42 lines
1.9 KiB
Dart
42 lines
1.9 KiB
Dart
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// lib/editor/canvas/pinch_scale_solver.dart
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//
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// Pure math for the pen canvas's absolute pinch-zoom. Extracted so the
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// re-baseline behavior (the subtle part) can be unit-tested without simulating
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// a flaky multi-pointer gesture.
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//
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// The pinch is driven ABSOLUTELY: the scale shown is always
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// scaleStart * (rawScale / rawScaleAtBaseline)
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// where `scaleStart` is the matrix scale captured at the current baseline and
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// `rawScaleAtBaseline` is the recognizer's cumulative `details.scale` at that
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// same baseline. Dividing by `rawScaleAtBaseline` re-normalizes the cumulative
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// scale so it reads 1.0 at the baseline instant.
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//
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// Why this matters: a baseline is captured at gesture start AND on every
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// pointer-count change (a finger blips 2→1→2, routine on Windows touch). At
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// gesture start `details.scale` is 1.0, so a naive `scaleStart * rawScale` is
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// correct. But at a MID-GESTURE re-baseline `details.scale` is whatever the
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// pinch has accumulated (e.g. 0.40) — multiplying the fresh `scaleStart` by
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// that stale 0.40 popped the zoom to a wrong scale and snapped back (the
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// reported flicker). Normalizing against `rawScaleAtBaseline` removes the pop.
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import 'package:flutter/foundation.dart' show clampDouble;
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/// Returns the absolute target scale for a pinch frame.
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///
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/// [scaleStart] — matrix scale captured at the current baseline.
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/// [rawScaleAtBaseline] — recognizer cumulative `details.scale` at that
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/// baseline (1.0 at gesture start; the live value at a re-baseline).
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/// [rawScale] — the recognizer's current cumulative `details.scale`.
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/// Result is clamped to [minScale, maxScale].
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double absolutePinchScale({
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required double scaleStart,
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required double rawScaleAtBaseline,
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required double rawScale,
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required double minScale,
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required double maxScale,
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}) {
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final double cumulative =
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rawScaleAtBaseline > 0 ? rawScale / rawScaleAtBaseline : 1.0;
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return clampDouble(scaleStart * cumulative, minScale, maxScale);
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}
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