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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