Symmetry: Reflection & Repetition#

The three registers gave you 20 bits — the left 20 pixels of a 40-pixel-wide screen. You never wrote the other 20. This page is where they come from.

The TIA generates the right half automatically from those same 20 bits, and a single flag decides whether it does so as a copy or a mirror. That is why the playfield is inherently symmetric: one write of PF0/PF1/PF2 per line paints the whole 40-pixel width, but both halves are built from the same bits.

Reflected vs. repeated (CTRLPF bit 0)#

How the right half is generated is set by bit 0 of CTRLPF, the reflect flag:

  • Repeated (D0 = 0): the right half draws the same 20 bits again, left to right — the left pattern simply copied across. The screen reads as two identical halves; good for tiled or repeating backgrounds.
  • Reflected (D0 = 1): the right half draws the 20 bits mirrored — a left-right mirror image around screen center. Good for symmetric shapes: the Christmas tree, a centered logo, a symmetric maze, a bowl-shaped arena.

Either way, both halves come from one set of register values. That is the cheap deal: a single write of PF0/PF1/PF2 per line paints the entire 40-pixel width. The cost is that with that one write you cannot show two different things on the two halves — only a pattern and its copy or mirror. (Breaking that limit is the asymmetric playfield.)

You author only the left 20 bits (solid); the TIA fills the right half for free (faded). The same bits become rolling hills or twin triangles when repeated, and a centered mountain or diamond when reflected — the whole difference is CTRLPF bit 0.

Choosing reflect or repeat#

  • Reflect when the content is naturally symmetric — which most playfields are, so reflection is the common default. A maze, an arena, a framed border, or a centered emblem all look right mirrored.
  • Repeat when you want a horizontally tiling texture — a repeating brick course, a banded background — where the same motif twice is exactly the intent.

The reflect bit lives in CTRLPF alongside other playfield controls — object priority and the score-mode coloring — and like any register it can be rewritten between scanlines, so one band of the screen can be reflected while another repeats.

In Practice#

  • Mind the center seam. Under reflection the screen mirrors around the middle, so the last bit of PF2 sits next to its own reflection — a lit pixel there reads as a 2-pixel-wide block straddling center. Plan shapes that meet in the middle with that doubling in mind.
  • The bit order still applies. Even for a symmetric shape, you’re laying out the left half into PF0/PF1/PF2 with PF0’s half-nibble and PF1 reversed; reflection only handles the right half for you, not the quirks of the left.
Backward The Three Registers The Asymmetric Playfield Forward