Re: [Openexr-devel] f-stops and steps. How are they related?

[Chris Cox]

> First of all, I think it is important to understand why we work with a linear 
> representation of light levels.  The reason is that most processing is 
> mathematical, and the math only works correctly with linear data.

That is incorrect.   Math can work in any domain.
For example:  c = a*b works in the linear domain just as well as c = 
a + b works in the log domain.

And many image processing algorithms are designed to work on gamma or 
log encoded images (relying on the perceptual uniformity of the 
encoding).

In many cases, it is far more efficient (processing and storage wise) 
to use images encoded as perceived brightness (gamma near 2.5) rather 
than something related to photon counts (gamma 1.0).


>  The same functions are applied to every pixel, regardless of light 
> level, so therefore
> the mapping from perceived light level to numeric code should be linear.

That is also incorrect (and really bad logic).

How the function is applied has nothing to do with the data encoding.

The end to end (photons -> sensor -> ADC -> storage ->DAC ->  CRT -> 
photons) transfer function can be close to linear - but since most 
images need to have dynamic range compression applied before going to 
a display or print device, the result is usually far from linear.


> Next, it is important to understand how a CRT works.  The digital codes 
> (usually 8 bits per component) are sent to DACs (Digital to Analog 
> Converters) 
> which convert to voltage.  In a CRT, these voltages drive electron guns aimed
> at phosphors.  The issue is that phosphor does not react linearly.

I believe the dominating factor is the electron gun, not the phosphor.



> Remember that you may be dealing with images that are truly linear, 
> or you may 
> be working with images that already have gamma applied so they will look good 
> on the average display attached to the average operating system.

That's only one reason to apply gamma encoding.
Storage efficiency is another important reason.  Without gamma 
encoding you'd need more bits to get the same quality of appearance - 
and thus larger file size.



> There have been standards proposed for TIFF to document the embedded gamma 
> with a tag.

It is better to document the appearance with an ICC profile - which 
includes the  transfer function (generalization of gamma) as well as 
color information about the image.


Please read http://www.poynton.com/GammaFAQ.html


Chris