ADR-003: Fill registry pattern in rasterizer

Date: 2026-03-23
Status: Accepted

Context

As fill effects multiplied, the rasterizer needed a way to dispatch from a fill name (string) to the correct function without a growing if/elif chain.

Decision

All fill functions are registered in a single dict _FILL_FNS in justdoit/core/rasterizer.py:

_FILL_FNS: dict = {
    "density":   density_fill,
    "sdf":       sdf_fill,
    "noise":     noise_fill,
    ...
}

The render() function dispatches via _FILL_FNS.get(fill).

Rationale

  • Single source of truth — adding a fill requires exactly one registration point, not scattered conditionals
  • CLI discoverability — _FILL_FNS.keys() can be used to enumerate valid --fill values
  • Testability — the registry can be inspected in tests to verify all fills are reachable
  • Extensibility — third-party code can register custom fills by importing and mutating _FILL_FNS

Consequences

  • Every new fill must be added to _FILL_FNS — it will be silently ignored otherwise
  • Fill keys are part of the public CLI API — choose them carefully, rename requires a deprecation path
  • The registry is imported at module load — circular imports from effects/ back to core/ are forbidden

Fill key naming convention

  • Short, lowercase, no hyphens: density, noise, truchet, lsystem
  • Descriptive enough to be guessable from the CLI: --fill truchet should be self-explanatory