![](http://datasheet.mmic.net.cn/30000/MC68322FT16_datasheet_2368702/MC68322FT16_144.png)
Graphic Operations
12-10
MC68322 USER’S MANUAL
MOTOROLA
12.6.4 Halftone Companion Tables
A halftone bitmap is automatically replicated in both the X and Y dimensions. For scanline
graphic orders, the task is complex since each scanline run describes an individual area on
which to be operated. For example, on 0
° pages after the displacement required by the bit
string specifier has been applied, the proper location in the halftone bitmap must be
calculated. For the 16-bit bit string specifier, this task is relatively straightforward and can be
carried out with a minimum number of cycles. For the 32- and 48-bit specifiers, the task is
considerably more complex because of the much larger displacement values associated
with them. The task can be accomplished, but at a significant cost in overhead cycles. To
reduce the penalty for the larger bit string specifier formats, a halftone companion table is
employed. This eliminates virtually all overhead cycles in return for only slightly higher
memory storage requirements within typical scanline tables.
The halftone companion table contains a list of corresponding halftone specifiers (similar in
composition to bit string specifiers) for each 32- and 48-bit bit string specifier in the scanline
table. There is no corresponding halftone specifier for 16-bit bit specifiers because they are
halftoned without assistance. The halftone specifiers use displacements instead of absolute
values. This minimizes the number of halftone tables to one per halftone pattern for each
halftoned character. Thus, if a character is halftoned, only one halftone table must be
constructed, regardless of the number of times the character is used. The same halftone
table for a particular character can be used with different halftone patterns if the dimensions
of each halftone bitmap are identical.
Each field of a halftone specifier corresponds to a similar field in the 32- or 48-bit bit string
specifier. As illustrated in Figure 12-6, the 32-bit halftone specifier contains two fields—a
halftone horizontal movement (HDX) and a halftone run length (HRL). HDX corresponds to
the 32-bit bit string specifier’s DX field and HRL corresponds to the RL field.
Figure 12-6. 32-Bit Halftone Specifier Format
HDX and HRL are defined as follows where HW is equal to the warp of the target halftone
bitmap:
HDX = DX mod HW (DX
≥ 0) V [HW – (– DX mod HW)] mod HW (DX < 0)
HRL = RL mod HW
+0
BYTE ADDRESS
OF WORD
+2
HRL
HDX
15
14
13
12
11
10
9
8
7
65432
1
0
NOTES:
HRL = Halftone Run Length (unsigned)
HDX = X Dimension Displacement—Horizontal (unsigned)
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Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
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