Re: Black Point Compensation (BPC) - another track
Re: Black Point Compensation (BPC) - another track
- Subject: Re: Black Point Compensation (BPC) - another track
- From: Graeme Gill <email@hidden>
- Date: Wed, 11 Feb 2009 10:09:07 +1100
Wilma Kay wrote:
Continuing on this black point
compensation thing, but on another track, I had a close look at two ICC v2 sRGB
profiles on the ICC's www.color.org site. These MatTRC profiles are named sRGB_IEC61966-2-1_noBPC.icc and
sRGB_IEC61966-2-1_withBPC.icc.
I've taken a close look at both of these in the past, and in my opinion
they are both faulty, and not compliant with the ICC V2 spec.
The sRGB_IEC61966-2-1_noBPC.icc profile doesn't appear to represent
the colorimetic behaviour of the IEC61966-2.1 space.
According to the ICC V2.4 spec, table 20 page 19,
a display profile using a matrix should represent colorimetric
information. The IEC61966-2.1 space is defined as having
XYZ value of 0 for RGB 0, while the sRGB_IEC61966-2-1_noBPC.icc
profile produces a non-zero XYZ for a zero RGB.
The sRGB_IEC61966-2-1_withBPC.icc profile correctly produces
XYZ 0 for RGB 0, but its black point tag is inconsistent
with this, and contains a non-zero value. The causes
a problem in the dark areas when this profile is gamut
mapped to an output space, where the gamut mapping
is relying on the black point to be truthful.
I'm not sure what the idea of the sRGB_IEC61966-2-1_withBPC.icc
profile is in any case. BPC is a gamut mapping technique,
and is therefore only applicable to perceptual or saturation
intents. Given that a matrix profile can only contain
one intent, and it has to be colorimetic, trying to
create an sRGB matrix profile with BPC would seem to be
non compliant with the ICC V2 spec.
If such a profile is wanted, then it should be a Lut based
profile, so that the BPC effect can be encoded in the
perceptual table, leaving the colorimetic information
in the colorimetric table.
I note that neither of these profiles is consistent with the
standard sRGB profile released by Microsoft and HP
on the www.srgb.com website and also distributed
with various copies of Microsoft Windows (the "sRGB Color Space Profile.icm"
file) thereby sowing considerable confusion about what sRGB actually is.
They fix the D65 MWP problem of the
old sRGB profile that Adobe applications aptly ignore but bring up some
questions that don't seem to be well answered by the associated ICC committee descriptions
(I think ICC folks sometimes obfuscate instead of clarify).
I'm unaware of any practical issued with the official sRGB profile as distributed
by Microsoft. It behaves very sensibly as far as I can see. There is a subtle
incompatibility if it is used with a CMM that employs the "Wrong Von Kries"
chromatic adaptation to recover absolute colorimetric behaviour, but on
the other hand it gives better color reproduction for the much more
commonly used relative colorimetric intent than a display profile created
using "Wrong Von Kries".
So if, as consensus seems to have it
from the Forum, BPC is a CMM operation, how could these sRGB source profiles
have properties related to BPC and be so named. Why would I want to have a "flare" factor added to my images
with the ..noBPC profile? How would one
actually use the …noBPC profile?
I wouldn't recommend using either profile.
Graeme Gill.
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