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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