Some questions had to be answered:
1.) Which object type for storing the png? There are many to choose: Image, pixbuf, pixmap. A long time I was focused on pixmaps, finally discovered, that the alpha channel was not copied, but just applied to the color values, when rendering the png into the background. Therefore a transfer from pixbuf to pixmap was not a good idea.
2.) To read out single pixel values is a litle complicated on pixbufs. Only way I could see: Change pixbuf to pixmap and thatone to image. (Unless one reads the values directly from memory. But that's not to prefer).
3.) How to get the transparency color into the alpha channel? The hint I got here
, thank you very much.
4.) Finally just use