be
a linear function.  Let's say we want to go from 1 to 10. If the card sends
a signal of 1, intensity of a pixel should be one, for a signal of value 2,
intensity should be 2, and so on up to 10.

But actually, monitors don't react with a linear function but with a
function we can write like "signal power gamma". So instead of the couples
1->1, 2->2...10->10, with a gamma of 1.2 we get 1->1, 2->2.3,
3->3.7,...6->8.6, 7->10 (10.33 actually but the monitor can go up to 10
only). So signals 7 to 10 will draw the same pixel. When you tell your
sofware that you have a monitor with a gamma of 1.2, it sends a signal
corrected as "signal power 1/gamma". For example, instead of sending the
signal 2, it computes 2 power 1/1.2 = 1.8 and sends 1.8 instead of 2. The
monitor does 1.8 power 1.2 = 2. So we have again a linear intensity.

What does it mean ? IF you configure your system with the good gamma, you
will have a linear reaction to a signal. If another system presents a
different gamma, it will display the same image than the first one, provided
it is well configured too.

So, you don't need to (you should not) correct pictures depending on the
gamma of your system.   Scan so you have a decent picture on YOUR screen
whatever machine you have. All other machines in the world (PCs or Macs)
should (theorically of course) display the same image.

A last word. I doubt that general sentences like "PC have a gamma of x, Macs
a gamma of y" could be true. IMHO, it depends a lot more on the monitor
model and the video card.

Hope this helps,



- --
Jean-Claude Berger (jcberger@jcberger.com)
Systems and RDBMS consultant (MCSE)
Lyon, France
http://www.jcberger.com

> -----Original Message-----
> From: owner-leica-users@mejac.palo-alto.ca.us
> [mailto:owner-leica-users@mejac.palo-alto.ca.us]On Behalf Of Pascal
> Sent: Sunday 28 February 1999 14:07
> To: Leica Users Group
> Subject: [Leica] OFF-TOPIC: gamma setting for webpages
>
>
> This is a question that is most likely to be of interest to many luggers.
>
> It happens that Macintosh and Wintel computers use a different gamma
> setting for monitor displays. Mac's have a gamma of 1.8 while Wintel's
> have a gamma of 2.2.
> This gamma setting affects the brightness of the picture when viewed on
> the other platform.
>
> The images on my web pages have been scanned at the gamma level of 1.8
> using the Nikon Super Coolscan LS-2000 (the older ones with a UMAX
> flatbed scanner). While they look good on Macintosh machines, I have
> noticed that they -obviously- display much darker on a Wintel machine.
> While this is not so much of a problem for average images, it could
> reduce images with a lot of shadow to something very dark and
> undistinguishable on Wintel while showing enough shadow detail on a Mac.
>
> About a week ago, I scanned a couple of images at the gamma level of 2.2
> as a test and put them on my web pages (two in the Elena section and
> another in the Belgium section- those with the Carl Zeiss Softar 3
> filter). Now, while they look good on Wintel (because they have been
> scanned at optimum gamma for Wintel), they look overexposed on Mac (where
> the screen is 1.8).
>
> How do others resolve this issue? Is there such thing as an optimum
> cross-platform gamma level where pictures are enjoyable on all computer
> platforms?
>
> Thanks for suggestions or experiences.
>
> Pascal
> NO ARCHIVE
>
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