Kenneth wrote:
> OK. So here are the facts so far: to make my POV "linear" grey band test
> reproduce in POV's preview window with a correct , real-world appearance,
> and using the recommended assumed_gamma of 1.0, then my grey values need to
> be specified in a NON-linear way--i.e., instead of <.5,.5,.5> being used to
> represent "half as perceptually white" as <1,1,1>, the values instead need
> to be raised to the power of [whatever system gamma I've chosen.]

Be careful with the word "linear" - what you mean is "linear to 
perceived brightness" (right?), whereas I (and again, I guess most other 
people with some background in the field) say "linear" when I mean 
"linear to physical light intensity". That could lead to 
misunderstandings - a linear relationship is always between two 
quantities, and saying that a quantity is "linear" without specifying to 
what other quantity it is linear only works as long as there's an 
implicit agreement about what that other quantity is. Obviously there's 
no such agreement between us two.

> Ah, but what about lighting?
> [...]
> Everyone, take a look at Ard's assumed_gamma of 1 image--that's an order!!--
> or just run such a test.  I'd really like to know how each of us perceives
> the illumined sphere...how the light reacts with the curving surface.

I just did the math, and if I assume that 1. the sphere is an ideal 
diffuse reflector and that 2. Ard's image is encoded with a gamma of 2.2 
(it is not specified in the PNG), I come to the conclusion that Ard's 
upper image (with assumed_gamma 1) is physically correct, while the 
lower one (without assumed_gamma) isn't.

If the lower image looks more aesthetically pleasing to you, that's 
probably because ideal diffuse reflectors aren't that common in the real 
world, so...

> And what we each do to correct for it...if in fact we DO correct for
> it. (I do...by using assumed_gamma of 2.0! Ard uses a different
> method.) But others may not do so at all...???

...you may change the sphere's reflectivity properties using the 
"brilliance" keyword. With "finish { ambient 0 diffuse 1 brilliance 2 }" 
(and assumed_gamma 1), a similar image to Ard's lower one is achieved. 
(I'm too lazy now to look up what brilliance does exactly, but it may be 
that  "brilliance <display_gamma>" exactly reproduces that image.)

What you're trying to do with your "assumed_gamma 2.0" is changing 
POV-Ray from taking linear-to-intensity numbers to taking 
linear-to-perceived-brightness numbers because these seem more natural 
to work with to you. However, assumed_gamma affects the output end of 
POV-Ray, not the input end, so you're also forcing POV-Ray to do its 
internal calculations with the linear-to-perceived-brightness numbers, 
and that just doesn't lead to physically correct results.

Inputting linear-to-intensity numbers makes POV-Ray do its calculations 
in a physically correct way, and using assumed_gamma 1.0 makes POV-Ray 
encode its results in the output file correctly (with your display_gamma 
value). As Christoph already said, don't misuse assumed_gamma to adjust 
your rendered image's brightness to a visually pleasing value. The 
proper way of doing that is changing the objects and light sources in 
the scene itself.

  -Christian

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