Sorry - this was sent yesterday, and has been held up by problems
contacting RunRev site - trying again ....
Date: Wed, 29 Jun 2005 22:38:50 +0100
From: Alex Tweedly <[EMAIL PROTECTED]>
Subject: Re: compileIt for revolution?
Jon wrote:
Derek:
Let's put it differently. My real interest is in seeing how to
optimize Rev code to access arrays in general, and image data in
specific, as rapidly as possible.
I trust you've already read all the "tricks to optimize code" mentioned
on the list last week (in particular, the ones from Ken's sonsothunder site)
http://www.sonsothunder.com/index2.htm?http://www.sonsothunder.com/devres/revolution/revolution.htm
Accessing arrays in Rev is slow-ish because they are associative. They
are quick for assoc arrays, but it's still a look-up.
Access to numbered chunks (e.g. line 3, item K, ...) is dependent on the
number being accessed - because it requires a pass through the variable
to count chunks. The one big exception to this is character access -
char N is a direct lookup (offset N from the start). So if you can
access "char N of variable" that is much faster ....
So the big message for image processing is - don't access it as an
array, access it as chars in a variable.
Your Brightness code took (for me, for one med size photo)
8.5 seconds to convert from tImagedata into your arrays
another 8 seconds to ComputeHistogram
By leaving it as tImageData, I could save the conversion time of 8.5
seconds.
*and* I could reduce the ComputeHistogram time from 8 seconds to around
2 seconds, by changing
repeat with r = 1 to imH
repeat with c = 1 to imW
put Brightness(c, r) into i
add 1 to histo[i]
end repeat
end repeat
to (NB uses the fact that the "spare" byte is always zero)
repeat for each char c in lImageData
add chartonum(c) to temp
add 1 to count
if count = 4 then
add 1 to histo[temp]
end if
put 0 into temp
put 0 into count
end repeat
I realized that this still requires an access to an array for each
increment to "histo". It would be great if there were a "direct access"
integer variable type, but there isn't. However, if you are desperate
enough to save time, you can use "char N" accesss to a temp variable,
and then you can convert that into array accesses: thus the section
if count = 4 then
add 1 to histo[temp]
end if
becomes
if count = 4 then
add 1 to temp -- because 1:256, not 0:255
put numtochar(chartonum(char temp of mytemp) + 1) into char temp
of mytemp
if chartonum(char temp of mytemp) = 255 then
add 255 to histo[temp-1]
put czero into char temp of mytemp
end if
put 0 into temp
put 0 into count
end if
remembering that at the end you need to roll them up
repeat with i = 1 to maxHist+1
add chartonum(char i of mytemp) to histo[i-1]
end repeat
This rather extreme measure reduces the time to 1.2 seconds.
So in some ways that's a good saving 8 seconds to 1.2 - but in other
ways it shows that it's probably flogging a dying horse.
Although I didn't complete it, I believe this technique applied
throughout your example would reduce the overall time for this photo of
mine from around 55 seconds to 10 or 12 seconds.But the cost is some
ugly, barely maintainable code, and it's still beyond the stretch of
what a user would be willing to wait for a simple transformation to be done.
--
Alex Tweedly http://www.tweedly.net
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