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Graphic Operations
12-6
MC68322 USER’S MANUAL
MOTOROLA
Scanline operations arise quite often from two sources. The first occurs from outline fonts,
which describe the outline of a set of characters via splines, lines, and arcs. The outlines are
scaled for the preferred point size through software algorithms. The result is a set of scanline
endpoints that must be filled to create a solid character. The second occurs from vector
images such as wire-frame diagrams, which are entered as either a series of line drawing
graphic orders or as a previously generated bitmap. Since these vector images contain a
high percentage of white space, they typically require less storage space when described
as a series of scanline operations. Scanline transfers operate on a destination bitmap and
specify a halftone bitmap to render grayed or patterned images. No source bitmap is
involved in a scanline transfer.
12.6.1 Scanline Tables and Bit String Specifiers
A scanline table consists of a series of bit string specifiers representing an image that has
been compressed using run-length encoding. For example, bitmap fonts can be converted
into this format to reduce memory requirements for font storage. A scanline table can also
be used to efficiently represent line art and filled polygon shapes.
Each bit string specifier describes a displacement along with a run-length encoded definition
of a graphics operation for the RGP to perform on a single scanline. The MC68322 supports
three different sizes of bit string specifier formats: 16-, 32-, and 48-bit. The smaller formats
help to reduce memory requirements when short displacements or scanline runs are
required. The larger formats allow any pixel in a bitmap to be reached with a single specifier.
Three bit string specifiers are supported:
16-Bit—Conditionally moves to the next scanline, goes a short distance left or right from
there, and then draws a line that is a maximum of 63 bits long.
32-Bit—Moves vertically up to three scanlines, goes a large distance left or right from
there, and then draws a line that is a maximum of 4,095 bits long.
48-Bit—Moves a very large distance in both X and Y dimensions and then draws a line
that is a maximum of 16,383 bits long.
All bit string specifiers are multiples of 16 bits and must always be located on word memory
boundaries (0 mod 2 byte addresses). Each bit string specifier consists of an ID field, an
unsigned run length (RL) field, and a signed displacement (DX, DY, or DZ) field. Figure 12-3
illustrates the three types of bit string specifier formats.