39586 22MAR88-1007 CoCo 3 Graphics
From: RONMUMMAW To: TONYZAMORA
Sorry it has taken me awhile to respond. I would assume you have
already gotten some information from others on MAC picture files.
I know Mike Andrews is very knowledgeable and helpful in this area.
He is the one who gave me the information I have. I’m afraid I can’t
tell you much. I know little or nothing about the first 768 bytes of
a MAC picture file. The picture’s name begins in about the second
or third byte of the file. The actual picture data starts in byte
769 of the file. Beginning with byte 769, if you find a byte value
greater than 128, subtract its value from 257 and that will tell you
how many times to copy the next byte onto the screen. If the byte is
less than 128 then add one and copy that many of the following bytes
from the picture data to the screen. For example, suppose starting
with byte 769 you found these values;
248 255 003 100 152 018 012 …
The first value was > 128 so 257-248=9 indicating that you need to
put 9 255’s (the next byte) on the screen. The next byte is a 3,
less than 128 so 3+1=4 so the next four bytes, the 100, 152, 018 and
the 12 would be put into the next four bytes. I might add that a
set bit(1) produces a lit pixel on the COCO and it must be a black
pixel on the MAC because you get a negative image on the COCO if you
don’t compliment a byte before storing it to screen memory.
Hope this is helpful to you. Good luck and sorry I can’t tell you
more. Let me know how it goes.