Turning imageData into alphaData (quickly)?

[Wilhelm Sanke]

On Mon Mar 13, 2006, Ian Wood revlist at azurevision.co.uk wrote:

> (snip)
>   A handler that goes through each RGB pixel and reduce it to a  
> grayscale value for use as alphaData was pretty easy, but for a  
> 200x200px image that's 40,000 repeats, and takes minutes on a G5.
>
> on maskStuff
>    put the seconds into tStart
>    put "" into tMaskData
>    put the imageData of img "mask" into tImgData
>    put number of chars of tImgData / 4 & return into fld "status"
>    lock screen
>    repeat with x = 1 to ((number of chars of tImgData) / 4)
>      if the mouse is down then exit to top
>      put charToNum(char (x*4)-2 of tImgData) into tR
>      put charToNum(char (x*4)-1 of tImgData) into tG
>      put charToNum(char (x*4) of tImgData) into tB
>      put the round of ((tR + tG + tB)/3) into tM
>      put numToChar(tM) after tMaskData
>      --put tM & return after fld "status"
>    end repeat
> (snip)
> Any ideas for a faster way? Cross-platform command-line tools?
>
> Ian Wood
> Panoramic photography, from web to billboard, sunrise to moonrise
> http://www.landmarksofbritain.co.uk



Hi Ian,

I modified your script to the solution below, i.e. I took the script of 
my "gray-scale" button of my ImageData Toolkit and adapted it to your needs.
The script is for a 640 X 480 image, meaning that more than about one 
million of chars have to be processed (640 * 480 * 3)
Running the script takes *one* second on my Windows XP computer (2 GHz):

 on mouseUp
  set the cursor to watch
  put the milliseconds into Start
  put the imageData of image 2 into iData
  put empty into tmaskdata
  put 2560 into re # i.e. 640 * 4
  repeat with i = 0 to 479
    repeat with j = 0 to 639
      put  chartonum(char (i*re + (j*4+2)) of idata) into tC1
      put chartonum(char (i*re + (j*4+3)) of idata) into tC2
      put  chartonum(char (i*re + (j*4+4)) of idata) into tC3
      put the round of ((tc1 + tc2 + tc3)/3) into tM
      put numToChar(tM) after tMaskData
    end repeat
  end repeat
  put the milliseconds - Start into fld "test"
end mouseUp

Best regards,

Wilhelm Sanke
<http://www.sanke.org/MetaMedia>
<http://www.sanke.org/ImageDataArt>