特定场景下SCNShape无法通过UIBezierPath绘制的技术求助

Possible Workarounds & Root Cause Analysis for SCNShape 3-Point Path Issue #

First off, thanks for flagging this and submitting a bug report to Apple—this kind of edge case is really helpful for polishing the framework!

Let’s dig into what might be going on and some fixes you can test out:

Potential Root Cause

The problem where SCNShape fails to render a stroked 3-point path (generated via CGPath.copyStrokingWithWidth) is likely tied to how SceneKit processes the path’s geometry. When working with 3-point segments, the stroked path can create self-intersections, zero-area faces, or non-manifold geometry that SceneKit’s renderer silently rejects. This usually happens when the stroke width is large relative to the angle of the middle point, or if the stroked path ends up with overlapping segments.

Workaround 1: Simplify the Stroked Path

Before creating your SCNShape, try cleaning up the stroked path to remove invalid segments or self-intersections. On iOS 14+, you can use the built-in path simplification API:

let originalPath = yourBezierPath.cgPath
let strokedPath = originalPath.copyStrokingWithWidth(
    yourStrokeWidth,
    lineCap: .round,
    lineJoin: .round,
    miterLimit: 10
)
// Simplify the path to fix geometry issues
guard let simplifiedPath = strokedPath.copy(simplifying: .default) else {
    // Handle failure
    return
}
let shape = SCNShape(path: simplifiedPath, extrusionDepth: 0.1)

Workaround 2: Build Custom SCNGeometry Manually

If path simplification doesn’t work, you can bypass SCNShape entirely by generating the stroke geometry yourself. Here’s a high-level approach:

• Split your 3-point path into two separate line segments
• For each segment, create a rectangular polygon that represents the stroke (adjusting the corners at the middle point to match your line join style)
• Combine these polygons into a single SCNGeometry using SCNGeometrySource and SCNGeometryElement

This gives you full control over the geometry and avoids any quirks with SceneKit’s path-to-shape conversion.

Workaround 3: Tweak Stroke Parameters

Sometimes adjusting the stroke’s line join or miter limit can resolve the rendering issue. Sharp miter joins are a common culprit, so try switching to a bevel join instead:

let strokedPath = originalPath.copyStrokingWithWidth(
    yourStrokeWidth,
    lineCap: .round,
    lineJoin: .bevel, // Use bevel instead of round/miter
    miterLimit: 1
)
let shape = SCNShape(path: strokedPath, extrusionDepth: 0.1)

If you can share the exact conditions that trigger this bug (the "且仅当……" part you mentioned), we can refine these suggestions even more!

内容的提问来源于stack exchange，提问作者Andrew