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Overlay Mode

In order to generate interactive moving cursor functions it is necessary to produce sprite like graphical out put. This requires an "XOR" modification of the displayed graphics. After a call to

tv_set_overlay(Tv *tv)
all drawing takes place in a colour called overlay (usually red) and uses rop_xor. This means that after drawing a sequence of primitives, the screen can be restored to its original state by repeating the draw sequence. This allows red sprites to wander over a fixed background display for example.

To return to the previous draw context call tv_reset_draw(tv) as above.

root 2017-09-21