Exporting transparent PNGs (again)

Chris Carroll-Davis asgg35 at dsl.pipex.com
Tue Jul 4 11:06:41 EDT 2006


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?

Help!

Chris




On 4 Jul 2006, at 12:54, Chris Carroll-Davis wrote:

> Hello again folks.
>
> I recently posted a question about Rev's weird behaviour when  
> exporting transparent PNGs from fields with Japanese text.  No-one  
> (understandably) was able to offer any help so, as is often the  
> case, I think I'll be looking for a workaround...
>
> ... Exporting non-transparent PNGs works fine so, in theory, should  
> I not be able to export the image (perhaps to clipboard or  
> variable) then apply this to a graphic as an alpha channel?  Then  
> export the resulting image as transparent PNG?
>
> So... two questions:
>   1)  Does the above workaround solution sound feasible?
>   2)  How the devil do I do it???!!
>
> I've had a few half-hearted attempts at using the alphadata  
> property, but I can't get it to work at all!  Can anyone point me  
> to some example stacks that use this property that I can dissect?
>
> I'd be grateful for any suggestions!!
>
>
> Chris
>
>
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