Bregalad
Account closed
Registered: Jul 2008
Posts: 42 |
Sprite multiplexing *with* priorities
I'm willing to port a game on the C64 but for sure I'll need more than 8 sprites per screen. The game uses a top-down perspective so sprites that are southern on the screen must show above the sprites that are nothern, else it will look weird.
Unfortunately, most sprite multiplexing tehniques I've seen arround the net seems to kill any priorities, and that's not good.
I guess I can come up with a solution, but I'm not sure how good it is :
- The screen is sepearated into 10 areas of 20 pixel tall
- There is a raster interrupt each 20 pixels that will handle the multiplexing, and the interrupt at line 250 will also be used for the other usual stuff (screen updates, sound code, etc...)
- Each sprite always crosses 2 areas no matter where they are positionned (this assumes Y expanding is never used)
Now during the frame :
In the first area (lines 0-20), the sprites are mapped to hardware sprites with regards to their sorted priorities. If there is more than 8 then the lowest pirority is discarded.
In the second area, the sprites that were already used in the first area cannot be used, so I map the sprites that are still unused to the sprites starting in the second area, regarding to their priorities.
In the third area, all sprites that were used in the first are done being displayed and I can re-map them to sprites of the third area according to their priorities, etc....
Now the priorities are not matched between areas, but at least they are inside each area. Also the number of interrupts is constant, and everything can be pre-calculed during the non-display time, so that interrupts just do a few writes each 20 scanlines which is not too bothersome and allow it to be mixed with other raster split without too much trouble.
I know Cadavers said it was a bad idea to split the screen with areas, but I can't come with anything else. I could use the generic method in a way so that top sprites are lowest priorities than bottom sprites, and hoping it won't look too weird. |
