🚀 go-pugleaf

In article <2693@mit-hermes.ARPA> jpexg@mit-hermes.ARPA (John Purbrick) writes:
>I have an Atari 1040ST, and I've been playing with its bit-mapped graphics.
>One thing that the Mac does when running MacPaint is let you start a line
>by clicking on a point, then flopping the other end around with the mouse
>until you anchor it by clicking again. If the display is bit-mapped, what I'd
>expect to see is that every time the flopping line leaves a point on the
>screen, that point would have to be redrawn or you'd see a whole row of blank
>points in every object crossed by the line. Yet this doesn't happen. Surely
>the whole screen isn't redrawn every time a line is moved? An alternative
>would be to have two planes which are automatically or-ed together to
>generate the display, so one could be permanent and one temporary, but
>the ST certainly doesn't do this--does Macintosh?
>					--John Purbrick

The technique used is called XOR (eXclusive-OR).  What happens is this:
once the first point of the line has been clicked (or "anchored"), then
everytime the mouse moves, a line is drawn from the anchor to the new
pointer position.  However, instead of blindly erasing whatever is underneath
the line, the screen's bit image is preserved by XORing the line's bit image
with the screen image.  Now, the _next_ time the mouse moves, the line is
again drawn using the XOR mask, thus restoring the background to its previous
state.  Once the mouse positions the pointer in its final location and is
clicked, then the line is drawn "permanently".  Of course, most programs
also use the "double-buffer" as well, so that any individual action can
be "undone".  Anyway, here's an example of how this works:

	Screen:	    Line:	    Screen XOR Line:

	0010000	    0000010		0010010
	0100010	    0000100		0100110
	0111000	    0001000		0110000 <-- note "hole" in this byte
	0000100	    0010000		0010100
	0001000	    0100000		0101000
	0000000	    1000000		1000000

    XOR Screen:	    Line:	    Original Screen:

	0010010	    0000010		0010000
	0100110	    0000100		0100010
	0110000	    0001000		0111000
	0010100	    0010000		0000100
	0101000	    0100000		0001000
	1000000	    1000000		0000000

Hope this helps you a bit (;-).
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