Genesis VDP Plane Priority Question

Hi Assembler,

I am investigating a bug in the dgen-sdl emulator.

Look at the attached screenshot. Notice the player sprite is partially over the scores.

The plane priority on the emulator is as follows:

---8<---
// Draw, from the bottom up
// Low priority
draw_plane_back1(line);
draw_plane_back0(line);
draw_window(line, 0);
draw_sprites(line, 0);
// High priority
draw_plane_front1(line);
draw_plane_front0(line);
draw_window(line, 1);
draw_sprites(line, 1);
---8<---

I think plane 0 is plane A here and 1 = B.

What we are seeing here is a low priority sprite rendering over the low priority window plane. It looks wrong and is (this doesn't happen on the real thing).

By drawing the window planes last, ie:

---8<---
// Draw, from the bottom up
// Low priority
draw_plane_back1(line);
draw_plane_back0(line);
draw_sprites(line, 0);
draw_window(line, 0);
// High priority
draw_plane_front1(line);
draw_plane_front0(line);
draw_sprites(line, 1);
draw_window(line, 1);
---8<---

then the game *appears* correctly.

Yet if we consult some docs (http://emu-docs.org/Genesis/Graphics/genvdp.txt):

---8<---
(back) (front)
A > B > C > D > E' > F' > G'

' = Denotes high priority
A = Backdrop color
B = Low priority plane B
C = Low priority plane A
D = Low priority sprites
E = High priority plane B
F = High priority plane A
G = High priority sprites
---8<---

And we are given some info about the interaction of window too:

---8<---
The window plane operates differently from plane A or B. It can be thought
of a 'replacement' for plane A which is used under certain conditions.
That said, plane A cannot be displayed in any area where plane W is
located, it is impossible for them to overlap.
---8<---

This implies the following plane ordering to me:

---8<---
Backdrop color
Low priority plane B
Low priority plane A
Low priority window
Low priority sprites
High priority plane B
High priority plane A
High priority window
High priority sprites
---8<---

This is exactly the ordering that causes the issue in the first place?!

Can someone clarify this. Should the window always be on top?

 

PS.

I have just been told that moving the "window" plane to the top breaks the cd player in lotus turbo challenge.

 

I'm quite suprised that no-one here has any ideas. Perhaps I posted to the wrong forum?

 

x