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Very clear and pragmatic, thanks a lot. One basic (embarrassing)
question: does this apply to image-based lighting only (FG), or should
we linearize textures for standard lighting using standard XSI lights ?
in that case what's the advantage over the good old method of judging by
eye, using color corrections in the rendertrees, playing with light
falloff etc... ?
Also, when opening a new file for painting in Photoshop, does choosing
"Don't color manage this document" result in a linear image?
Christian Rittener
Guy Rabiller wrote:
Hi Bradley,
../.. So it's clear with regards to treating
source textures that you need to remove any
gamma bias from the camera or say, from
something painted in photoshop.
One way to, perhaps, clearly understand this is that we cheat ourselve
to produce an image that looks "correct" on a monitor wich has a non
linear response. An Artist creating an image in photoshop ( with
standard color scheme ) will set the colors according to what *he sees*
on screen. So the Artist is implicitly creating a non-linear image,
meant to be displayed on a non-linear response monitor.
Same is true for rendering. If you don't apply a rendering gamma, you
will set lights in your scene so the rendered image looks "good" to your
eyes, viewed on a non-linear response monitor. What this means is that
you'll "boost" your light to see "something", especialy in the low key
areas.
For the sake of simplicity, let's say that this non-linear response is
called "gamma 2.2", and forget about inverse gamma ( image versus
display response ). Simply accept that we work in "gamma 2.2".
Ok so the display is "gamma 2.2". A video camera will produce "gamma
2.2" images ( forget about the inverse gamma! ).
In this case, no problem. Both match.
A Photoshop Artist will create "gamma 2.2" picture implicitly, because
he "sees" a gamma 2.2 image on screen.
A Lighting Artist will, actualy, create a "gamma 2.2" image, despite the
fact that he might use or not a rendering gamma of 2.2 ( without knowing
it that said )! Because he "sees" a gamma 2.2 rendered image to his eyes.
Ok so, everything is ok, everybody produce "gamma 2.2" images !?
*YES*, and don't worry about it, the flame artist will not change the
gamma of your rendered image because it is implicitly "gamma 2.2".
Now, what's the deal with "linear lighting" ? The trick is in *what* the
artist *has to do* to produce this "gamma 2.2" image:
He cheats!
To achieve a what seems to him be "half light power", he wont set the
light power to 0.5 but perhaps 0.75. So even the settings he use are
non-linear. And that's the problem. Because mathematicaly, a renderer
works in linear space.
To correct this, you have to enter the *settings* in linear space, and
for this, you have to compensate the rendered image or the viewing of
the rendered image to be "gamma 2.2". This way, when your eyes will tell
you it's "half light power", you will have entered a setting of 0.5,
this time.
With this, you will still generate "gamma 2.2" images, except that this
time, you wont cheat, you will use linear *settings*.
To resume:
Whatever you do, you will allways produce "gamma 2.2" images because you
are creating it on a "gamma 2.2" display.
( as long as you think the image "looks good" to you ).
The problem is not in the gamma of the created image, but in "what you
did" to create this "gamma 2.2" image.
The degree of "cheating" if you want.
And when you start cheating a linear renderer with your non-linear
settings, this where you'll have non accurate lighting response, and
something will be "wrong" in your image. The image somehow looks good
but there is still something wrong.
Monitor set to gamma 1 ../..
This means NO gamma correction. It doesn't mean your monitor has a gamma
of 1.0.
CRT and LCD ( setted on sRGB ) allways have a gamma ~2.2/2.5.
So setting your monitor to gamma 1.0 means you have a gamma of 2.2/2.5!
So don't touch your display, it's gamma 2.2/2.5 anyway, don't try to
"correct it".
( Choose the sRGB profil if you have the choice in your monitor OSD
settings, NOT in the display/card settings on your computer OS, here
allways set gamma 1.0 ).
Render your image with a gamma of 2.2 and light according to what you
see at the end.
As your texture images have a gamma 2.2 ( created implicitly this way by
artists on their gamma 2.2 monitors ), you have to convert them to
linear gamma prior using those to satisfy your "linear gamma" renderer.
After all, I'm not sure it's clearer than other explanation :-)
Tell me.
--
guy rabiller | 3d technical director (at) LaMaison
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