  Color separations (most often cyan, magenta, yellow, and black) are commonly
used in printing to reproduce multi-color images. For mechanical reasons, these
color separations are generally not perfectly aligned with respect to each
other when they are rendered by their respective imaging stations. This
phenomenon, called color plane misregistration, causes gap and halo artifacts
in the printed image. Color trapping is an image processing technique that aims
to reduce these artifacts by modifying the susceptible edge boundaries to
create small, unnoticeable overlaps between the color planes. We propose three
low-complexity algorithms for automatic color trapping which hide the effects
of small color plane mis-registrations. Our algorithms are designed for
software or embedded firmware implementation. The trapping method they follow
is based on a hardware-friendly technique proposed by J. Trask (JTHBCT03) which
is too computationally expensive for software or firmware implementation. The
first two algorithms are based on the use of look-up tables (LUTs). The first
LUT-based algorithm corrects all registration errors of one pixel in extent and
reduces several cases of misregistration errors of two pixels in extent using
only 727 Kbytes of storage space. This algorithm is particularly attractive for
implementation in the embedded firmware of low-cost formatter-based printers.
The second LUT-based algorithm corrects all types of misregistration errors of
up to two pixels in extent using 3.7 Mbytes of storage space. The third
algorithm is a hybrid one that combines LUTs and feature extraction to minimize
the storage requirements (724 Kbytes) while still correcting all
misregistration errors of up to two pixels in extent. This algorithm is
suitable for both embedded firmware implementation on low-cost formatter-based
printers and software implementation on host-based printers.
