Perimeter strategy (compose.perimeter / code.perimeter)

August 30, 2026 · View on GitHub

Artifact: io.github.bodenberg:appdimens-kmp-perimeter:3.2.0 (:library-perimeter) See MODULES.md · README installation

Same API surface as scaled with prefixes prsdp / prhdp / prwdp / prssp / … — see COMPOSE-API-CONVENTIONS.md §3.

What it is

Perimeter scales by the normalized screen perimeter: sum of shorter and longer side in dp, divided by reference 833 dp (perimeter reference aligned with 300 + 533 in DesignScaleConstants). It reflects “width + height” together, unlike diagonal or a single axis.

Calculation used

Mathematically:

  • scale = (shorter + longer) / BASE_PERIMETER_DP with BASE_PERIMETER_DP = 833 (DesignScaleConstants, aligned with 300 + 533).
  • out = base × scale

Implementation note: scale is derived from the immutable per-window snapshot (DimenCache.currentMetrics) at resolution time — no process-global pre-computation. The formula matches (shorter + longer) / 833; the default path does not re-sum sides on every call.

  • With a: multiply by the pre-computed aspect-ratio factor (DimenCache.currentAspectRatioMul); custom sensitivity uses 1+k×logNormalizedAr1 + \text{k} \times \text{logNormalizedAr}.

Implementation: calculatePerimeterDpCompose in DimenPerimeterDp.kt.

How to use

import com.appdimens.kmp.compose.perimeter.prsdp

Modifier.size(72.prsdp)

Prefixes: prsdp, prhdp, prwdp (+ Sp, px, variants).

Code: com.appdimens.kmp.code.perimeter.

Why use it

When width and height together should drive size (frame-like cards, grids that represent total area).

When to use it

  • Layouts that think in total area but want a linear formula on the sum of sides (vs the square root in diagonal).
  • A perceptual alternative to diagonal when you want different weighting across dimensions.

Advantages and trade-offs

  • Pros: simple; folds both axes into one scalar.
  • Cons: two screens with similar diagonal can differ from diagonal-only metrics — align with design on which metric is source of truth.

Use on card / thumbnail families; avoid mixing at the same hierarchy level with diagonal without a clear guideline, or scales may clash.

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