Re: Default colors vs. gamma

[Alexander Malmberg]

Pascal Bourguignon wrote:
> And for gamma-newbies around here like me, why should the gamma be 1.6
> rather than 1.0?  Why shouldn't we keep 'absolute' values for colors
> (gamma=1.0) everywhere?

Well, I'm no expert on this, but as I understand it:

We really want to use linear colors; ie. when we say that the system 
gray is 0.667 gray, we want a color that is two-thirds of the light 
intensity of full white. The problem is that if we give a signal to the 
monitor that is two-thirds of the signal for full-white, various 
non-linear phenomenon in the monitor will cause the resulting color to 
be 0.667^gamma times the intensity of full white.

The 'gamma' in this function depends on all kinds of things (monitor, 
monitor settings, ambient light, etc.), but it's usually somewhere 
around 2.0. Thus, using 2.0, our 0.667 gray will turn out as 0.667^2.0 
~= 0.44, less than half the intensity of full white. (Note that 
brightness and contrast controls on a monitor can't fix this. They can 
change how bright black is, and how bright white is, but they can't 
change the ratio of intermediate colors to full white/black.)

To compensate for this, you do various gamma adjustments. A nice and 
simple approach is to make the video card do it. If we know that the 
effective gamma of our monitor is, say, 2.0, we can have the video card 
apply an inverse transform when it reads the framebuffer and sends the 
signal to the monitor (this is what 'xgamma' does). The video card will 
then read 0.667 from the framebuffer, apply the inverse transform, and 
really send 0.667^(1/2.0) ~= 0.82 to the monitor. In the monitor the 
0.82 ends up as 0.82^2.0 ~= 0.67, which is just what we wanted.

Since most users haven't told X to do this, other desktop environments 
make their colors brighter than they "should" be so that they'll look 
right on these systems. This is what my patch does. Instead of making 
our gray 0.667, it changes this to 0.667^(1/1.6) ~= 0.78, so that's what 
we'll write to the framebuffers. If the video card isn't doing any 
adjustments, it will send 0.78 to the monitor. which is pretty close to 
what we want.

1.6 is just a reasonable guess; it probably isn't the right gamma, but 
it'll be a lot closer than 1.0 (unless the user did the right 
adjustments, in which case it'll be all wrong).

(According to people in #GNUstep, GNOME, KDE, and Windows have their 
default interface colors set pre-adjusted for a 1.6 gamma, but they do 
not otherwise adjust colors.)


I was asked in #GNUstep to provide screenshots of this, so:

http://w1.423.telia.com/~u42308495/alex/gamma_colors.png

The right instance of TextEdit is without my patch (or with my patch, 
but with the default set to 1.0). The left instance is with my patch and 
the default settings (ie. colors adjusted for a 1.6 gamma). Thus, the 
right instance is using linear colors and should look "correct" on 
displays that are gamma adjusted, while the left instance should look 
"correct" on unadjusted displays. (When displaying the image, be wary of 
programs that do their own adjustments.)


After looking at the mails and #GNUstep discussion, I get this list (if 
I misplaced or forgot someone, please correct me):

Against:
Dennis Leeuw
Fred Kiefer
Gürkan Sengün
(_maybe_ Adam Fedor, not sure how to read that mail)

For:
Nicolas Roard
Alex Perez [1]
Jeff Teunissen
Riccardo Mottola


Which doesn't really get us anywhere, except that it's now late and I 
need to sleep. :)


[1] After discussion in #GNUstep, it turned out that Alex's earlier 
mails here were based on a misunderstanding of what the patch did.

- Alexander Malmberg