Exporting transparent PNGs (again)

[Dave Cragg]

On 4 Jul 2006, at 16:06, Chris Carroll-Davis wrote:

> Hello again.
>
> Well, I've made some progress!  I can now stick an image on a card  
> and use that image to make an alpha channel for a second image.  
> Hurrah.  Trouble is it is  S--L--O--W!  It takes over 10 seconds to  
> process a tiny 50-pixel square image!!!  The utility I'm making  
> needs to export about 3000 - much larger - graphics!
>
> The problem is that ImageData uses 4 bytes per pixel (even if it is  
> a 8-bit greyscale image) but AlphaData only uses 1 byte per pixel,  
> so I have to strip the data out in a repeat loop that is huge; even  
> for moderately sized graphics!    I started with a 300 x 300  
> graphic, but of course for AlphaData that is 300x300 bytes = 90,000  
> repeats.  Yuk!  There must be a better way
>
> Is there any lightning fast way of manipulating the binary data  
> without resorting to a repeat loop?
>

I'm not sure what calculations you are doing in the loop, but it  
sounds too slow. The following routine creates alphaData from a 300 x  
300 grayscale image in less than a second on my not so fast machine.  
(You'll probably need to substitute the calculation inside the loop  
with your own.)

I'm sure others will step up with faster alternatives. :-)


on mouseUp

   put the imageData of image 1 into tImageData
   put the milliseconds into tStart ## for speed calc
   put 0 into tCount
   repeat for each char tChar in tImageData
     add 1 to tCount
     if tCount = 4 then
       put numToChar(255 - charToNum(tChar)) after tMaskData
       put 0 into tCount
     end if
   end repeat
   put the milliseconds - tStart && length(tMaskData) ##speed score
   set the alphaData of image 1 to tMaskData
end mouseUp

Cheers
Dave