A very interesting read, and it fits nicely into your expertise.
However, you use "put into" in the last example (repeat for each).
This negates the speed gain you'd get by using "for each". try to use
"put after" or "put before". I made some tests, and it increased the
speed of the loop alone by up to 10% for me. Of course it also needs
double the ram :)
The code i used:
on mouseup
answer "Select threshold to create the black-and-white image:"
with "80" or "100" or "128" or "150" or "180"
wait 3 milliseconds
put it into threshold
set the cursor to watch
put 1 into x
set the saved of image x to the filename of image x
put the imageData of image x into iData
put 0 into counter
put the milliseconds into bvgTimestamp
repeat for each char C in idata
add 1 to counter
if counter mod 4 = 2 then # the red pixel
put chartonum(C) into tC
if tC > threshold then
put 255 into tC
else
put 0 into tC
end if
end if
put numtochar(tC) after idata2
--put numtochar(tC) into char counter of idata
end repeat
put the milliseconds - bvgTimestamp
set the imageData of image x to iData2
end mouseUp
On 13 Feb 2008, at 17:25, Wilhelm Sanke wrote:
There is only sparse documentation about "imagedata" in Revolution.
Without the more detailed information and sample stacks from the
websites of other experienced members of this list I would probably
never have got a start to learn and apply some of the possibilities
of imagedata handling.
Therefore it is praiseworthy that he Revolution team now tries to
provide us with more useful information in Newsletter #48 and
introduce us to the "use of thresholding for image manipulation".
They also apply modern "state-of-the-art" pedagogy, in so far as
they bundle their example with kind of a joke for better learning.
If such an approach isn't over-used it might indeed work.
My "Imagedata Toolkit" stack (last public version of May 2007
<http://www.sanke.org/Software/ImagedataToolkitPreview3.zip>)
among the more than 200 filters for manipulating imagedata already
contains 7 filters using the threshold principle (see menu-button
"thresholds").
I created another button in this stack adapting the script
information in the newsletter to my environment to compare the new
threshold filter.
First thing I noticed was that only 25% of an image is affected.
This holds for all of the three script examples displayed in the
newsletter. One important factor is missing in all these examples,
which I leave to you to find out for yourselves (another instance of
modern pedagogy).
I then downloaded the sample stack to have a look whether its
scripts worked better, but for some reason I was unable to un-zip
the stack on Windows, even after downloading it again. I transferred
the archive to my Macbook and finally succeeded in extracting the
stack using Stuffit Expander. Looking at the relevant script
revealed that here the author of the stack had indeed added the
missing factor.
Re-tranferring the unzipped stack back to Windows did not work
first, until I found out that Windows did not like the question-
mark as part of the stack name.
I removed the question-mark, and then was able to get the stack
onto my Windows computer, however, it was impossible to open it in
Revolution.
Therefore I only extracted the necessary script from the stack on my
Macbook and put it into my Imagedata-Toolkit stack on my Windows
machine.
Now I was able to compare the threshold script with my own 7
threshold buttons. The solution found by the author - even
disregarding the entertainment factor - was fine and its effects
different, but somewhat similar to those of my own buttons that use
two or more thresholds.
However, I found the script to run much slower due to the many
computations inside the nested repeat loops.-
======================
In the following, I use a step-by-step approach to show how the
speed of the script execution could be improved. To do this I show
only one of the thirteen script lines inside the loop that -
concerning the definition of the imagedata chars - is identical to
the other 12 lines.
"repeat with y = 1 to pHeight
repeat with x = 1 to pWidth put charToNum(char ((y - 1)
* pWidth * 4) + ((x - 1) * 4) + 2 of pImage) into tRed"
The script as it is takes 3500 milliseconds to execute for an image
640x480 on my Windows computer.
Step 1: Remove the two "- 1" inside the loop and change the x and y
start and end values accordingly;
new script:
"repeat with y = 0 to pHeight - 1
repeat with x = 0 to pWidth - 1 put charToNum(char (y *
pWidth * 4) + (x * 4) + 2 of pImage) into tRed"
Speed gain here is about 300 milliseconds (now 3219).
Step 2: Compute "pwidth * 4" outside the loops;
new script:
"put pwidth * 4 into tpwidth
repeat with y = 0 to pHeight - 1
repeat with x = 0 to pWidth - 1 put charToNum(char (y *
tpWidth) + (x * 4) + 2 of pImage) into tRed"
Speed gain another 300 ms (now 2932).
Step 3: Combine y with tpwidth;
new script:
"put pwidth * 4 into tpwidth
repeat with y = 0 to pHeight - 1
put y * tpwidth into typwidth
repeat with x = 0 to pWidth - 1 put charToNum(char
typWidth + (x * 4) + 2 of pImage) into tRed"
Step 4: put "x*4" into tx;
new script:
"put pwidth * 4 into tpwidth
repeat with y = 0 to pHeight - 1
put y * tpwidth into typwidth
repeat with x = 0 to pWidth - 1
put x * 4 into tx put charToNum(char typWidth + tx + 2
of pImage) into tRed"
Overall speed gain from beginning: 1 second (now 2585)
Step 5: Combine typwidth and tx;
new script:
"put pwidth * 4 into tpwidth
repeat with y = 0 to pHeight - 1
put y * tpwidth into typwidth
repeat with x = 0 to pWidth - 1
put x * 4 into tx put typwidth + tx into tyx put
charToNum(char tyx + 2 of pImage) into tRed"
Speed gain: another 100 ms (now 2461)
Step 6: Remove the irrelevant line accessing the alpha value of the
imagedata char, i.e.
remove line (this is the original form)
"put charToNum(char ((y - 1) * pWidth * 4) + ((x - 1) * 4) + 1 of
pImage) into tAlpha"
Speed gain: again 100 ms (now 2340).--
Overall speed gain is now 33% compared to the speed of the original
script.-
==========================
As an additional script example - which is not among the 7 scripts
in my Imagedata stack - I tried to find a very simply structured
script using "for each" (which is by the way - in this case - slower
than a nested repeat loop), applying only one threshold, and using
only the red value to create a black-and-white image:
"on mouseup
answer "Select threshold to create the black-and-white image:" with
"80" or "100" or "128" or "150" or "180"
wait 3 milliseconds
put it into threshold
set the cursor to watch
put the imageData of image x into iData
put 0 into counter
repeat for each char C in idata
add 1 to counter
if counter mod 4 = 2 then # the red pixel
put chartonum(C) into tC
if tC > threshold then
put 255 into tC
else
put 0 into tC
end if
end if
put numtochar(tC) into char counter of idata
end repeat
set the imageData of image x to iData
end mouseUp"
(Pay attention to possible line breaks of this script in this post.)
I hope I have added some useful information and thus have supported
the efforts of the Rev team to familiarize us with imagedata
manipulation.
Regards,
Wilhelm Sanke
<http://www.sanke.org/MetaMedia>
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