[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.
Cheers,
b&
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