Re: [Openexr-devel] magic numbers

[Peter Kümmel]

Peter Kümmel wrote:
> Florian Kainz wrote:
>  > Mapping pixel value 0.18, or middle gray, to 0.883 times the
>> monitor's maximum brightness means that diffuse white objects
>> do not map to the monitor's maximum brightness; there's some
>> headroom left for specular highlights, which are often much
>> brighter than diffuse white objects.  As I mentioned in my
>> earlier mail, the decision to display images this way is
>> pretty arbitrary.
> 
> It was 0. 0 882: sqrt(x)/x=middle with x=128.
> 
> I've figured it out, where 0.18 comes from:
> 
> sqrt(x)/x = x^(-1/2) = middle
> x^(1/2) = 1/middle
> x = (1/middle)^2
> 
> Because of this using 0.18 as middle value is
> equivalent to assume a contrast of only 1:31.

The 0.18 is the rounded value of a contrast
of 5 f-stops:

sqrt(32)/32 = 0.177

> 
> Seems this is a more realistic contrast of
> current monitors. Or I am completely wrong?
> 
> Peter
> 
> 
> 
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