Rune wrote:
> In POV-Ray 3.5 antialiasing was done after clipping the color to the range
> 0...1. In POV-Ray 3.6 this has been changed so that antialiasing is done
> before clipping.

That is not quite correct, POV-Ray 3.6 had clipping/gamma correction 
before antialiasing with non-adaptive aa (which contained a bug causing 
trouble with gamma correction).  3.6.1 fixed this bug by moving the 
clipping to the very end of the whole process (i.e. before the file 
writing) which among other things makes the HDR output much cleaner.

The whole issue has really been discussed a lot in the past i think but 
limiting it to the clipping and where to apply it does not really cover 
the whole issue sufficiently.  The problem occurs with all kind of 
non-linear tone mapping functions, no matter if gamma correction, 
clipping, film exposure (in MegaPOV) or anything else.  Since it is very 
common (and will become even more common with HDR output) to post 
process these tone mapping effects applying them before the antialiasing 
step is not always an option.

An idea i already drafted some time ago to allow efficient antialiasing 
with HDR scenes was to use an estimation of the final tone mapping used 
(the simplest would be an exponential function like in the film exposure 
feature) for the color averaging in the antialiasing code and to 
afterwards apply the inverse tone mapping function to restore the HDR 
color values.  The problem about the clipping in this concern is that it 
in not invertible - once clipped you can't restore the old color values.

I think clipping has only two purposes:

- to make the values fit into the range of image file formats.  This 
should always be done directly before file writing (and is now done this 
way in POV-Ray 3.6.1).
- to serve as an artistic/technical element (in fact the clipping itself 
is not the artistic element but the use of colors brighter than <1,1,1> 
for pure white).  What you complain about not being possible any more is 
exactly this kind of trick.  An additional clipping option might be 
useful now but it should be clear that this would be an additional 
artistic feature (just like reflection exponent and radiosity 
max_sample) and by default it should be turned off.

Christoph

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