Pixel and stroke geometry

Pixel and Stroke Geometry

If your app draws on a grid, canvas, or any pixel-aligned surface, the rendering rules below are non-obvious and produce a class of bugs that are otherwise mysterious. This skill covers the rules and the recipes that prevent the bugs.

When to Use

• Building a pattern editor, illustration tool, or any pixel-grid drawing app
• Implementing selection rectangles, marching ants, or grid overlays
• Drawing custom gauges, dials, charts, or progress rings
• Stroking paths in SwiftUI.Canvas or as Shape overlays
• Diagnosing "phantom pixel flicker" or "selection edge looks wrong" bugs

If your app is purely text-and-form-based, you can skip this skill.

The One Rule That Causes 80 % of Stroke Bugs

SwiftUI's .stroke(lineWidth: N) paints CENTERED on the path. A 2pt stroke covers 1pt INSIDE the path and 1pt OUTSIDE.

For Rectangle().path(in: rect).stroke(lineWidth: 2):

• Inside the rect: 1pt of stroke covers the rect's edge cells
• Outside the rect: 1pt of stroke extends past the rect's edge

This is correct rendering — exactly what CGPath does — but it's almost never what you want when you draw a border around a region.

Outset vs Inset — Which to Use

You have three choices for where the stroke sits relative to the rect's logical boundary:

You Want	Recipe
Stroke entirely OUTSIDE the rect	rect.insetBy(dx: -lineWidth/2, dy: -lineWidth/2) then stroke
Stroke entirely INSIDE the rect	rect.insetBy(dx: lineWidth/2, dy: lineWidth/2) then stroke
Stroke centered on the rect's edge (default)	No inset — .stroke(lineWidth: N)

The most common case for app developers: a selection border that should NOT visually overlap the cells inside the selection. That's "stroke entirely outside" — insetBy(dx: -lineWidth/2, dy: -lineWidth/2) to outset before stroking.

let strokeWidth: CGFloat = 2
let strokeRect = cellRect.insetBy(
    dx: -strokeWidth / 2,
    dy: -strokeWidth / 2
)
Rectangle()
    .path(in: strokeRect)
    .stroke(Color.white, lineWidth: strokeWidth)

Marching Ants and Animating Dash Phase

A common pattern: an animated dashed border (marching ants) for a selection. The animation is implemented via a TimelineView that updates StrokeStyle.dashPhase over time.

When the stroke is centered on the path AND anti-aliasing is enabled (the default for SwiftUI Canvas), the sub-pixel coverage of the stroke at the cell boundary changes every frame as dashPhase shifts. The result: diagonal pixel flicker emanating from the corners of the selection, looking exactly like a Core Animation rendering bug.

It's not a Core Animation bug. It's the stroke painting 1pt INSIDE the cell, combined with anti-aliased sub-pixel coverage of the cell's edge changing every frame.

The fix:

TimelineView(.animation) { timeline in
    let phase = CGFloat(timeline.date.timeIntervalSinceReferenceDate
                            .truncatingRemainder(dividingBy: 1.0)) * 10.0
    let strokeWidth: CGFloat = 2
    let outsetRect = cellRect.insetBy(
        dx: -strokeWidth / 2,
        dy: -strokeWidth / 2
    )
    ZStack {
        Rectangle()
            .path(in: outsetRect)   // OUTSET so stroke draws outside the cells
            .stroke(.white,
                    style: StrokeStyle(lineWidth: strokeWidth,
                                       dash: [6, 4],
                                       dashPhase: phase))
        Rectangle()
            .path(in: outsetRect)
            .stroke(.black,
                    style: StrokeStyle(lineWidth: strokeWidth,
                                       dash: [6, 4],
                                       dashPhase: phase + 5))
    }
}

The black layer is offset by 5pt of dash phase so its dashes fall in the gaps of the white dashes — the marching ants stay legible against any background.

1pt vs 0.5pt Strokes on Retina Displays

iOS devices since iPhone 6 are 2× or 3× retina. A 1pt stroke is 2 or 3 device pixels wide; a 0.5pt stroke is 1 device pixel wide on @2x displays (and 1.5 device pixels on @3x).

When to choose what:

Stroke Use	Recommended lineWidth
Hairline separator (List rows, Form sections)	0.5pt — the iOS system separator
Subtle outline that should read as "1 pixel"	0.5pt on @2x, 1pt elsewhere — usually just 1.0 / UIScreen.main.scale clamped
Visible outline / button border	1pt minimum
Selection / interaction affordance	2pt minimum (visible without squinting)
Hero stroke (chart axes, large gauges)	3pt+

If you find yourself using 0.5 for a stroke that should be visible at arm's length, you're solving for retina but ignoring legibility. Bump to 1pt.

Anti-Aliasing — When to Embrace, When to Disable

SwiftUI's Canvas and Shape views use anti-aliasing by default. AA is correct for:

• Curves (circles, rounded rectangles, paths)
• Diagonals (any non-axis-aligned line)
• Anything that needs to look smooth at small sizes

AA is the problem for:

• Pixel-art-style apps drawing 1×1 cells (you want hard edges, not soft)
• Grid overlays where alignment is the whole point

To disable AA on a SwiftUI Canvas, draw with the Canvas rendering closure's context.stroke(...) and set the path's blend mode or use the context.withCGContext escape hatch to call cgContext.setShouldAntialias(false) directly:

Canvas { context, size in
    context.withCGContext { cgContext in
        cgContext.setShouldAntialias(false)
        cgContext.setLineWidth(1)
        // … draw paths …
    }
}

For most apps, leave AA on. Only disable if you've confirmed AA is the source of a specific visual problem.

Pixel Alignment for Hairline Strokes

A 1pt stroke at a non-pixel-aligned coordinate produces a 2-pixel-wide soft line on @2x retina (the stroke's center falls between pixels, AA spreads the coverage). Aligning the stroke to a half-pixel makes it land exactly on one device row:

let scale = UIScreen.main.scale       // 2 or 3
let pixelAligned = round(coord * scale) / scale

Use this for hairline separators, chart gridlines, and any 1pt stroke where you want a crisp single-pixel line. Skip for thicker strokes — the AA benefit is invisible past 2pt.

Common Symptoms → Likely Cause

Symptom	Likely Cause
Selection border appears to "eat" the edge cells of the selection	Stroke is centered, not outset; insetBy(-lineWidth/2)
Animated dashed border has diagonal pixel flicker emanating from corners	Stroke + AA + animating dashPhase + cell-boundary path
1pt line looks 2pt wide and blurry	Stroke center falls between pixels; pixel-align
Pixel-art cells look soft / blurred	AA enabled on a render meant to be pixel-perfect
Stroke disappears entirely on @3x devices	lineWidth < 1.0 / 3.0 ≈ 0.33pt; bump to 0.5 minimum

Anti-Patterns

Temptation	Why It Fails
Trusting .stroke() defaults for selection borders	Strokes are centered on the path; for selection borders you want outset
Eyeballing whether a stroke is "off" by a pixel	Verify with a 10× zoom on simulator; sub-pixel issues are invisible at 1×
Disabling AA "to be safe"	AA is correct 99 % of the time. Disable only with evidence
Using 0.5pt for a button border	Reads as a hairline ghost; bump to 1pt for legibility

Verification

For any custom drawing code:

1. Render at 1× and at 5–10× zoom on simulator. Pixel issues are invisible at 1×.
2. Switch to dark mode and re-check — strokes that "looked fine" in light may have insufficient contrast in dark.
3. Animate any animating strokes for 5 seconds and watch for flicker. Flicker means stroke + AA + boundary issue.

Time Estimate

15 minutes to read and internalize. Saves multiple hours of debugging when the bugs in the symptoms table inevitably show up.