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Fix voltage spike at segment boundaries in smooth plot mode
In smooth (rise-time) mode, drawSampledWave samples totalVoltageAt at
evenly-spaced z values including exact boundary points (e.g. z=0.5 for
N=2 segments). At those points both the ending wave of the left segment
and the starting wave of the right segment passed the ±1e-9 range checks
in waveVoltageAt, doubling the displayed voltage — a visible spike.
Fix: add segmentForZ(z, N) which maps each sample to its owning segment,
and filter launchedWaves in totalVoltageAt to only sum waves whose segIdx
matches. The +1e-9 nudge in segmentForZ handles floating-point imprecision
when z lands exactly on a boundary (e.g. 200/400 = 0.5 exactly).
The bug was absent in step mode because totalSegmentsForWaves uses midpoints
between breakpoints, never probing the exact boundary z. It was also subtle
with 3 segments (boundary at 1/3) since 1/3·400 is never an integer.
Adds two regression tests that directly catch the spike.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
+83
-3
+18
-2
physics.js
+18
-2
physics.js
···
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}
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}
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// Map z ∈ [0,1] to the index of the segment that owns it.
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// Boundaries are assigned to the right-hand (higher-index) segment; z=1 goes to the last.
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// The +1e-9 nudge handles floating-point imprecision when z is exactly i/N.
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function segmentForZ(z, N) {
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if (z >= 1 - 1e-9) return N - 1;
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return Math.min(N - 1, Math.floor(z * N + 1e-9));
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}
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// Sum of all wave contributions at position z.
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function totalVoltageAt(z, launchedWaves, tau, tr = 0) {
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// For multi-segment models (N > 1), only waves whose segIdx matches the segment
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// that owns z are summed. This prevents double-counting at segment boundaries
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// where adjacent waves share a z value (a plotting artifact, not a physics error).
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function totalVoltageAt(z, launchedWaves, tau, tr = 0, N = 1) {
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const segIdx = N > 1 ? segmentForZ(z, N) : 0;
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let V = 0;
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for (const wf of launchedWaves) V += waveVoltageAt(wf, z, tau, tr);
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for (const wf of launchedWaves) {
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if (N > 1 && wf.segIdx !== segIdx) continue;
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V += waveVoltageAt(wf, z, tau, tr);
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}
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return V;
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}
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···
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riseShapeLinear,
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waveVoltageAt,
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totalVoltageAt,
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segmentForZ,
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computeDynamicState,
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};
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})();
+64
physics.test.js
+64
physics.test.js
···
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sumEventsWithLinearRamp,
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riseShape,
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riseShapeLinear,
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totalVoltageAt,
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computeDynamicState,
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} = globalThis.TLPhysics;
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// Helper: assert two numbers are within `tol` of each other.
···
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// PULSE( 0 1 0 100p 100p 1u 2u ) → linear ramp, TR = 100ps = 0.1·τ_d
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//
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// TSV columns: t[s] v(a) t[s] v(b)
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// ────────────────────────────────────────────────────────────────────────────
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// ────────────────────────────────────────────────────────────────────────────
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// totalVoltageAt: no spike at segment boundaries (multi-segment plotting bug)
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//
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// In smooth mode, drawSampledWave evaluates totalVoltageAt at exact boundary
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// z values (e.g. z=0.5 for 2 segments). Without segment filtering, the parent
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// wave (dir=+1, zEnd=boundary) and the transmitted wave (dir=+1, zStart=boundary)
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// both pass the range check, doubling the voltage at the boundary.
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// ────────────────────────────────────────────────────────────────────────────
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test("totalVoltageAt: no spike at segment boundary (2 segments, step mode)", () => {
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// Z0=40 → Z0=60, Rg=50, RL=∞.
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// Γ_bound = (60-40)/(60+40) = 0.2. V1 = 1 * 40/(50+40) = 4/9.
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// At tNorm=0.6 the boundary crossing (tNorm=0.5) is complete:
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// seg-0 has incident(4/9) + reflected(0.2·4/9), seg-1 has transmitted(1.2·4/9).
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// Correct V at z=0.5 = (1+0.2)·(4/9) = 4.8/9 = 8/15.
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// Buggy code (no segIdx filter) would give 2·(8/15) = 16/15 — a visible spike.
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const model = { Vg: 1, Rg: 50, RL: Infinity,
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segments: [{ Z0: 40 }, { Z0: 60 }], reflectTol: 0.001 };
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const waves = computeWaveParams(model);
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const bounce = buildBounceSeries(model, waves);
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const dyn = computeDynamicState(0.6, bounce, 0, 0);
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const N = model.segments.length;
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const eps = 1e-4;
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const vL = totalVoltageAt(0.5 - eps, dyn.launchedWaves, 0, 0, N);
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const vB = totalVoltageAt(0.5, dyn.launchedWaves, 0, 0, N);
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const vR = totalVoltageAt(0.5 + eps, dyn.launchedWaves, 0, 0, N);
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// Correct value: (1 + 0.2) * (4/9) = 8/15
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near(vB, 8 / 15, "V at boundary = (1+Γ)·V1", 1e-6);
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near(vB, vL, "no spike: V(boundary) ≈ V(boundary-ε)", 1e-6);
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near(vB, vR, "no spike: V(boundary) ≈ V(boundary+ε)", 1e-6);
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});
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test("totalVoltageAt: no spike at boundaries with 4 segments", () => {
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// 4 equal segments: boundaries at 0.25, 0.5, 0.75.
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// tNorm=0.3 puts the front in seg 1; all three boundary z-values should be spike-free.
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const model = { Vg: 1, Rg: 50, RL: 100,
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segments: [{ Z0: 50 }, { Z0: 75 }, { Z0: 50 }, { Z0: 75 }],
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reflectTol: 0.001 };
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const waves = computeWaveParams(model);
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const bounce = buildBounceSeries(model, waves);
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const N = model.segments.length;
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const eps = 1e-4;
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for (const tNorm of [0.3, 0.6, 1.2, 2.0]) {
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const dyn = computeDynamicState(tNorm, bounce, 0, 0);
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for (const zB of [0.25, 0.5, 0.75]) {
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const vL = totalVoltageAt(zB - eps, dyn.launchedWaves, 0, 0, N);
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const vB = totalVoltageAt(zB, dyn.launchedWaves, 0, 0, N);
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const vR = totalVoltageAt(zB + eps, dyn.launchedWaves, 0, 0, N);
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// No spike: boundary value must lie between its two neighbours (within tolerance).
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const lo = Math.min(vL, vR) - 1e-6;
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const hi = Math.max(vL, vR) + 1e-6;
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assert.ok(vB >= lo && vB <= hi,
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`spike at z=${zB}, tNorm=${tNorm}: V=${vB}, neighbours ${vL}..${vR}`);
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}
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}
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});
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+
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// ────────────────────────────────────────────────────────────────────────────
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test("SPICE comparison: linear ramp, open-circuit load (tline-oc.sp)", () => {
+1
-1
render.js
+1
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render.js
···
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const y0 = (V) => top0 + 4 + (vHi - V) / (vHi - vLo) * (panelH - 8);
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drawPanelFrame(top0, bot0, y0, "Sum (all waves)", vLo, vHi);
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if (smooth) {
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drawSampledWave(ctx, xOfZ, y0, (z) => totalVoltageAt(z, launched, riseTimeTau, riseTimeTr), theme.ok, 2.4);
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drawSampledWave(ctx, xOfZ, y0, (z) => totalVoltageAt(z, launched, riseTimeTau, riseTimeTr, segments.length), theme.ok, 2.4);
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} else {
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drawPiecewise(ctx, xOfZ, y0, sumSegs, theme.ok, 2.4);
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}