On 1/27/2011 1:03 PM, Ive wrote:
> Am 27.01.2011 21:20, schrieb stbenge:
>> So, what would be a safe value to use? I heard that HDR images support
>> 32 bits for each color channel.
>
> Err, OpenEXR and Radiance do use a 16bit *floating point*
> representation. There is also TIFF that would support up to 32bit float
> HDR images (and more!) but this is currently not supported by POV-Ray.

Well, in the case of Rune's illusion.inc, I /could/ implement different 
default values for both OpenEXR and HDR...

> The problem is: to say what maximum is possible is not so simple. E.g.
> you can represent light intensity of 50000:1 but cannot represent at the
> same time details within dark regions with enough precision.

Ack, I didn't think about that :(

> So a value of 4,294,967,295 would work,
>> right?
>
> No!!!
>
> My suggestion of using 10000 is a kind of educated guess (to find the
> exact value one had to look up how many bits are used for mantissa and
> exponent within the 16 bit (half) floating point format as used by
> OpenEXR in compare to the 32bit float format to find out what maximum
> value can be used without starting to loose precision.

I think that might be a little beyond my capabilities ATM, but could it 
be done in-POV without testing every single pixel?

> But this would still not mean that you can cover high dynamic ranges
> like 50000:1 AND preserve precision! Thats the kind of nature of
> floating points ;)

Can I at least assume 10000 would be a visually lossless value under 
most circumstances?

> So sorry, there is no value that will work in all cases until you
> examine the image itself and use the maximum that is used there.

OK, so if a person *knows* what the brightest region in their scene is, 
can a good value be derived from that knowledge? Especially if the 
bright spots are below 10000?

This subject is more complex than I thought it was :S

Sam

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