Folks:
Before I bang my head against the wall too much, I'd like to make sure I'm
approaching this task correctly.
I have a digital elevation file consisting of 16 bit 2's complement data. I
first determine the position of the data for the desired location, then I do a
binary read.
put getAFilePath("files/dems/") into fPath
put fPath&"ETOPO5.DAT" into theFile
open file theFile for binary read
if the result <> empty then
answer "Couldn't open DEM file"
exit readDemTest
end if
--Then i calculate the byte position where the data should be, then read it
using:
read from file theFile at thePos for 8 --read 8 chars beginning at
thePos (I should only need 2 to get the value)
put it into theDepths
put byteToNum(byte 1 of theDepths) into b1
put byteToNum(byte 2 of theDepths) into b2
--Then, because I don't remember the byte order, I'm trying each one to see
which gives me sensible results
--It should have a sign bit, so only 7 bits of the most significant byte should
count, and the first one is the sign bit.
if b1 > 127 then --sign is negative. numbers from 128 to 255 should be
negative
put -256+b1 into msb
put 256*msb - b2 into depth1
else --sign is positive
put b1 into msb
put 256*msb + b2 into depth1
end if
--try the other way
if b2 > 127 then
put -256+b2 into msb
put 256*msb - b1 into depth2
else
put b2 into msb
put 256*msb + b1 into depth2
end if
What I'm wondering before I spend more time on this, is whether the livecode
commands are the correct ones for this. I have old C code that does some of the
same processing, but I can't run it now to test.
Thanks,
Bill
William Prothero
http://es.earthednet.org
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