Re: Matrix profile with negative "Red colorant tristimulus"
Re: Matrix profile with negative "Red colorant tristimulus"
- Subject: Re: Matrix profile with negative "Red colorant tristimulus"
- From: Graeme Gill <email@hidden>
- Date: Wed, 17 Sep 2008 11:04:52 +1000
Mark wrote:
I opened the profile with ColorSync Utility and see the it has
negative values for the "Red colorant tristimulus" Y (-0,005) and Z
(-0,000).
Hum, this is wrong - right? How can the primaries have negative values?
A matrix is really a transformation into another 3D coordinate system,
and it's quite possible for that system to have primaries that are
at imaginary, non-real world coordinates (the XYZ coordinates do this
themselves). If the RAW RGB values from your camera have been manipulated
by various processing stages in the camera, it is quite possibly that
the best fitting matrix is one that has primaries that are non-real,
and even have -ve XYZ values.
Unfortunately the ICC format didn't anticipate this situation, and while
the XYZ values are encoded as signed values, it forbids XYZ values
being negative, and some CMM's may enforce this, making the correct
fitting profile matrix illegal as far as they are concerned. Other
CMM's take a more pragmatic view, and allow -ve XYZ values.
Graeme Gill.
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