i don't understand your question... you draw something and lay a mask on it which is transparency somewhere.

1.) the image
2.) the mask - white means transparency (not masked), black not (masked). gray is somewhere between.
3.) the mask on the image. the mask is colored red instead of black and white now. parts which are masked appear red. where the mask is transparent, the image below appear in normal colors. in the shadow you can see, a little bit from the black background appear. because the mask is gray there (pic 2)
4.) png image combined with mask in front of a white background. in the nav system, the image is shown in front of the map of course.





hope you asked something like this...