[argyllcms] Re: DeviceLink Profile refining

marcel nita wrote:
> Hello all,
>
> I have tested device link improvement many times. I am using an Epson
> Stylus Pro 4800 and EyeOne Pro with Contract Proof Paper and the IT8
> target.
>
> As the input profile I am using ISO Coated and the printer profile was
> created with Profile Maker.
>
> The results are always the same: the white point and yellow are
> increasing after the first step( the other primaries and other ink
> combinations are really improving ).
>
> Basically, the process is:
>    - print the IT8 target ( with the device link profile );
>    - measure the target;
>    - create the abstract profile( using a previous correction profile
> if not the first step );
>    - re-create the device link profile using the abstract profile: ISO
> Coated -> Abstract Profile -> Printer Profile.
>
> Could you please tell me if there are there any known problems with
> refining or if you have an explanation for my results? I do not expect
> extraordinary results from the process, I do not expect it to do magic
> things, but I think at least they shouldn't that get worse.
>
> Also, in tweak/refine.c, in function PCSp_PCSp(void *cntx, double
> *out, double *in), would it be correct to test(something like dE) the
> new correction and the compund correction against the corresponding
> target reference data just to see which is getting too far from
> reference? Then the correction with the smaller delta will be
> considered( think it as a way to ignore the previous abstract profile
> for certain patches, that are getting worse ). I am still not sure if
> this approach would be correct, so it would be very helpful if someone
> can correct me if I am wrong( preferably Graeme :) ).

The error which needs to be compensated is always the difference between
the actually measured values of the target patches (printed with the
either the original or with the last, previously refined profile) and
their corresponding reference values. And if the measured values differ
significantly from the reference, then this simply implies that the B2A
table of the original (or the previously refined) profile is inaccurate,
or that the printer's behavior has drifted significantly since the
previous refinement, and the newly established correction profile will
attempt to fix this (at least partially; it may take several refinement
rounds until convergence).

On the 2nd and on subsequent rounds, the old correction profile is
merged into the new one, since the new correction profile is intended to
be used in conjunction with the _original_ device profile only (i.e. you
will always apply just fixN.icm + device.icm (e.g. when you run
icclink), but not the complete chain of profiles fixN.icm + ... +
fix2.icm + fix1.icm + device.icm).

Btw, are the colors you're complaining about actually in-gamut on the
proof printer (or on the used paper)? There is of course no way to fix
out-of-gamut colors.

Regards,
Gerhard


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