[stella] VDEL problems again

[Glenn Saunders <cybpunks2@xxxxxxxxxxxxx>]

After several months I spent a bit of time going over the DD code again and 
I have to admit I'm a little rusty.

I still can't understand how to properly implement VDEL.  (Here we go again).

I really am trying, though.

I took a look at the Combat disassembly and I aligned my sprite writes 
similarly:

GRP0 is written right after the WSYNC on the first of the two 
line-pairs.  GRP1 written close to the end of the 2nd line-pair.  I'm 
assuming that the GRP1 write timing is not critical, just GRP0.

I'm also assuming that regardless of the desired Y position of GRP0 or GRP1 
that I can leave it like this, or am I going to have to have a separate 
kernel where GRP1 is written first, and then GRP0?  I didn't see that sort 
of thing in Combat.

Now, as long as I set both sprites to the same Y position, enable VDEL for 
P0 and disable VDEL for P1, the cars line up properly even though I'm 
writing on alternate lines.

However, when I use the code below to automate the settings for VDEL, I can 
no longer get the cars to align at 1-scanline accuracy:

        LDA P0_YFullres         ;vertical delay becomes
        EOR #1
        STA VDELP0      ;least significant bit

        LDA P1_YFullres         ;of player position
        EOR #1          ;(thoughtfully placed in bit 0
        STA VDELP1      ;by Stella's designers)

Even if I experiment with hardcoded values I can't seem to figure out how 
this should be working.

Anybody know what I'm doing wrong?

I can't wait to get this behind me so I can try to wedge the missiles in 
and then start working on animation routines.


Glenn Saunders - Producer - Cyberpunks Entertainment
Personal homepage: http://www.geocities.com/Hollywood/1698
Cyberpunks Entertainment: http://cyberpunks.uni.cc

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