SYNOPSIS:
    backingstore: Eccet can use the depth and class fields as
                  additional storage for operations that need
                  separate target fields.

DESCRIPTION:
    Eccet can use the class and depth fields to store results of
    operations that cannot run in place (in situ).
    Several filters (all filters whose name begins with FILTB)
    use this backing store as the target for their operations.
    The backing store can be manipulated with the commands
    COPY_GREYTOBACK, COPY_BACKTOGREY, XCHG_GREYANDBACK,
    MIX_BACKTOGREY and ERASE_BACK.

SEE ALSO:
    FILTB_GAUSSMEDIAN, FILTB_MEDIAN, FILTB_NLC, FILTB_NLC2,
    FILTB_NLD, FILTB_NLD_ADD, FILTB_NLD_POSADD, FILTB_NLD_NEGADD,
    COPY_GREYTOBACK, COPY_BACKTOGREY, XCHG_GREYANDBACK,
    MIX_BACKTOGREY, ERASE_BACK
    class, depth, grey, norm, field
