Object Priority#
When two objects cover the same pixel, one of them has to win — and which one shows in front is object priority. This is a separate question from collisions: collision detection only tells you that two objects overlapped, never which was drawn on top. Priority is what decides the picture.
The default order#
The TIA ranks the six things it can draw in a fixed order. Highest priority is drawn in front:
| Priority | Objects |
|---|---|
| 1 (front) | P0, M0 |
| 2 | P1, M1 |
| 3 | BL, PF |
| 4 (back) | background |
Two pairings fall straight out of the missile and ball color-borrowing: a missile shares its player’s priority, and the ball shares the playfield’s. So missile 0 is always exactly as “in front” as player 0, and the ball sits at the same level as the walls it’s usually colored like.
The practical upshot of the default order: players and missiles pass in front of the playfield. A sprite walking across a wall is drawn over it.
Putting the playfield in front (CTRLPF bit 2)#
Set bit 2 (D2) of CTRLPF and the playfield and ball jump ahead of the players:
| Priority | Objects |
|---|---|
| 1 (front) | PF, BL |
| 2 | P0, M0 |
| 3 | P1, M1 |
| 4 (back) | background |
Now a sprite slides behind the playfield. That’s how you get a character disappearing into a tunnel, walking behind a pillar or bridge, or threading a maze whose walls are drawn over it — the effect in Pitfall!’s underground and many maze games.
Note there’s no per-object switch: priority is a single global ordering, flipped as a whole by D2. Layering one specific sprite behind the walls while another stays in front means changing D2 mid-frame, on the scanlines where you want the other rule.
CTRLPF at a glance#
Priority is one of several controls packed into the single CTRLPF register. The whole register, and where each part is covered:
| Bit | Field | Covered in |
|---|---|---|
| D0 | playfield reflect / repeat | Symmetry |
| D1 | score mode (color halves in player colors) | Building a Scoreboard |
| D2 | playfield/ball priority (this page) | here |
| D4–D5 | ball width (1/2/4/8 clocks) | Missiles & the Ball |
Like any TIA register, CTRLPF can be rewritten between scanlines, so all four of these can differ from one band of the screen to the next.
In Practice#
- Priority decides the picture; collisions decide the rules. They’re independent — a hit is registered whether or not the object that hit was the one drawn on top. Reach for Collisions for the latter.
- The two players are not equal. P0 (and M0) always beat P1 (and M1). If two characters can overlap and you care who looks “nearer,” assign the front one to player 0.
- D2 is all-or-nothing per line. Because it reorders everything at once, getting “this wall in front but that sprite still on top” usually means toggling D2 across scanlines rather than per object.