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) :

sphere {
  <1,1,0>, 1
  pigment { rgb <112,57,57>/255 }
}

sphere {
  <-1,1,0>, 1
  pigment { image_map { png "img_col.png" } }
}

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.

All that seems strange to me, maybe I missed something obvious, if you guys
know something about it, please tell me. Tx

JC

Post a reply to this message

PNG has gamma correction built in. I'm not exactly sure how POV-Ray handles
this, but it might possibly be fixed if you set the gamma settings described
in the documentation?

Personally, I'd like a way to turn off the PNG's gamma correction, since I'd
rather just use a simple linear color scale until the final image is
produced.

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

Post a reply to this message

You should try putting "global_settings { assumed_gamma 1 }" in your
scene.
  I had once the same problem of an image map looking a lot different
in POV-Ray than it looked when viewed separately with an image viewer,
and the problem was the default assumed_gamma.
  (IMHO the default assumed_gamma should be 1, not 2.2 (or whatever it is).
I don't understand why it's 2.2.)

-- 
#macro N(D)#if(D>99)cylinder{M()#local D=div(D,104);M().5,2pigment{rgb M()}}
N(D)#end#end#macro M()<mod(D,13)-6mod(div(D,13)8)-3,10>#end blob{
N(11117333955)N(4254934330)N(3900569407)N(7382340)N(3358)N(970)}//  - Warp -

Post a reply to this message

Warp wrote:

> You should try putting "global_settings { assumed_gamma 1 }" in your scene.
> I had once the same problem of an image map looking a lot different
> in POV-Ray than it looked when viewed separately with an image viewer,
> and the problem was the default assumed_gamma.
> (IMHO the default assumed_gamma should be 1, not 2.2 (or whatever it is).
> I don't understand why it's 2.2.)


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.

The manual mentions that display_gamma is ignored if you don't specify
the assumed_gamma, so I guess there would be a change if you add
{ assumed_gamma 1 } even if the default was 1? For some reason it's
difficult for me to hold the meaning of POV-Ray's gamma settings in
my head, so correct me if I'm confused...

When I created an image for the IRTC on my Mac, I had to keep in mind
that most people would be viewing the image on a PC, and it would look
darker than what I was seeing.

-Mark Slone

Post a reply to this message

"Renderdog" <slo### [at] hiwaaynet> wrote in message
news:web.3e43d1f3c25eeb4b7ba9929f0@news.povray.org...
>
> 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.
>
> The manual mentions that display_gamma is ignored if you don't specify
> the assumed_gamma, so I guess there would be a change if you add
> { assumed_gamma 1 } even if the default was 1? For some reason it's
> difficult for me to hold the meaning of POV-Ray's gamma settings in
> my head, so correct me if I'm confused...
>

IIRC, assumed_gamma 1.0 (or whatever) will only work correctly if you have
specified the _correct_ gamma of your monitor in your povray.ini (or
.povrayrc) to start with. The quick way (as we all know) is to use gamma.gif
to guesstimate it. Not perfect, but works well enough for most people.
What I'd really like to see is an extension to Display_Gamma so that we
could set r, g & b separately. (my monitor's g is slightly different to r &
b thus giving me a slight magenta cast in my images (unless I remember to
correct for it)

John

--
Run Fast
Run Free
Run Linux

Post a reply to this message

> When I created an image for the IRTC on my Mac, I had to keep in mind
> that most people would be viewing the image on a PC, and it would look
> darker than what I was seeing.


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.

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

Post a reply to this message

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/

Post a reply to this message

In article <3e43e012@news.povray.org>, "Slime" <slm### [at] slimelandcom> 
wrote:

> Create the image with assumed_gamma 1.8 and display_gamma 1.8.

Wrong: assumed_gamma 1 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.

Correct.

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

Post a reply to this message

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 :-)

Post a reply to this message

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

Post a reply to this message