Threshold filters (Rev Newsletter #48)

Björnke von Gierke bvg at mac.com
Wed Feb 13 12:46:21 CST 2008


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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