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2008-12-12 00:46

Codey

Registered: Oct 2005
Posts: 79

multiplexer question

once displaying a sprite, do you have to wait a full 21 scanlines before displaying a new image of the same sprite?

2008-12-12 01:03

ptoing

Registered: Sep 2005
Posts: 271

Eh? Afaik you can have 8 sprites per scanline, if they overlap does not matter at all. I think.

Some coder will probably explain it properly.

2008-12-12 01:34

doynax
Account closed

Registered: Oct 2004
Posts: 212

Right, you'll have to wait until the end of the last sprite before the channel can be reused somewhere else further down. That is you cannot arbitrarily break off a sprite in order to display something else, nor can you make the VIC chip render the same channel twice on a single scanline (in any meaningful way.)
The VIC triggers the display of a sprite by comparing the current scanline to the sprite's y-coordinate, and that's the registers only function. And unlike the other attributes (e.g. x-coordinate, shape, color, and so forth) rewriting the y-coordinate in the middle of a previous sprite instance has no immediate effect. In other words if you set a new y coordinate less than 21 lines down from the last one the VIC simply won't display anything at all.

Technically you can also cut down the height of a sprite down to 17 scanlines by toggling y-expansion at just the right cycles, but that's of little practical use for a general-purpose multiplexer.

2008-12-12 01:47

ptoing

Registered: Sep 2005
Posts: 271

Ah, now I get what he meant. Heh, feeling stupid T_T

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