[comp.sys.amiga] Rastport masking

dillon@CORY.BERKELEY.EDU (Matt Dillon) (01/01/87)

	I wanted to some display double buffering by openning a normal
2-bit plane screen and setting the color map such that one plane would
be ignored and the other displayed (then vise versa for the double buffer
flip).  There don't seem to be any 'OR' drawing modes... if you set APen
to 2 and write, it will clear plane 0 and set plane 1.  COMPLEMENT doesn't
work either because I want 'OR' not 'XOR'.

	But the rastport structure has a neat variable called 'Mask',
mentioned only fleetingly in the documentation.  You simply use JAM1 or JAM2,
and set the Mask variable to the planes which you want to write to.  The
default is -1 (write to all planes).  In anycase, the information may be
helpful to some of you so I decided to post it.

			-Matt