Topic       : The GFA-Basic Compendium
Author      : GFA Systemtechnik GmbH
Version     : GFABasic.HYP v2.98 (12/31/2023)
Subject     : Documentation/Programming
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View Ref-FileGRAPHMODE n

n: iexp

The command GRAPHMODE determines the way in which graphics is output to the
screen with relation to what is already there and is important when pictures
are to be drawn on top of one another. Four possible modes can be represented
by the numerical expression n:

    1  -->  Replace
    2  -->  Transparent
    3  -->  Xor
    4  -->  Reverse transparent (inverted and transparent)

When 'n' has the value 1, the new drawing is simply drawn over the existing
screen contents obliterating it completely.

When n is equal to 2, the new drawing is ORed with the existing one, which
means that the old picture can still be seen behind the new one.

With n equal to 3, the new drawing is XORed with the existing one. This means
that, at each pixel where a graphic point was already present, that graphic dot
is deleted, and for all other points, the image is drawn normally. The
importance of this mode is that the process is reversible. By XORing the new
drawing with itself the original screen is restored. Thus, by using this mode,
animation is possible by repeated drawing and 'undrawing' of a figure as it is
moved around the screen.

In the case where n = 4, the new drawing is inverted and then ORed with the
existing one. In this way, a display similar to mode 2 is produced (n=2), but
the new picture is shown in reverse video.

Example:

    FOR i%=1 TO 4
      GRAPHMODE i%
      DEFFILL 1,3,8
      PBOX 150*i%-100,10,150*i*,100
      DEFFILL 1,2,10
      PBOX 150*i%-140,50,150*i%-40,150
    NEXT i%

--> Four filled rectangles are drawn, and a further four are drawn partly
    overlapping the previous ones. Each pair is combined using a different
    GRAPHMODE setting.

    Internally calls vswr_mode().

vswr_mode()+