Phase 2-C: Text mobjects via CoreText outlines

[ronaldmannak]

Phase 2-C: Text mobjects via CoreText outlines

Scope

Mobject.text("Hello") — real vector text that morphs, fades, and transforms like any other mobject, by extracting glyph outlines with CoreText and converting them to the existing cubic-Bézier representation. create on stroke-less filled mobjects (like text) now draws a temporary borrowed-fill-color outline (Manim's Write behavior) so the draw-in phase is visible.

Public API

#if canImport(CoreText)
extension Mobject {
    /// A filled, strokeless text mobject centered at `position`.
    /// fontSize is Manim-style: 48 (the default) spans one scene unit
    /// per em, so default text is a bit under one unit tall.
    public static func text(
        _ string: String,
        fontName: String? = nil,     // nil → system UI font
        fontSize: Double = 48,       // Manim-style; 48 → 1 unit per em
        at position: Vec2 = .zero,
        color: ManimColor = .white
    ) -> Mobject
}

extension BezierPath {
    public static func text(_ string: String, fontName: String?, fontSize: Double) -> BezierPath
}
#endif

Design notes

• Platform gating: the whole file is #if canImport(CoreText), so the package still builds on Linux CI — text is simply unavailable there. Tests are gated the same way (they run on the macOS job, no-op on Linux).
• Pipeline: CTLine → runs → glyphs + positions → CTFontCreatePathForGlyph → CGPath.applyWithBlock → subpaths. Quadratic segments are elevated to cubics, close-path elements emit explicit closing edges so the morph system sees closed subpaths, and the element switch has an @unknown default.
• Sizing (Manim parity): glyphs are laid out at 48 pt and scaled so fontSize 48 = one scene unit per em (unitsPerPoint = (fontSize / pointSize) / pointSize); fontSize scales linearly from there.
• Centering: the mobject recenters on its visual bounding box, so at: places the text's optical center, consistent with shapes.
• Fill-only by default (stroke width 0) — matching how Manim renders text. The create animation borrows the fill color as a temporary outline during draw-in and fades it out as the fill arrives, ending exactly at the authored state.
• The run-font lookup type-checks via CFGetTypeID before casting — Swift 6 rejects as? CTFont ("conditional downcast to CoreFoundation type will always succeed") so a conditional cast is not an option here.

Test strategy

TextMobjectTests.swift (CoreText-gated): glyph outlines are non-empty and closed, em sizing, linear scaling between font sizes, visual centering, whitespace advances, align/morph round-trip against a shape, .create on text, and the temporary draw-in outline (createShowsTemporaryOutlineForStrokelessText).

Concurrency

CoreText objects never escape the factory — everything returned is the existing Sendable value types. No @unchecked Sendable.

Stack

Independent — based on main.

[gemini-code-assist]

Code Review

This pull request introduces text mobjects to the PicoManim library by extracting glyph outlines using CoreText and storing them as Bézier subpaths, enabling text to be drawn, morphed, and styled like other mobjects. Comprehensive unit tests are also added to verify layout, scaling, and animations. The review feedback suggests improving safety by avoiding force-casting when extracting font attributes from CTRun, and enhancing maintainability by replacing magic numbers with the defined pointSize constant in the scaling calculation.

Important

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[gemini-code-assist]

Casting the run attributes to [NSAttributedString.Key: Any] and then force-casting the font with as! is verbose and potentially unsafe. We can cast to [AnyHashable: Any] and safely cast the font using as? CTFont with the CoreText key kCTFontAttributeName directly.

Suggested change

	let attributes = CTRunGetAttributes(run) as? [NSAttributedString.Key: Any]
	let runFont = attributes?[NSAttributedString.Key(kCTFontAttributeName as String)]
	.map { $0 as! CTFont } ?? font
	let attributes = CTRunGetAttributes(run) as? [AnyHashable: Any]
	let runFont = attributes?[kCTFontAttributeName] as? CTFont ?? font

[ronaldmannak]

Partially fixed in 6d83c0a + a16396c. The force cast is gone, but the suggestion as written does not compile: Swift 6 rejects as? CTFont outright — the macOS CI job failed with "error: conditional downcast to CoreFoundation type 'CTFont' will always succeed" (run for 6d83c0a, which tried the conditional-cast form). a16396c replaces it with a CFGetTypeID(value as CFTypeRef) == CTFontGetTypeID() check followed by the (now type-verified) unconditional cast, with a source comment explaining why the conditional cast can't be used.

(Source: comment r3533121355 on PR #13 — the as? CTFont form is a compile error in Swift 6, so please don't re-suggest it.)

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[gemini-code-assist]

Avoid duplicating the magic number 48 for the font size and scaling. Using the defined pointSize constant makes the scaling formula more maintainable and self-documenting.

Suggested change

	let pointSize: CGFloat = 48
	let unitsPerPoint = (fontSize / 48) / 48
	let pointSize: CGFloat = 48
	let unitsPerPoint = (fontSize / Double(pointSize)) / Double(pointSize)

[ronaldmannak]

Fixed in 6d83c0a — the scale formula now uses the pointSize constant ((fontSize / Double(pointSize)) / Double(pointSize)) instead of repeating the literal.

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[chatgpt-codex-connector]

💡 Codex Review

Here are some automated review suggestions for this pull request.

Reviewed commit: f699af7cba

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[chatgpt-codex-connector]

Make text visible during create draw-in

When callers animate default text with .create, the first half of the animation only advances strokeEnd before the fill fade starts; checked ManimScene.apply's .create case, and it doesn't reveal fill until progress exceeds 0.5. Because text is initialized here with both zero stroke alpha and zero stroke width, Mobject.text("Hi") remains completely invisible during that draw-in phase and then just fades in, so the new text mobject does not actually draw in like the other path mobjects.

Useful? React with 👍 / 👎.

[ronaldmannak]

Fixed in 6d83c0a — ManimScene.apply's .create case now borrows the fill color as a temporary outline whenever the target has no visible stroke and a visible fill (Manim's Write behavior): the borrowed stroke is fully opaque during the draw-in phase, fades out exactly as the fill fades in, and is gone at progress 1 so the end state matches the target bit-for-bit. This benefits any stroke-less filled mobject, not just text. Test added: createShowsTemporaryOutlineForStrokelessText (visible stroke at t = 0.25, no stroke and full fill at the end).

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[chatgpt-codex-connector]

Scale fontSize as scene units

When callers use the advertised API where fontSize is the em height in scene units, this applies an extra / 48: Mobject.text("A", fontSize: 1) lays out a 48-point glyph and then scales it to roughly 1/48 of a scene unit instead of one unit, making the documented default/non-default sizing effectively tiny. The fixed CoreText point size should only determine outline extraction; the scene scale needs to be based on fontSize / pointSize.

Useful? React with 👍 / 👎.

[ronaldmannak]

Not a code bug — the PR description was wrong, and I've corrected it. The code and its doc comments consistently implement Manim-style font sizes: fontSize defaults to 48, and 48 → one scene unit per em (unitsPerPoint = (fontSize / 48) / 48), so Mobject.text("A") is about one unit tall, matching Manim's proportions. The "fontSize is the em height in scene units / default 1" wording in the PR description was a drafting error on my side; the description now states the Manim-style contract the code actually implements. The tests (defaultSizeSpansAboutOneUnitPerEm, fontSizeScalesLinearly) pin that behavior.

(Source: comment r3533141965 on PR #13 — resolved by fixing the PR description, not the code.)

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[ronaldmannak]

@codex review

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Generated by Claude Code

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Already looking forward to the next diff.

Reviewed commit: a16396c562

ℹ️ About Codex in GitHub

Your team has set up Codex to review pull requests in this repo. Reviews are triggered when you

• Open a pull request for review
• Mark a draft as ready
• Comment "@codex review".

If Codex has suggestions, it will comment; otherwise it will react with 👍.

Codex can also answer questions or update the PR. Try commenting "@codex address that feedback".

[ronaldmannak]

Ready for human review/merge

Phase 2-C (text mobjects via CoreText outlines) is merge-ready.

• Focused diff — CoreText glyph-outline extraction (Mobject.text / BezierPath.text) plus the Write-style temporary outline for stroke-less create, all gated behind #if canImport(CoreText).
• CI green — test-macos and test-linux both pass on the head commit (a16396c); the platform gate keeps the Linux build clean while the text tests run on macOS.
• Tests — glyph outlines, em sizing, linear scaling, visual centering, whitespace advances, align/morph round-trip, .create, and createShowsTemporaryOutlineForStrokelessText.
• Automated review resolved — Codex reviewed the head commit clean; the earlier Gemini/Codex comments were each answered (draw-in visibility fixed, pointSize constant, font cast reworked to a CFGetTypeID check after the as? CTFont form failed to compile, sizing clarified in the description).
• PR description updated to state the Manim-style sizing contract the code implements.

Independent — based on main. Not merging; awaiting your go-ahead.

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