[Openicc] New options on the mainline

[Graeme Gill]

Kai-Uwe Behrmann wrote:
> Am 21.01.08, 14:31 +1100 schrieb Graeme Gill:

>> It depends how much the calibration is an add-on, or whether it is
>> fundamental to the pipeline. If one were starting with a blank sheet,
>> then really there should only be one curve needed for each channel.
>> More interesting questions are things like how the target response
>> should be established. What response attribute, and should it
>> be relative or absolute ? The former is good for "pretty pictures",
>> and getting the maximum gamut for a given printer. The latter allows
>> for matching different instances of printers, and is better things like
>> proofing, but the trade off is reducing the target to be the common
>> denominator gamut.
> 
> Would the later mean to target at the proofing device?

Yes.

>> My current favourite idea for linearizing each channel would
>> be to measure the L*a*b* for a step wedge test chart, and then
>> linearize with respect to the distance along the response locus.
>> This evens out the delta E change for a change in the channel input.
> 
> Yes, just correct for lighness deviations and ignore the CIE*ab 
> components.

No, that's not my suggestion. Some colorants (ie. Yellow)
do not cause big changes in L*, most of the change is
in the other components. Throwing away a*b* would
reduce the signal to noise ratio of the readings to
an unacceptable degree. The full L*a*b* should be
used to linearise with respect to delta E.

>> One simply links the target response curve with the inverse of
>> the measured response curve to arrive at a device to device
>> per channel transform that is the calibration curve. Often
>> it's better to store the device response as the "calibration" and
>> compute the actual transform on the fly. This permits having
> 
> You refere here to the relative versus a absolute calibration target?

No, I'm referring to the process of turning a calibration target
and readings from a calibration chart into the final device to device
calibration mapping.

Graeme Gill.