Re: Spectro question - try again
Re: Spectro question - try again
- Subject: Re: Spectro question - try again
- From: Robin Myers <email@hidden>
- Date: Tue, 03 Sep 2002 14:51:53 -0700
- Organization: Robin Myers Imaging
email@hidden wrote:
>
>
James Reswick Jr. asks some interesting question and received several
>
answers - which - IMHO all missed the point he was driving at. This still
>
leaves a good question on the table. Let me restate his questions the way I
>
understand it, and see if there are any takers.
>
>
If the data (in the target) being sent to the output device for profiling
>
is CMYK numbers, and the data being captured by the measuring device is LAB
>
space numbers, how can the profile building software compare these two
>
different types of numbers in a meaningful way?
Andrew and Bruce are correct in stating that two profiles are necessary
to make a complete transformation. However, there are a few things that
perhaps are being overlooked.
To produce a printer profile a target is printed with known CMYK
combinations. These CMYK defined patches are read with an instrument as
L*a*b* (or XYZ) values. By knowing the requested CMYK values and the
resulting L*a*b* (or XYZ), the profiling software makes a CMYK-to-L*a*b*
(or CMYK-to-XYZ) table.
However, this is not useful yet since the color matching method (CMM)
takes L*a*b* (or XYZ) values and converts them to the necessary CMYK
values to get the matching colors. So, the CMYK-to-L*a*b* (or
CMYK-to-XYZ) table is inverted (usually through a bunch of
interpolations) to result in a L*a*b*-to-CMYK (or XYZ-to-CMYK) table.
This table inversion is the extremely important piece missing from the discussion.
Both of these tables (e.g. CMYK-to-L*a*b* and L*a*b*-to-CMYK) are stored
in the ICC profile where they are known as AtoB and BtoA tables (I've
left off the number designations deliberately folks, so please take it
easy on the napalm). As an example of their use consider that going from
an RGB image to a CMYK printer the CMM does the following
1. Converts the RGB image to L*a*b* values using the AtoB (e.g.
RGB-to-L*a*b*) table in the input profile.
2. Converts the L*a*b* values to CMYK printer values using the BtoA
(e.g. L*a*b*-to-CMYK) table in the output profile.
BTW, I've included the XYZ here for a reason. The ICC allows for either
L*a*b* or XYZ values to be used in the conversion matrices. There are
reasons for allowing both of them in the format, and even reasons to use
both within the same profile (I don't think the ICC allows this).
Robin Myers
_______________________________________________
colorsync-users mailing list | email@hidden
Help/Unsubscribe/Archives:
http://www.lists.apple.com/mailman/listinfo/colorsync-users
Do not post admin requests to the list. They will be ignored.