I noticed when I use an image map in POVray, that the colors in the
resulting image differ from the original. I understand that the brightness
may differ, due to lighting conditions, but I would expect that the
proportion of red/green/blue stays the same.
However, the resulting image contains less red.
I am using white light and no special atmospheric conditions. What could be
the cause of this ?

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> I noticed when I use an image map in POVray, that the colors in the
> resulting image differ from the original.

The only time I've ever experienced this is when using PNG files. In this
case, gamma correction is causing the color changes. This can be fixed by
adding

global_settings {
assumed_gamma 1
}

so that gamma correction has no effect.

If you're not using a PNG file, then this probably won't help.

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

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Slime wrote:
>>I noticed when I use an image map in POVray, that the colors in the
>>resulting image differ from the original.
> 
> The only time I've ever experienced this is when using PNG files. In this
> case, gamma correction is causing the color changes. This can be fixed by
> adding
> 
> global_settings {
> assumed_gamma 1
> }
> 
> so that gamma correction has no effect.

Sorry but this is nonsense.  The above will apply normal gamma correction.

If you experience unexpected colors in image maps you have to remember 
that POV-Ray does not invert gamma correction in images so you should 
use not gamma corrected, linear images for image maps.

The only special thing about PNG images is that they can store gamma 
correction information.  Some programs (including POV-Ray) read and 
apply this information when present.

Christoph

-- 
POV-Ray tutorials, include files, Sim-POV,
HCR-Edit and more: http://www.tu-bs.de/~y0013390/
Last updated 23 Sep. 2004 _____./\/^>_*_<^\/\.______

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> Sorry but this is nonsense.  The above will apply normal gamma correction.

Well, I'm not sure exactly what's going on internally. All I know is that
when I don't have the assumed_gamma line, my PNG images are darker then
they're supposed to be in POV-Ray. Maybe this is because gamma correction
*isn't* being applied, or something - I don't know. All I know is that PNG
images are the only image format I use that have gamma correction
information stored in them, and they're the only type of images that give
wrong results when I don't have that line. So I assume that the problem is
caused, one way or another, by gamma correction doing what I don't want it
to do, and when I add that line (it's supposed to be there anyway I
believe), the problem goes away.

Also notice that I said "so that gamma correction has no effect," not "so
that there is no gamma correction." I realize that gamma correction is still
taking place, it's just that this way it produces the desired results. I
wasn't implying that POV-Ray is doing anything wrong. Maybe it's photoshop's
fault (when it saves the image). I assume it's *my* fault because I'm not
telling POV-Ray what the assumed gamma of my scene is.

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

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Slime wrote:
> 
> Also notice that I said "so that gamma correction has no effect," not "so
> that there is no gamma correction." I realize that gamma correction is still
> taking place, it's just that this way it produces the desired results.

Without an assumed_gamma line POV-Ray does not apply any gamma 
correction to the render.  But all this has nothing to do with the use 
of image maps in POV-Ray.

What might lead to confusion is that POV-Ray applies gamma information 
stored in images when they are used in image maps which other programs 
don't do.

Christoph

-- 
POV-Ray tutorials, include files, Sim-POV,
HCR-Edit and more: http://www.tu-bs.de/~y0013390/
Last updated 23 Sep. 2004 _____./\/^>_*_<^\/\.______

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Not sure. Suppose you might see some shift with certain default or explicit
finishes.

What happens if you change the finish for the object with the image map
applied to { ambient 1.0 diffuse 0.0 } ?  Do you still see a color shift?

Are you looking at the original image and ray traced image on the same
computer monitor?

"Brigitte" <nomail@nomail> wrote in message
news:web.41a64323cc213c51cfb58d550@news.povray.org...
> I noticed when I use an image map in POVray, that the colors in the
> resulting image differ from the original. I understand that the brightness
> may differ, due to lighting conditions, but I would expect that the
> proportion of red/green/blue stays the same.
> However, the resulting image contains less red.
> I am using white light and no special atmospheric conditions. What could
be
> the cause of this ?
>
>
>

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"William Pokorny" <pokorny_at_epix.net> wrote:
> Not sure. Suppose you might see some shift with certain default or explicit
> finishes.
>
> What happens if you change the finish for the object with the image map
> applied to { ambient 1.0 diffuse 0.0 } ?  Do you still see a color shift?

I've now used an image map (jpeg image) that contains one single color
rgb<94/255,46/255,51/255>
If I apply no lighting, and finish{ambient 1.0 diffuse 0.0},
the resulting color is rgb<162/255,115/255,122/255>, so it's a lot brighter
than the original.

I've also tried to find a value for ambient where the brightness is more or
less the same as in the original image
finish{ambient 0.3 diffuse 0.0} gives a color rgb<93/255,67/255,70/255> so
inexplicably the result contains proportionally less red.

>
> Are you looking at the original image and ray traced image on the same
> computer monitor?
>
I have measured the colors both of the original image and the POV ray result
using paint shop pro, and I am watching them on the same monitor ...

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Brigitte wrote:
> 
> I've now used an image map (jpeg image) that contains one single color
> rgb<94/255,46/255,51/255>
> If I apply no lighting, and finish{ambient 1.0 diffuse 0.0},
> the resulting color is rgb<162/255,115/255,122/255>, so it's a lot brighter
> than the original.

Did you read what i wrote?  This is perfectly normal and intended 
behaviour.  POV-Ray applies gamma correction to the rendered image 
according to the display_gamma setting (which is usually 2.2 on PC 
Windows/Linux) if an assumed_gamma statement is present in the scene. 
This is explained in detail in the docs.  Your numbers comply with this 
since

(94/255)^(1/2.2) = (162/255)

For image maps you therefore have to use images without gamma correction 
(i.e. linear images).  If your image is gamma corrected (like when you 
get it from the web or a (cheap) digital camera) use the gamma 
correction function of your imaging program for this.

Christoph

-- 
POV-Ray tutorials, include files, Sim-POV,
HCR-Edit and more: http://www.tu-bs.de/~y0013390/
Last updated 23 Sep. 2004 _____./\/^>_*_<^\/\.______

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Cool. So we can be reasonably sure Brigitte's scene file has an
assumed_gamma statement other than 1.0 or that her image_map jpg specifies a
gamma correction. I was under the impression jpg files do not store gamma
correction information. If true when Brigitte correctly sets the
display_gamma and assumed_gamma to 1.0 she will see the proper color for her
image mapped object.

Is it still true many of the older textures shipped with POV-Ray will then
not be properly corrected for display because they expect an assumed gamma
of something like 2.2?

The documentation mentioned can be found at:
http://www.povray.org/documentation/view/3.6.1/218/

Bill

"Christoph Hormann" <chr### [at] gmxde> wrote in message
news:coa3g4$d45$1@chho.imagico.de...
>
> Did you read what i wrote?  This is perfectly normal and intended
> behaviour.  POV-Ray applies gamma correction to the rendered image
> according to the display_gamma setting (which is usually 2.2 on PC
> Windows/Linux) if an assumed_gamma statement is present in the scene.
> This is explained in detail in the docs.  Your numbers comply with this
> since
>
> (94/255)^(1/2.2) = (162/255)
>
> For image maps you therefore have to use images without gamma correction
> (i.e. linear images).  If your image is gamma corrected (like when you
> get it from the web or a (cheap) digital camera) use the gamma
> correction function of your imaging program for this.
>
> Christoph
>
> -- 
> POV-Ray tutorials, include files, Sim-POV,
> HCR-Edit and more: http://www.tu-bs.de/~y0013390/
> Last updated 23 Sep. 2004 _____./\/^>_*_<^\/\.______

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in news:41a8bf51$1@news.povray.org William Pokorny wrote:

> Cool. So we can be reasonably sure Brigitte's scene file has an
> assumed_gamma statement other than 1.0 or that her image_map jpg
> specifies a gamma correction. [...]

No.

If an image looks fine in an imageviewer on a 'gamma 2.2' monitor, you 
can be sure that a gamma correction is applied to it. An image with a 
gamma of 1 would look dark on that monitor.

Now if we take that gamma 2.2 image and use it as an imagemap in POV-Ray 
it will look to bright if the scene is rendered to a gamma of 2.2. The 
gamma correction is now applied twice to the initial image. If the scene 
were renderd to a linear gamma, the image map would look right. Brigitte 
can try this by removing the line 'assumed_gamma = 1' from her scene.

To use images for imagemaps, you have to convert them to a linear gamma 
(gamma=1).


Ingo

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