Slime wrote:

>The solution to that:
>
>Create the image with assumed_gamma 1.8 and display_gamma 1.8.
>
>Then render the final version of the image with display_gamma 2.2, but
>assumed_gamma left alone. The image will then look on PCs the way it looked
>on a mac while you were creating it.

Would it be better to create the image with assumed_gamma 1.0 and
display_gamma 1.8, so the image maps look correct, and then render
at display_gamma 2.2? How would the image look different between
these cases? Are there other reasons to prefer assumed_gamma 1.0?

BTW, I'd probably produce the final image with a display_gamma 2.0,
a compromise between the two, as I don't want to totally abandon my
fellow Mac users :-)

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Exether wrote:
>I noticed recently that the color I get with an image map is not exactly
>what I expected. So I did a few tests with the following (with a basic
>camera and light) :
....
>Where img_col.png is a uniform image of the <112,57,57> color (#703939) made
>with GIMP. What I see is that the mapped sphere is darker when I expected
>it to be the same. Then I try to get the same color by changing the first
>sphere's color to 0.5*<112,57,57>/255 so that they are about the same
>brightness. But then the mapped sphere seems to be more Red that the other
>one.


Does GIMP have a gamma setting?  When I was creating a background with image
maps made with Terragen I had the same problem, they didn't look the same
in POV.  Then I found Terragen's default gamma setting was 1.5 and I had
POV set to 1.  After I compromised and set them both to 1.275 everything
looked right.

RG

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> Wrong: assumed_gamma 1 and display gamma 1.8.

Actually, it won't make a difference just as long as you use the same
assumed_gamma when rendering each time. You could use assumed_gamma 20 if
it's easier for you.

The point of using assumed_gamma equal to the display_gamma that you develop
on is that the color values you type in will be proportional to their
brightness on the screen, which makes it easier to guess colors when making
your textures.

 - Slime
[ http://www.slimeland.com/ ]

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> Actually, it won't make a difference just as long as you use the same
> assumed_gamma when rendering each time. You could use assumed_gamma 20 if
> it's easier for you.


I take it back. While the above sentence is true, I thought about it for a
bit, and realized that if you develop with an assumed_gamma other than 1,
the fact that you're specifying colors weirdly makes things react
incorrectly in certain parts of raytracing.

For instance, a full white should appear twice as bright as a mid gray when
they're multiplied by 0.5 (for instance, when they're reflected off a
surface with a reflection value of 0.5). But if you're developing with an
assumed_gamma other than 1, which means your mid gray isn't rgb(.5,.5,.5),
the multiplication by .5 brings the white and gray down to other areas of
the 0-1 range, where they may no longer appear one twice as bright as the
other.

I'm glad I figured that out. Thanks for pointing this little fact out to me.

How frustrating! Now I realize that I should actually use gamma correction!

 - Slime
[ http://www.slimeland.com/ ]

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What about if you are doing an animation and you want the brightness to be
the same on the projection screen as you see it on your Mac or PC?
What's the best advice for such a scenario?
Also, if I see that a video projection of my animation is dark overall, can
I rerender the scenes and use the gamma setting to "adjust" the overall
color brightness next time around (same question as above, I guess.)
thanks - this is a biggie.
D.

"Christopher James Huff" <cja### [at] earthlinknet> wrote in message
news:cja### [at] netplexaussieorg...
> In article <web.3e43d1f3c25eeb4b7ba9929f0@news.povray.org>,
>  "Renderdog" <slo### [at] hiwaaynet> wrote:
>
> > If I understand correctly, when I create an image with { assumed_gamma
1 }
> > on my Mac (display's gamma 1.8), and someone runs the same code on their
> > PC (display's gamma 2.2), they would generate a brighter image that
would
> > look the same on their monitor as my image looked on my monitor. This
> > is good, but they may wonder why the image they generate is brighter
> > than the one I created.
>
> No assumed_gamma, no gamma correction.
>
> assumed_gamma 1, the display gamma is used to correct the image, it will
> display correctly on the machine it was rendered on if Display_Gamma was
> set correctly.
>
> Other assumed_gamma values are mainly useful for scenes designed without
> gamma correction. You should use assumed_gamma 1 for new scenes, adjust
> the display gamma if you are rendering to display elsewhere.
>
> --
> Christopher James Huff <cja### [at] earthlinknet>
> http://home.earthlink.net/~cjameshuff/
> POV-Ray TAG: chr### [at] tagpovrayorg
> http://tag.povray.org/

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In article <3e44778d$1@news.povray.org>, "Slime" <slm### [at] slimelandcom> 
wrote:

> That would allow people to use any assumed_gamma they wanted for
> development. They could make it match their display_gamma, so colors would
> be easy to come up with, and the result would still be correct.

I think you are trying to come up with an overly complex solution to a 
nonexistent problem.
If assumed_gamma is equal to display_gamma, no correction will be done. 
Then it will only display correctly on linear devices.

-- 
Christopher James Huff <cja### [at] earthlinknet>
http://home.earthlink.net/~cjameshuff/
POV-Ray TAG: chr### [at] tagpovrayorg
http://tag.povray.org/

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In article <3e443858$1@news.povray.org>,
 "Dennis Miller" <dhm### [at] attbicom> wrote:

> What about if you are doing an animation and you want the brightness to be
> the same on the projection screen as you see it on your Mac or PC?
> What's the best advice for such a scenario?

Set the display gamma to the gamma of the projection screen. If it is 
different from the monitor, you won't be able to have it display the 
same on both.
PNG can store gamma information, which would allow properly designed 
software to display the image correctly, but I don't know if either POV 
or any common software actually uses it.

-- 
Christopher James Huff <cja### [at] earthlinknet>
http://home.earthlink.net/~cjameshuff/
POV-Ray TAG: chr### [at] tagpovrayorg
http://tag.povray.org/

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Thanks. I never know exactly what the setup will be on the other end when I
send out my animations (and they can be on DV, DVD, VHS. etc.) so I will try
to find out if there is a standard for most common projectors (but then
again, there are different types of projectors!).
Best,
D.

"Christopher James Huff" <cja### [at] earthlinknet> wrote in message
news:cja### [at] netplexaussieorg...
> In article <3e443858$1@news.povray.org>,
>  "Dennis Miller" <dhm### [at] attbicom> wrote:
>
> > What about if you are doing an animation and you want the brightness to
be
> > the same on the projection screen as you see it on your Mac or PC?
> > What's the best advice for such a scenario?
>
> Set the display gamma to the gamma of the projection screen. If it is
> different from the monitor, you won't be able to have it display the
> same on both.
> PNG can store gamma information, which would allow properly designed
> software to display the image correctly, but I don't know if either POV
> or any common software actually uses it.
>
> --
> Christopher James Huff <cja### [at] earthlinknet>
> http://home.earthlink.net/~cjameshuff/
> POV-Ray TAG: chr### [at] tagpovrayorg
> http://tag.povray.org/

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On Fri,  7 Feb 2003 05:37:42 EST, "Exether"
<exe### [at] demoxandmailcom> wrote:

>Where img_col.png is a uniform image of the <112,57,57> color (#703939) made
>with GIMP. What I see is that the mapped sphere is darker when I expected
>it to be the same. Then I try to get the same color by changing the first
>sphere's color to 0.5*<112,57,57>/255 so that they are about the same
>brightness. But then the mapped sphere seems to be more Red that the other
>one.
>

It's gamma correction.  The PNG file format has gamma information
in it.  This is theoretically a good idea but in practice it seems
that many programs either have proken file gamma support or they
each interpret gamma differently.

I use a 2-step method of working around this:

(1) use a file format with no gamma (such as TGA or BMP).
(2) set the assumed_gamma in the global_settings part
    of your POV file to be the same as the display_gamma
    setting in the povray.ini file

This disagrees with what is considered proper in theory and
in the documentation, but it works.  I don't argue with
what works.

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"Exether" <exe### [at] demoxandmailcom> wrote:
> I noticed recently that the color I get with an image map is not exactly
> what I expected.

Pls see my "image_map and gamma" post/thread in this group.

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