My iterator function, called via sp1_IterateSprChar() each time round the game loop, contains this:
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if( ++c->attr == INK_WHITE )
c->attr = INK_BLUE;
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sp1_InvUpdateStruct(c->update);
I found the answer: use sp1_IterateUpdateSpr(), where the iterator function takes a struct sp1_update *u and the character cells can be invalidated with
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sp1_InvUpdateStruct(u);