LOD Properties — Reference Sheet

A catalog of node and subtree properties where a zoom-aware Level-of-Detail (LOD) decision can reduce per-frame work. Pairs with item 51 (Subpixel LOD Culling) in optimization.md, which drops entire leaves whose projected bounds collapse below a threshold.

This document defines what is LOD-able, what the decision metric is, and where in the pipeline the decision applies. It does NOT prescribe specific thresholds or promise specific wins — both require empirical verification per backend.

Principles

1. LOD decisions are camera-zoom-indexed. A node's visual significance depends on how it projects to device pixels.
2. Two kinds of LOD:
   • Skip work — eliminate draw dispatches entirely (safe, portable)
   • Replace with cheaper primitive — swap a complex draw for a simpler one (requires per-backend validation; modern GPUs may already short-circuit)
3. The only trustworthy reason to implement a rule is a measured win. Categories that "look" cheap on paper may already be handled by the underlying graphics backend.
4. Threshold policy is pluggable, not hard-coded. Per-property thresholds live in a runtime config so they can be tuned per backend / per fixture / per workload.

Notation

• z — camera zoom (device pixels per world unit)
• px(x) = x · z — project a world-space length to device pixels
• A property is "subpixel" when its projection falls below a threshold matching the backend's AA resolution (typically 0.5 px for coverage, 1.0 px for structural features)

Pipeline Stages

Each LOD decision applies at one of three stages:

Stage	Work avoided	Constraint
Frame plan	skip node / subtree entirely	needs zoom + bounds at plan time
Picture record	emit cheaper primitives into cached SkPicture	per-node pictures must become zoom-variant
Draw time	dynamic per-frame decision against current zoom	cheap decision; compatible with cached pictures

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Catalog

A. Geometric node / bounds

ID	Property	Metric	Action
A1	render bounds	both axes projected < ε	cull leaf ✅ item 51
A2	render bounds area	area·z² < ε²	cull leaf
A3	render bounds diagonal	diag·z < ε	cull leaf
A4	stroke-only contribution	stroke_w·z < ε	drop stroke paint, keep fill
A5	subtree cumulative area	Σ child area·z² < ε²	cull subtree

B. Corner & rounding

ID	Property	Metric	Action
B1	corner radius (rect)	r·z < ε	RRect → Rect
B2	corner radius (path)	r·z < ε	drop corner arcs → polyline
B3	stroke join miter	miter·z < ε	force bevel fallback

C. Stroke & outline

ID	Property	Metric	Action
C1	stroke width (thin)	width·z < ε	skip stroke draw
C2	stroke width (hairline)	width·z ≈ 1 px	clamp to width=0 hairline path
C3	dash segment length	dash·z < ε	replace with solid stroke
C4	dash gap length	gap·z < ε	replace with solid stroke
C5	variable-width amplitude	amp·z < ε	collapse to constant stroke
C6	marker size	marker·z < ε	omit marker

D. Path / vector complexity

ID	Property	Metric	Action
D1	segment chord length	chord·z < ε	drop consecutive near-coincident pt
D2	bezier flattening tol	tolerance = 1/z	coarser curve tessellation
D3	sub-path bbox area	bbox·z² < ε²	drop sub-path
D4	near-coincident points	d·z < ε	merge points

E. Effects (save_layer / filter avoidance)

ID	Property	Metric	Action
E1	drop-shadow blur radius	r·z < ε	skip shadow
E2	drop-shadow offset	|offset|·z < ε	fold color into fill
E3	inner-shadow radius	r·z < ε	skip
E4	layer blur sigma	σ·z < ε	skip blur
E5	backdrop blur sigma	σ·z < ε	skip backdrop blur
E6	glass displacement	d·z < ε	skip
E7	noise grain scale	grain·z < ε	skip

F. Opacity & blend

ID	Property	Metric	Action
F1	alpha near zero	opacity < 1/255	cull node
F2	opacity × area	α·w·h·z² < ε	cull node
F3	blend on tiny subtree	subtree area·z² < ε²	force Normal blend

G. Fills

ID	Property	Metric	Action
G1	gradient projected span	span·z < ε	averaged solid
G2	gradient stop density	stops > pixel span	collapse to average
G3	image fill size	img_display_px < ε	center-pixel solid
G4	pattern tile size	tile·z < ε	tile-averaged solid
G5	occluded paint	opaque paint above	skip occluded paints

H. Text

ID	Property	Metric	Action
H1	font size (cull)	font·z < ε_cull	skip text entirely ✅ item 52
H2	font size (greek)	ε_cull ≤ font·z < ε_greek	render as SkRect(s) ✅ item 52
H3	line height	lh·z < ε	collapse to thin rect
H4	glyph advance	adv·z < ε	merge adjacent glyphs
H5	attributed run span	run·z < ε	merge runs
H6	decoration thickness	thickness·z < ε	skip decoration
H7	text-shadow blur	r·z < ε	skip

I. Clip & mask

ID	Property	Metric	Action
I1	clip path area	bbox·z² < ε²	drop clipped subtree
I2	clip complexity	many segments, low z	replace with bbox clip
I3	mask area	bbox·z² < ε²	drop masked subtree

J. Container / subtree

ID	Property	Metric	Action
J1	subtree cumulative area	Σ children·z² < ε²	rasterize once as snapshot
J2	container vs sparse children	children « container	skip container paint
J3	nested container depth	depth > N at low z	flatten subtree to image

K. Render-surface backing

ID	Property	Metric	Action
K1	surface backing resolution	bounds·z	allocate at projected size
K2	filter quality	surface_px small	nearest sampling
K3	compositor promotion	cost estimate at z	don't promote if blit ≥ live

L. Devtools overlays

ID	Property	Metric	Action
L1	frame title label	node_w·z < label_w	hide label
L2	selection handles	node_area·z² < ε²	hide handles
L3	hit badges	density at z	cluster badges

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Verification

Each property must be verified before implementation. Two checks:

1. Skia cost probe — measure the raw per-primitive cost of the operation to be avoided OR of the replacement primitive. If the backend already short-circuits the condition, the LOD rule is moot or regressive. See examples/skia_bench/* for the probe pattern.
2. Scene-level bench-report diff — run with/without the LOD rule across a diverse fixture set, compare per-stage timings.

Two independent sources of possible redundancy:

• Skia's existing fast paths (e.g. SkRRect::isRect() for r=0)
• GPU driver's analytic-coverage shaders that early-exit on sub-pixel inputs (varies per backend — Metal, Ganesh GL, Graphite, WebGL, …)

Rules that skip work entirely (A, E, H1/H2, F1, G5) are generally safe to implement without per-backend validation: they remove draw dispatches the backend would otherwise execute.

Rules that replace with a cheaper primitive (B, C, D, G1–G4) need per-backend measurement because modern analytic-AA shaders may already handle the sub-pixel case efficiently.

Applied Findings

Findings are tracked inline in optimization.md (numbered items) and in per-property verification notes alongside their benchmarks.

• Item 51 (A1) — implemented. Subpixel leaf-bounds culling.
• Item 52 (H1) — implemented. Text font-size-below-threshold cull.
• B1 (RRect → Rect) — measured via skia_bench_rrect_vs_rect. Needs per-backend decision; on some backends the analytic rrect shader is already cheaper than drawRect at sub-pixel radii.