Only possible with two source windows loaded, this feature allows merging of a part of one source window onto another source with the result in the destination.
The alignment of the two sources prior to merging is easy! Simply scroll the sources individually or alternatively open the merge-align window or press the space bar in the destination window (see the help file for more details). This displays a simple overlay of the two sources. Drag the left/right mouse button over this window to align the first/second source image. In both cases all of the windows are updated in real-time - whichever window position or sizing you adjust.
When the alignment is complete decide which one of the source images you want to have as a 'base' in the destination by pressing the 'M' button and selecting 'copy 1 only or 'copy 2 only'.
Drag out a 'contort zone' in the destination then press one of the two portion-merge buttons a few times to merge in one of the two sources.
That was one of many ways of portion merging windows. Try realigning the windows with different main merge switches. Try different mouse-drag rectangles.
The final example shows another portion merge of mona into the sea, but only after she had been colored up to match that of the sea. This was achieved by sending mona to the destination, then changing the color by an option in the edit menu, then final sending that destination back to source 2.