imagedata and alpha byte

[jbv]

Hi list,
Still struggling with imagedata, I am trying to modify the transparency
of an image by manipulating pixel by pixel the alpha byte.
I tried with numToChar(0) and binaryEncode("B*","00000000") but to no
avail...
OTOH manipulating the RVB channel works fine.

Any idea on how modify the alpha channel of the imagedata of a image,
and then exporting the image as PNG with the modified transparency ?

Thanks in advance.
jbv