AN2621 Freescale Semiconductor / Motorola, AN2621 Datasheet - Page 4

• Manufacturer Part Number: AN2621
• Description: MPC8220i PF300 Image Coprocessor Operation
• Manufacturer: Freescale Semiconductor / Motorola

Color Conversion Unit (CCU)
Both tri-linear and tri-nonlinear interpolations can be implemented. The algorithm allows non-uniform grid
point spacing (that is, the table indices must be located on a three-dimensional grid, but the grid can be either
irregular or regular, and the table need not be of equal size in each of the three address dimensions). This
allows selection of grid points to coincide precisely with certain important colors including the 16
“Neugebauer” primaries: white paper, C, M, Y, K, CM, CY, CK, MY, MK, YK, CMY, CMK, CYK, MYK,
and CMYK. It is important to locate at least some of these colors on table grid points so that (a) they are
interpolated with zero interpolation error, and (b) the color gamut of the printer is fully utilized by assuring
that the “tent poles” of the interpolating table are located at the extremes of the interpolation space. The use
of irregular grids also allows for finer table granularity in regions of color space where the color conversion
function has the most curvature. This reduces possible interpolation error while maintaining a relatively
small table size.
Following the three-dimensional lookup and interpolation each output channel is further processed by its
unique one-dimensional look up table to affect tonal corrections prior to piping the converted data to the
next processing block, the Screening Unit (SU).
2.2
Each of the output colors is represented by a sparse three-dimensional (meaning that they are addressed in
three-dimensions) color lookup table that can be addressed by 8-bit values in each of the three independent
variables that constitute its “input” space. The color lookup tables are stored in system RAM. Not all of the
256
“sparse” table. Instead, each of the color tables consists of 8-bit colorant output values for each of M table
entries. The table entries are indexed in each of the three input dimensions by indices (I
have values (1, 2, … N
CCU is to receive a triplet of 8-bit RGB values, to find the eight table entries that surround it, to get the 8-bit
color values of each of the surrounding table entries, and to compute the interpolated result for the RGB
triplet.
Dedicated lookup tables are supplied to contain the indices and the remainders for each possible input data
value in each of the three input dimensions. To effect linear interpolations the tables of indices are
programmed with the index number corresponding to the next lower input (address) value that has a table
entry, and the tables of remainders are programmed with the distances between the input (address) value and
the next lower input (address) value that has a table entry. The remainders are expressed as the fractional
distances between the input (address) value of the next lower address with a table entry and the one next
higher with a table entry encoded as 8-bit values. To facilitate the calculations, the remainder table is
programmed with the bit-wise complement of the computed remainder (contents = 255 – remainder). An
example of typical remainder and index tables are shown in Table 1.
4
Address
3
0
1
2
3
4
possible inputs to the table are represented in the table, however, which is why it is referred to as a
Computational Details
Index Table
Table 1. Example of CCU Index and Remainder Tables for Linear Interpolation
Contents
1
1
1
2
2
i
). The total number of entries in the table is M (= N
Remainder
170
MPC8220i PF300 Image Coprocessor Operation
Freescale Semiconductor, Inc.
85
28
0
0
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Remainder
Contents
Table
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255
170
255
227
85
Start of range of addresses associated with first index
One-third distance through span
Two-thirds of distance through span
Start of range of addresses associated with second table entry
One-ninth distance through span
Comments
1
× N
2
× N
3
). The function of the
1
, I
2
, and I
MOTOROLA
3
), which