Image Masks - Completely lost

[Bob Warren]

Recently, Scott Rossi wrote:

 >I think your problem is the image you are using for the mask contains 
 >*no* transparency information.  If you want to use images directly as 
 >masks, they must be created with transparency information included. 
 >The white area in your circle image is read as color, not transparency.

 >Execute the following in your message box and see if the stack 
 >demonstrates what you are trying to do.

 >  go url "http://www.tactilemedia.com/download/masktest.rev"

 >The stack contains two images, both of which were created in Photoshop 
 >with transparent backgrounds; the top image is a GIF with a 1 bit 
mask, >the bottom is an image with an 8 bit mask.  Notice that the 8 bit 
mask >allows for a smoother edge appearance.

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A single example is worth a thousand words. Thank you very much Scott!

I now see that statements like the following are perfectly possible:

   set alphaData of img display to alphaData of img disc.gif
   set imageData of img display to imageData of img grad.png

I suspected that it might be something like that, but part of my 
confusion comes from VB where the definition of "mask" is somewhat 
different (i.e. the white area in my circle image CAN be read as 
transparency).

Regards,
Bob