[argyllcms] Re: Profile input white not mapping to output white

  • From: Ben Goren <ben@xxxxxxxxxxxxxxxx>
  • To: argyllcms@xxxxxxxxxxxxx
  • Date: Sat, 24 Nov 2012 18:47:54 -0700

On 2012-11-24, at 5:12 PM, Graeme Gill wrote:

> How do you know what is "pure" white and black input, without making
> assumptions about the device ?

Well, the assumption I'm making is that the camera is going to capture a 
smaller range of values than exists in the original scene -- sometimes a much 
smaller range, as is the case here where I'm shooting against a white seamless 
background and intentionally overexposing the background to force it to render 
as pure white. Similar situations happen all the time in outdoor photography.

I can't even imagine any photographic situation where you'd have all three 
channels saturated / clipped / maxed out in the original where you'd want those 
areas to get rendered other than as pure white. Indeed, one of the big problems 
with RAW processing is where there's clipping in one or two channels and the 
result gets rendered as something other than pure white. A great many lines of 
code have been devoted to gracefully dealing with such situations, but the only 
desirable options are generally something between pure white and the 
surrounding colors -- and all three channels saturated always gets rendered as 
pure white.

For a scanner or for a camera operated on a carefully-controlled copy table, I 
can understand how you might want to be wary of extrapolating beyond the gamut 
of the target. But for a camera off the copy table, I really don't think 
there's any valid assumption other than that the maximum possible input value 
should map to the profile's white point. Whatever the absolute color of that 
object, the camera saw it as pure white, and I think the profile should see it 
that way, too.



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