Subject: Re: vic gamma control question

From: [email-masked] (R. P. Channing ["Rick"] Rodgers)
Date: Thu, 18 Sep 1997 12:19:12 -0400

Hugh,

Thanks for the response.  however, it misses the mark.  I understand
what gamma is, I have read all of Charles Poynton's nice articles.
The best single Web site I've found on the gamma issue is:

   http://www.cgsd.com/papers/gamma.html

What is *not* clear is how one controls the gamma on the Ultra 2 using
the ffb frame buffer.

A summary of where things stand.  I'e read the section "SMCC Frame
Buffers" from page 33 onward, in the "Solaris 2.5.1 on Sun Hardware
AnswerBook." On pg. 41, they recommend trying to run openwin as:

   openwin -dev /dev/fbs/ffb0 defdepth 24

to get 24-bit TrueColor.  They warn that some applications may break
with a "BadMatch" error message, requiring a return to the default
8-bit PseudoColor setting of the Creator ffb.  On page 33, they try
to convince the reader that the ffb is in many ways a combination of
the features of all the recent previous high-end frame buffers from Sun.
On page 34, they recommend using 24-bit TrueColor to reduce colormap
flashing, and start listing the 11 "visuals" supported by ffb.  On page
35, they state that these fall into two categories, "linear" or
"gamma corrected" (by which I am inferring that they actually mean
a gamma of 1.0, though they do not define their terms in this document)
and non-linear (which they also call "not gamma corrected").  This
notion is expanded from pg. 37 on, in a section called "Gamma
Correction."

I've called Sun with what I consider to be two distinct problems:

1) The Sun document does not define its terms, and is unclear with
   respect to what gamma values are associated with the 11 "visuals"
   that the ffb supports, and how they may be set.

2) Assuming that "linear" means gamma=1.0, and that non-linear means
   gamma!=1.0, what is gamma in the "non-linear" regimes, and can it
   be controlled?

Finally, a tip for other Ultra users about a document available from
SunSupport via http://sunsolve.sun.com/ (requires that you have a
password, which you should have if you have a support contract).
Select "simple search" and then search for the pattern "cde 24"
wherupon you find document 13458 entitled "CDE: Running in 24 bit mode,"
which tells you how to invoke the 24-bit TrueColor visual for
OpenWindows if you are using the dtlogin login manager like me.
 
Turning up monitor intensity is in general *not* an appropriate way of
dealing with gamma problems.  The gamma problem relates to perceived
*relative* intensities among different colors; changing overall monitor
intensity does not address this (it inreases all intensities by the same
proportion), and there are compelling reasons (re. Charles Poynton)
to leave intensity at a comfortable (generally low) level.

Cheerio, Rick Rodgers

> From [email-masked] Thu Sep 18 11:37:40 1997
> 
> Someone has probably already answered this for you, but, if not,
> I can't tell you the "ultimate" reason for the design decision,
> but, the short answer is that the vic gamma setting only works
> on 8-bit framebuffers.  You are lucky to have a 24-bit framebuffer.
> 
> Probably, what you need to do is adjust your monitor's overall
> brightness.  If you do a web search for a guy named Poynton (sp?), 
> I think, and gamma, I think you will find a procedure to "correctly"
> set the gamma, as well as an online gamma treatise from his textbook.  
> 
> Or, you can just turn the contrast knob on your monitor up until 
> it looks brighter...

> Hope this helps,
> 
>   Hugh LaMaster, M/S 258-5,     ASCII Email:  [email-masked]
>   NASA Ames Research Center     Or:           [email-masked]
>   Moffett Field, CA 94035-1000  No Junkmail:  USC 18 section 2701
>   Phone:  415/604-1056          Disclaimer:   Unofficial, personal *opinion*.
> 
>