Am 11.09.2011 15:00, schrieb Warp:
>    The main reason why assumed_gamma 1.0 ought to produce more accurate
> results has to do with a physical concept called irradiance. Despite the
> fancy name, irradiance is simply the amount of energy that a certain amount
> of light carries. This is measured in watts per square meter (in other
> words, how many watts of energy a certain amount of light carries to
> each square unit of a surface).
>
>    Now, when light hits a surface, some of it is reflected. How much is
> reflected and to which direction is an extremely complicated function,
> but for the sake of simplicity let's assume the following scenario:
>
>    A fully white (rgb 1.0) light source, and a fully white (rgb 1.0) diffuse
> surface, oriented so that it's facing at 60 degrees from the light source.
> Due to this orientation the surface will reflect exactly 50% of the incoming
> light to all directions (including the camera, of course).
>
>    (The reason why it's exactly 50% at 60 degrees is due to the cosine law,
> that says that the amount of reflected light is proportional to the cosine
> of the angle of the incoming light, and cos(60) = 0.5.)
>
>    In other words, assume that the light source were emitting 10 watts/m^2
> of light. The surface would hence emit 5 watts/m^2. Hence the brightness
> of this surface would be 5 watts/m^2 (ie. exactly half of that of the light
> source).
>
>    Now, how do we *draw* this surface? The problem is that the relationship
> between irradiance and the brightness perceived by the human eye is far from
> linear. In other words, the surface might be emitting 50% of the incoming
> light, but it will not *look* half as bright as a fully-lit white surface.
> In fact, rather than looking 50% gray, it will look approximately 73% gray,
> because that's how the human eye perceives it.
>
>    In other words, when we draw this surface, it has to *look* like a
> 73% gray rather than a 50% gray, because that's the perceived brightness
> of half of the full irradiance.
>
>    That is what the assumed_gamma 1.0 is doing. It's the reason why a "rgb 0.5"
> will look about 73% gray with that setting (rather than 50% gray).
>
>    Hence if you render for example a diffuse sphere like this, it ought to
> be more accurate in terms of brightness than with assumed_gamma 2.2. (The
> parts of the sphere that are facing at 60 degrees from the light source
> should look about 73% gray, rather than 50% gray, if the physics are correct.)
>
>    However, this causes a practical problem when specifying colors. Namely,
> do you want the color definition "rgb 0.5" to mean "half of full irradiance",
> or do you want it to mean "50% gray"? With assumed_gamma 1.0 it will mean
> the former (while with assumed_gamma 2.2 it will mean the latter).
>
>    As said, however, "half of full irradiance" corresponds roughly to about
> 73% perceived brightness (compared to the brightness of "rgb 1.0"). In other
> words, "rgb 0.5" will *look* significantly brighter than half-gray.
>
>    The relationship between irradiance (the absolute amount of energy that
> is carried by light, measured in watts/m^2) and the brightness that is
> *perceived* by the human eye is roughly logarithmic, and the exponent is
> approximately 2.2. (The estimation is probably very rough, though.)
>
>    Now, displays also have a non-linear relationship between raw pixel values
> and the irradiance emitted by those pixels. In other words, a pixel with
> values (128,128,128) will not emit 50% of the irradiance of a fully-white
> pixel (255,255,255). Instead, this relationship is also logarithmic, with
> an exponent of, curiously (although I don't know if coincidentally), 2.2
> in most systems.
>
>    What this means is that a pixel with value (128,128,128) will *look*
> approximately 50% gray (even though the monitor is only sending about 22%
> of the light, as measured in watts/m^2). This is actually extremely
> convenient when dealing with bitmaps: There's an almost linear relationship
> between pixel values and perceived brightness.
>
>    This is the reason why image manipulation programs will use (128,128,128)
> for half gray (because it certainly looks half gray, which is convenient).
>
>    This is also the explanation of the "assumed_gamma" keyword: It is assumed
> that the color was specified in an environment with gamma 2.2 (which is the
> most common). In such an environment (128,128,128) does look 50% gray, and
> hence if you use "assumed_gamma 2.2" in povray, the equivalent "rgb 0.5"
> will also look 50% gray.
>
>    The problem is that if you try to do this when assumed_gamma 1.0 has been
> specified in povray, the result will be completely different. In that case
> the colors are assumed to be linear (in terms of irradiance).
>
>    As said, with assumed_gamma 1.0 "rgb 0.5" does not mean "50% gray", and
> instead it means "half of the full irradiance" (which is approximately the
> same as 73% gray).
>
>    If you want "50% gray", you need to pre-gamma-correct the color. Basically,
> you need to calculate pow(0.5, 2.2), getting you about "rgb 0.218", which
> would be about 50% gray.
>
>    Of course there are still some problems left. Most prominently, if you
> want a gradient from one color to another that *looks* linear (rather than
> being linear with respect to irradiance), there's currently no easy way to
> achieve that, when using assumed_gamma 1.0. There are many other situations
> as well, related to color maps and other such maps. This can make designing
> textures a bit difficult.
>
>    An easy way around this problem is to simply use assumed_gamma 2.2 and
> accept that the end result might not be physically as accurate. After all,
> the human brain is quite forgiving of such small inaccuracies, and nobody
> will notice in practice.
>

While what you write about irradiance is true I completely disagree with 
all conclusions you draw from this.
The main misconception seems to be that you assume there is something 
like a color that is the inherent property of an object. What the color 
within a pigment statement actually describes is the way (diffuse, 
simplified I know) light is reflected. And BTW you always seem to assume 
white light while e.g. the color from a light bulb is far away from this.
And all I have to say about "small inaccuracies" and "nobody will notice 
in practice" is that my experience simply shows the opposite.

But I believe you and I know it is possible to create great and 
realistic looking scenes with POV-Ray following your advice but I am 
also quite sure there is a lot more fiddling with colors and settings 
involved to make it *look* right instead (as I propose) trying to feed 
POV-Ray with "real-world" values and simply let POV-Ray *calculate* it 
right. This has also the great advantage (I am very lazy) that I can 
reuse my own object within different scenes and completely different 
lighting setups and they look always right and as expected.

-Ive

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