Building Printer/Output Profiles - Color Management for Photographers - page 229

disposal frequently will begin the editing process too soon after building
an output profile. Profile editing is time consuming and requires a great
deal of output be produced during this editing process. First evaluate the
output profile using the files discussed in Chapter 9 before considering
the use of a profile editor.
If a problem exists with a profile that affects both the preview and
output tables, fix the preview portion of the profile first. The reason for
this approach is that editing an output profile is based on both numeric
feedback and the soft proof generated by this profile. If the soft proof isn't
correct, editing the output table of the profile will be conducted using
an incorrect color appearance generated by the profile. Therefore, it is
always a good idea to decide what part of a profile needs editing, and
then decide if all the rendering intents share a problem. Only at this point
does it make sense to plan the order of the profile editing process.
Sidebar
A to B and the Alphabet Soup Confusion over Tables: I mentioned that printer/output pro-
files have a preview portion and an output portion within each ICC profile. These two areas are
controlled using two groups of profile tables. The ICC specifies the tables and the direction in
which they apply their color transformations in a somewhat confusing manner. Most profile
editors use the same methods to allow the user to select what portion of the table they will
edit. In Chapter 1 I discussed the differences between LUT or table-based profiles and matrix-
based profiles. All our output profiles are table-based. Each profile contains multiple tables. A
look-up table is used for conversions between the device color space (RGB, CMYK, etc.) and
the PCS (Profile Connection Space, usually LAB) for each rendering intent. A profile has two
directions to account for—data coming in for conversion to the PCS and data going out from
the PCS to the device color space of the profile (RGB, CMYK, etc.). These tables are referred to
as the AtoB and BtoA tags. There are six tags in each printer profile. The six tables are:
• AtoB0Tag (device to PCS, perceptual)—soft proof
• AtoB1Tag (device to PCS, colorimetric)—soft proof
• AtoB2Tag (device to PCS, saturation)—soft proof
• BtoA0Tag (PCS to device, perceptual)—output
• BtoA1Tag (PCS to device, colorimetric)—output
• BtoA2Tag (PCS to device, saturation)—output
The important item to keep in mind is the direction (device color space to PCS or PCS to device
color space). When the direction is from the device color space to PCS, that table ultimately
affects the soft proof. This is also known as the Forward transform or AtoB table. When the
direction is from the PCS to device color space, that table controls the output portion. This is
also known as the Inverse transform or BtoA table. When you edit the Inverse transform (BtoA),
both the soft proof and the output will be affected by the edit since both need to be updated.
When you edit the Forward transform (AtoB) only the soft proof will be affected. When you edit