You want to mask out parts of a picture using the mask function of
the Frame editing module and therefore need a mask? – Select the
1 bit and set the magnification to 1:1. In the Frame
editing module, change the image control curve so that parts to be
masked will look black, all other parts white. Now convert the data
without deleting the original frame. Restore the original control
curve in the original frame, then mask the picture with the new bitmap
Warning: Such bitmap masks cannot be used as masks for PostScript output. You will have to transform the b&w graphic with Speed~Line or by hand (in the Vector module) into a vector mask, as described in the tutorial above!