Re: Understanding an ICC profile
Re: Understanding an ICC profile
- Subject: Re: Understanding an ICC profile
- From: Mark <email@hidden>
- Date: Fri, 8 Jun 2007 08:43:27 +0200
Thanks Clemens,
wow, there's a huge difference in the ITU-R BT 709-5 and ITU-R-709-2
primaries, I was not aware of that, and this is what was confusing me
the most.
I still have some question:
Is there some way to tell what standard a profile followed, can I
read somewhere that the Apple profile used ITU-R-709-2 ?
How does the Media White Point Tristimulus value (found on most ICC
profiles, if not at all) relate to a white illuminant (say D65) - if
at all? I'm confused again....
About gamma: yes, I was aware of the so called "simple" gamma
approximation. But how would an ICC profile describe the complex form
of gamma (I mean the formula) - I've never seen such a formula on an
ICC profile. Would that be described with a LUT?
And at last: what is the Chromatic Adaptation Matrix?
Sorry about all the newbie questions.
Cheers
Mark
On 08.06.2007, at 00:44, Clemens Beisch wrote:
Hi Mark,
the "sRGB Profile.icc" from Apple is not based on the "ITU-R BT
709-5" primaries
it is based on "ITU-R-709-2" primaries like the original "sRGB
IEC61966-2.1" (sRGB Color Space Profile.icm) profile
made by HP in 1998.
Here you will find the following primaries:
Red
X: 0.4361 ; Y: 0.2225 ; Z: 0.0139
Green
X: 0.3851 ; Y: 0.7169 ; Z: 0.0971
Blue:
X: 0.1431 ; Y: 0.0606 ; Z: 0.7141
sRGB dosn't has a gamma of 2.2!
It has a curve function that is close to 2.2 but not the same.
Function:
if ( x <= 0.03928)
y = y / 12.92
else
y = ((0.055 + x) / 1.055) ^ 2.4
This is the biggest difference between the original profile and the
one apple has made.
Take the same image and assign both profiles and you will see the
difference in the
very dark colors.
I hope this helps a little bit.
Best regards,
Clemens Beisch
Am 07.06.2007 um 23:11 schrieb Mark:
Hello list,
I'm currently reading some documents about color spaces. I have
the in formation about various color spaces in front of me and I
just thought, cool, now let me check how this looks like in reality.
So I opened up the /System/Library/ColorSync/Profiles/sRGB
Profile.icc profile.
To my surprise none of the values I have about sRGB can be found
on the profile - how can that be - can someone demystify this to me?
Here's what I have on sRGB (and that I mostly cannot find on the
profile):
1) Primaries / Phosphors (ITU-R BT 709-5)
R G B
X 0,64 0,30 0,15
Y 0,33 0,60 0,06
Z 0,03 0,10 0,79
2) White Illuminant (D65)
X 0,3127
Y 0,3290
Z 0,3583
3) XYZ to RGB Matrix
X 3,2405 -1,5371 -0,4985
Y -0,9693 1,8760 0,0416
Z 0,0556 -0,2040 1,0572
4) RGB to XYZ Matrix
X 0,4125 0,3576 0,1804
Y 0,2127 0,7152 0,0722
Z 0,0193 0,1192 0,9503
5) Gamma
2,2
Regards
Mark
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Mit freundlichem Gruß,
Clemens Beisch
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