I sent this message Saturday, and it just came back to me as bounced
(because the list-server was off-line).
So here it is again - with an addendum ...
Geoff Canyon wrote:
On Jun 24, 2005, at 4:26 AM, Jon wrote:
With all due respect, Jim, if you are trying to do even simple math
on a large array of numbers (like computing a histogram of image
data), Rev is simply too slow to use. It has nothing to do with
data structures: it has to do with slow pCode trying to run tight
loops.
As stated previously, I cannot accept this statement/conclusion
without seeing the code in question. There are simply too many ways
to write slow code to assume that Rev can't do a task based solely on
a very general description -- unless the description involves real
time first-person gaming ;-)
Well, the problem Jon mentioned (histogram of image data) is pretty well
defined. To be usable, you need to do it for reasonable size pictures in
less than 1-2 secs. (where "reasonable" is defined to be whatever your camera takes, since
that's the data you're most likely to want a histogram of, in
preparation for controlling contrast, colour balance, etc.)
Naive code to do it is something like
put 0 into count
repeat for each char c in the imageData of img "Image"
switch count mod 4
case 0
break
case 1
add 1 to tRed[chartonum(c)]
break
case 2
add 1 tGreen[chartonum(c)]
break
case 3
add 1 to tBlue[chartonum(c)]
-- put -1 into count
break
default
put "error " && count into msg
end switch
add 1 to count
end repeat
displayHistogram tRed, tGreen, tBlue
For me, on my fairly puny laptop, that takes around 175 seconds, so I
only need a 300x speedup.
And with an equally naive C external, it takes about 400 ms - about a
quarter of which is formatting the result back suitable for Rev, and
then Rev reformatting it suitable for my displayHistogram function.
I tried some other Transcript approaches (I think the fastest one I
found was
replacing the unused bytes by 'cr's
sorting lines by char 1 of each (remembering to set caseSensitive to
true first !!)
scanning through the lines, counting as you go until char 1 changes,
then putting that count into the appropriate array element
and repeat the sort + scan for char 2 and char 3 in turn
This brought the time needed for a small photo (800x600) from 14 secs
down to 7; unfortunately on a large photo that took more than 5 minutes,
and increased my Windows Virtual Memory twice. But even on the small
photo the time taken is still enough to make it unusable.
--
Alex Tweedly http://www.tweedly.net
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