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OK. Let me try again to get my mind around this.
- the gamma isn't stored as part of most images except png.
- when you talk about gamma correction being already applied to the image,
what you are essentially saying is that the image has been adjusted to
display properly on a 'gamma 2.2' display system - not that the gamma is
stored with the image file itself. (My failing this time around is here. I
have been for a long time taking images as "linear images" because there is
no gamma information stored as part of the image)
- the POV-Ray recommendation for all of us is to use assumed_gamma of 1.0
for reasons both internal to the POV-Ray engine and for distribution of
source code to others.
- Using an assumed_gamma of 1.0 will brighten any image used as an image map
set up to display correctly on any system where the true display gamma is >
1.0 and the display gamma is either defaulted or set to the true gamma of
the display system where POV-Ray is run.
- These implications apply to all color maps, and pigment maps that were
derived from images that looked good on display systems with a gamma > 1.0.
- The most general solution is then to convert all mapped color information
so it will display correctly on a display system with a gamma of 1.0 (what I
understand now Christoph was suggesting), set the display_gamma correctly
for every system and use an assumed_gamma setting of 1.0.
- Many of the default textures supplied with POV-Ray have NOT been modified
to display correctly if using and assumed_gamma of 1.0 and the correct
display_gamma as recommended. Is there a reason the developers could not
supply an updated set of textures either as part of the standard package or
as an adjunct package?
Do I have this right or am I still confused?
Bill
"ingo" <ing### [at] tag povrayorg> wrote in message
news:Xns95AECE85E992Eseed7@news.povray.org...
> 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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in news:41a906fc$1@news.povray.org William Pokorny wrote:
> - the gamma isn't stored as part of most images except png.
Yes
> - when you talk about gamma correction being already applied to the
> image, what you are essentially saying is that the image has been
> adjusted to display properly on a 'gamma 2.2' display system - not
> that the gamma is stored with the image file itself.
Yes
> [...] the POV-Ray recommendation for all of us is to
> use assumed_gamma of 1.0 for reasons both internal to the POV-Ray
> engine and for distribution of source code to others.
Yes
> - Using an assumed_gamma of 1.0 will brighten any image used as an
> image map set up to display correctly on any system where the true
> display gamma is > 1.0 and the display gamma is either defaulted or
> set to the true gamma of the display system where POV-Ray is run.
Assumed gamma is just a factor. The essential part of the gamma
correction is the Display_Gamma setting (defaults to 2.2 in POVwin). For
what assumed_gamma does exactly check doc-section "3.3.3.3
Assumed_Gamma"
> - These implications apply to all color maps, and pigment maps that
> were derived from images that looked good on display systems with a
> gamma > 1.0.
Yes
> - The most general solution is then to convert all
> mapped color information so it will display correctly on a display
> system with a gamma of 1.0 (what I understand now Christoph was
> suggesting), set the display_gamma correctly for every system and use
> an assumed_gamma setting of 1.0.
Yes. Kari Kivisalo once posted an equation for such corrections, but I
cant find it anymore.
> - Many of the default textures
> supplied with POV-Ray have NOT been modified to display correctly if
> using and assumed_gamma of 1.0 and the correct display_gamma as
> recommended. Is there a reason the developers could not supply an
> updated set of textures either as part of the standard package or as
> an adjunct package?
I guess so, yes.
On a side note: If you use POV-Ray to render images for use as
hightfields in another scene, you should render to a gamma of 1, so
comment out the 'assumed_gamma' line.
Ingo
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