[stella] Crazy Scan Line Jumping Sprite!!!

Subject: [stella] Crazy Scan Line Jumping Sprite!!!
From: Ruffin Bailey <rufbo@xxxxxxxxxxxxx>
Date: Wed, 26 Aug 98 13:56:02 -0000
Well, I've finally gotten to the point of frustration with that cycle 
counting mess you hear old 2600 programmers talking about.  Take a look 
at this bin.  It should be UUEncoded.  Please lemme know if it ain't.  
The demo is simply a sprite (looks a little like a tiny blue ladder) that 
you can use the P0 joystick to move 'round the screen.  The PF drawing 
routine is pretty snazzy, imo, but pretty time consuming.  And thus the 

Player1 is being used to display the xpos of the critter in binary.  The 
blue fellow starts near the right edge of the screen.  Move him to the 
right, and poof, the yellow P1 disappears as P0 is completely on the left 
edge of the screen.  Call this x=0.  Move him over one HMOVE worth and P1 
displays one line.  The P1 graphic is now simply the binary of the 
x-position of P0.

Come x-position 44 (binary: 00101100), he starts to screw up.  Come line 
53 (binary: 00110101), he's back together again.

You'll also notice that if the blue guy overlaps one of the platforms, 
that line is repeated!?!  I apparently am using more than 76 cycles in 
the line, right?  Anyway, I haven't found what gives here, but I'll keep 

So what can I do to keep my sprite from jumping up a scan line between 
x-positions 44 and 53?  I tried VDEL, hoping everything would get itself 
together during the line, but no luck.  

One guideline:  I am dead-set on keeping single scan-line resolution.  If 
I have to deal with this guy partially moving up a line at x=44, I'll 
write some crazy code to make the player graphics change at this point to 
make up for it.  I just need to know if that's the problem and if, with 
the "snazzy" screen drawing routine, the problem is a necessary evil.

Thanks a lot!

Ruffin Bailey           |  Write Atari 2600 games on your Mac!  
rufbo at bellsouth.net  |  http://members.aol.com/~mactari

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