4 - Forth Interest Group

none were found for bit accessing, so we use C@ L and C ! L.
C@L expects paragraph and offset on the stack, and returns
a byte. C ! L expects a byte, paragraph, and offset, and stores
the byte.
For navigation (see code at the end of the article), we
define three SEGMENTS: HEADER. SEG, IMAGE. BOT . SEG,
and IMAGE. TOP. SEG. IMAGE. BOT . SEG holds image rows
879 through 440, while IMAGE. TOP. SEG holds rows 439
through zero. (Recall that Paintbrush reverses the order of the
image rows.)
To get an image of the Paintbrush .BMP file into memory,
we use GET-IMAGE. First, GET-IMAGE uses HS-Forth's
OPEN-Rto find the file whose file-spec address is passed, and
OPEN-R obtains a file handle. The handle is stored on the
return stack.
ymax
Using HS-Forth's READH command, GET-IMAGE then
fills the respective memory areas with a file's contents.
READH expects a memory paragraph segment, offset, file
handle, and number of bytes to read. It returns on the stack
the number of bytes it read into the memory location.
GET-IMAGE "anchors" the image at IMAGE. BOT . SEG. In
other words, after the first half of the image is loaded into
memory, the location of the second half is computed relative
to the first. (Math for the last segment takes advantage of the
fact that eighty divided by sixteen equals five.) CLOSEH
consumes the file handle as it closes the file.
PUT-IMAGE works analogously to GET-IMAGE. Neither
command tests the header or image, but simply moves the
data. Both commands need rewriting for better error han-
dling, if needed.
The image is accessed relative to its first row, which
happens to be its last row in memory. The arcane method of
calculating the paragraph's segment of the first row of the
image is shown next in the code. Fetching the
IMAGE. BOT . SEG paragraph starts the process. Indexing to
the very first image row requires moving the pointer down
879 rows. One VAR receives the segment value, another
receives the offset.
Mappingx and y coordnates to the image byte containing
the pixel uses the command XY->ADR. Notice the use of
high-level words to write a machine language command
using the HS-Forth OPT" command. XY->ADR converts y
into a paragraph segment by multiplying y by minus five and
adding the result to the location of row zero (which is last in
memory).
Dividing x by eight converts the pixel number into the
byte number in the row. Adding that to the offset indexes into
the image row. (For several reasons, the beginning offset of
each of the image rows is zero, so some of these steps are
superfluous for the images we are accessing.)
The remainder of x divided by eight points to the pixel,
as shown below.
bit position 7654321 0
The above is performed inside the PIXEL command by
8 MOD (obtaining the bit position) and the data structure
CLEAR. BIT. CLEAR. BIT provides a mask for ANDing the
bit clear (i.e., to zero), thus forcing the pixel lit. In our case,
Paintbrush thinks a zero pixel is lit (black) while a pixel equal
to one is white. (PIXEL performs no range checking, so
either make your own, or be careful.)
O+ xmax
ymax
The PIXEL? command works like PIXEL except a flag
is returned instead of the pixel being set. PIXEL? selects a
bit, then tests it. When the pixel tested is zero, we say it is lit,
or true. Otherwise, PIXEL? returns false.
Finally, we have filename buffers and (rather uninterest-
ing) test code. TEST1 fills an entire Paintbrush page with
black pixels in about twenty-five seconds on my system.
O+ xmax
ymax
TEST2, TEST3, and TEST4 draw lines. TI sets the filenames
buffers and loads an image. GBS is set to a blank page
previously created in Paintbrush, while PBS is set for keeping
results.
These tests were used to verify PIXEL and, oddly
enough, GET-IMAGE and PUT-IMAGE. Strangely, I still
haven't completely figured out HS-Forth's SEGMENT com-
November 1993 December 28 Forth Dimensions