Martino Ferrari
0a209becb1
A measurement is a statistic of the window the pane is *about*, and on a pinned view -- a held event, a paused stream, an opened file -- that is the frame, not the part of it on screen. The pane followed the viewport instead, so a wheel on a latched event moved `average` and `top`: the reported "when measuring a still frame, moving the zoom makes measurements such as average or top change". Nothing about the data had changed; the user was only looking closer. The window is `RowSet::pinned_range()` while the pin frames the view and the viewport otherwise, and the choice is made in `update_measures()` -- the call site -- rather than inside `MeasureWindowReader`, because it is *booked* as well as read: that window is what the "it moved" test compares against `App::measure_window`, so a gesture inside the pin spends no recompute either, and a reader that chose its own would leave a pinned view recomputing every gesture frame for a number that cannot change. `pinned_range()` is now the one expression of the pin's extent, shared with the framing (`hold_pinned_window()`, `fit_x()`), and the reader is handed the window instead of reading the plot. Live is unchanged: there the window really moves and the statistic follows it, and the accumulator's rule about what resets the history is about neither. `UDPSCOPE_PROBE_MEASUREPIN_MS` is the guard, three legs of before/after a gesture on the row's own axis. On a latched event zoomed to a quarter (`view=[-1..1]` -> `[-0.25..0.25]`, `pin` unchanged): `same=4/4 for_same=1 pass_delta=0`, every value bit-identical -- `average=0.000000 top=0.957975 peak to peak=2.000000 amplitude=1.915950` both sides. The same on a pause taken from it. Since the key `for=` follows `pin=` now, `[measure-trace]` gained the field, so a held run reads `for==pin` with `swin` free to sit inside it. On a live view a zoom to an eighth reads `same=0/4 for_same=0 pass_delta=3 span_after=0.125000 follows=1` with the values moving -- the case the rule must not swallow. Before the fix the same probe reads `same=1/4 for_same=0 pass_delta=1 span_after=0.500000` on the held event (`top` `0.957975` -> `0.957966`) and `same=0/4 span_after=0.125000` on the pause (`top` `0.957944` -> `0.358614`). Live reads the same either way. Also fixed, found by the pause leg: pausing while an event is *held* froze an **empty** picture. `on_pause_toggled()` read `stream_at_x0()` after setting the pause state, and that accessor follows the display -- `paused` with no pause view published yet answers `-display_epoch()`, which on a held event is 0, so the cut searched the ring at edge-relative x, found nothing, and the frame bound an empty snapshot: every measurement `--`, every ruler gone (`ev=650001` on the probe's line now, `ev=0` before). The offset and the window are read first, which is the offset the axis is actually displaying. The live case is unaffected, which is why the pause probe, which pauses a live view, never saw it. Docs corrected where they asserted the old half-truth: the invariants now separate the accumulator's rule (unchanged) from the window a value is computed over (this change), the trigger/pinning file names the pin as load-bearing for both the framing and the numbers and records the pause ordering rule, `verification.md`/`probe-readings.md` describe the new probe and `pin=`, and the README and the manual say what a still frame measures. Checked: `cmake --build build --clean-first -j4` 0 errors and the documented three warnings; `make -C build test-ui` 13 ok; `make -C build test-help` green; an exit-dump run 0 Gtk-WARNING/Gtk-CRITICAL and `gtk warnings: 0`; `[measurepin-probe]`, `[pan-probe]` (`held=1`, `allowed=1`, `limited=1`), `[pause-probe]` (`event_size` non-zero exactly while paused, `axis` inside `frozen_x`, `skips` 0 live / 30 paused), `[stats-probe]` (`dn=1` across the pause), `[zoom-probe]` (`gap_px=0.0` at every step) and `[measure-probe]` + the dump's `measures window:` (all-types run `marks_in_window=4/4`, `pin=off` live) on the fast mock. `measure cost:` is unchanged in kind: `raw=37298 (0.0%)` of `covered=121704537`, 1.04 ms a pass -- the runs that read higher are the probe's own zoomed-to-a-slice windows, where the window is legitimately shorter than a bucket.
158 lines
11 KiB
Plaintext
158 lines
11 KiB
Plaintext
<page xmlns="http://projectmallard.org/1.0/"
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type="topic" style="task"
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id="measurements">
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<info>
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<link type="guide" xref="index#analysis"/>
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<link type="seealso" xref="cursors"/>
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<desc>A measurement is a statistic of one signal over the window the pane is about — the
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visible range, or the frozen frame while a view is pinned — with optional
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on-plot marks and a per-measurement statistics line.</desc>
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<credit type="author copyright">
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<name>Martino Ferrari</name>
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</credit>
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</info>
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<title>Measurements</title>
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<p>A measurement is a statistic of one signal over the window the pane is about, recomputed as
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that window moves. The pane lists them with their values; each can also draw its result on
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the plot.</p>
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<p>That window is the <em>visible range</em> — so on a live view the numbers follow what you
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pan and zoom to. On a <em>still frame</em> — a held event, a paused stream, an opened file — it
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is the frame's own extent instead, so panning or zooming inside it changes nothing: the data
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being measured has not changed and you are only looking closer.</p>
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<section id="reach">
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<title>How to reach it</title>
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<steps>
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<item>
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<p>Press the measurements button in the toolbar, or <key>m</key>.</p>
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</item>
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<item>
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<p>The pane's <gui>+</gui> adds one; clicking a row opens the dialog where the signal,
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the type, the threshold share or shares and the on-plot marks are set, with
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<gui>Save</gui> and <gui>Delete</gui>.</p>
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</item>
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</steps>
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</section>
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<section id="knowing">
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<title>Worth knowing</title>
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<list>
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<item><p>The types are average, top, bottom, min, max, peak to peak, amplitude, above,
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below, between, pulse duration, period and frequency. <gui>top</gui> and <gui>bottom</gui> are the waveform's
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<em>flat</em> high and low levels: the level the signal dwells at, taken from the densest
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band of the window on each side of the split. A band-limited edge overshoots and rings —
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a nominal 2 V square reaches about ±1.18 at its edges — and a ripple or an overshoot is
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therefore <em>not</em> what these read. <gui>amplitude</gui> is the delta between those two
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levels, and <gui>peak to peak</gui> is the swing between the extremes (so on a rippled
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square <gui>amplitude</gui> reads 2 V where <gui>peak to peak</gui> reads 2.36 V).</p></item>
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<item><p><gui>above</gui>, <gui>below</gui> and <gui>between</gui> are the opposite kind of
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number: the plain <em>mean of the samples</em> above a share, below it, or between two
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shares. That is a different question from <gui>top</gui>/<gui>bottom</gui> even about the
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same split, and it is the older one — on a unit sine <gui>above 50%</gui> reads 0.64 V
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(2/π, the mean of a half cycle) where <gui>top</gui> reads 0.96 V (near the peak, which is
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where a sine dwells most), and on a band-limited square <gui>above 50%</gui> reads 0.99 V
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where <gui>top</gui> reads the plateau, 1.00 V: the edge's ring is inside the first and not
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inside the second. <gui>between</gui> takes <em>two</em> shares — <gui>from %</gui> and
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<gui>to %</gui> — and reports the mean of the samples inside that band, which no single
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split can answer; the order you type them in does not matter, since a band between 80 % and
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20 % is the band between 20 % and 80 %.</p></item>
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<item><p>A mean over a side carries whatever is on that side, so these three are the right
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numbers when you want the <em>energy</em> or the <em>offset</em> of part of the waveform,
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and the wrong ones when you want the level a plateau sits at. Like every other measurement,
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one with nothing to measure reads <code>--</code> rather than a made-up number: a share at
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100 % has no sample above it, and a band can hold none at all — on an ideal
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(non-ringing) square every sample is exactly ±1, so a band between 50 % and 75 % is
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empty.</p></item>
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<item><p><gui>top</gui> and <gui>bottom</gui> also read <em>near the peak</em> on a
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waveform that has no flat part, which is the honest reading rather than a failure: a unit
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sine has no plateau, so the densest band is at its peak and these read about ±0.96 — not
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the 0.64 a mean over each half would give, and not ±1 either. Only a level the signal
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actually dwells at is a level.</p></item>
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<item><p>The <em>split</em> says which side a level is read from, so any share between the
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two levels reads the same number; it is not a boundary the value depends on. The one case
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where it is read directly is a window the scope cannot summarize by level — a window
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shorter than a level's own resolution, or one read through <gui>hf</gui> — where
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<gui>top</gui>/<gui>bottom</gui> fall back to the mean of each side of the split and
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<gui>amplitude</gui> to the window's peak to peak.</p></item>
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<item><p>The on-plot marks come in two kinds and the mix is the reading: a <em>level</em>
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mark is a ruler at a value, a <em>time</em> mark is a marker at a position. An average
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draws one level mark; a max draws a level mark <em>and</em> a time mark at the first
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sample that reached it; <gui>peak to peak</gui> draws a ruler at each extreme and
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<gui>amplitude</gui> a ruler at each of its two flat levels — the pair is the span, so
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the two rulers <em>are</em> the measurement.</p></item>
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<item><p>Marks cannot be dragged. A measurement mark says what the measurement measured,
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so it is created unselectable — as is the trigger marker.</p></item>
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<item><p>A pass is throttled to a cadence rather than run every frame, so a ruler over a
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live view is <em>stepped</em>. The number and the ruler come from the same pass, so they
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always agree.</p></item>
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<item><p>An invalid pass keeps its <em>level</em> marks and drops its <em>time</em>
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marks. A level is a property of the signal and outlives the window; a position inside
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the window does not. That is what stops the rulers blinking off at a mode change.</p></item>
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<item><p>A crossing is counted with <em>hysteresis</em>, not on a bare level: the
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thresholds are a tenth of the window's peak to peak either side of the share, and an
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edge is counted only once the signal has passed the far one. Period, frequency and pulse
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duration all depend on it. The mid level is exactly where a waveform moves fastest, so
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any interferer faster than the fundamental recrosses a single level several times per
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real crossing — a 5 Hz sine with a 500 Hz tone at a tenth of the amplitude reads 39 Hz
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without the band and 5 Hz with it. The band holds to an interferer a third of the
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waveform's own size.</p></item>
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<item><p>The band is clamped to stay inside the data, which is what keeps a threshold set
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near an extreme usable: a fixed band there would sit above every sample and the
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measurement would read nothing. A share near an extreme still leaves the band little
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room, so it rejects less — which is one reason period and frequency offer no share to
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set.</p></item>
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<item><p>A window with too few crossings reads <code>invalid</code> / <code>--</code>
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rather than a plausible invented number. Frequency over a window holding one edge is the
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usual case.</p></item>
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<item><p><gui>frequency</gui> is reported in Hz and <gui>period</gui>/<gui>pulse
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duration</gui> in seconds; every other type carries the channel's own unit. A value is
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written with a <em>metric prefix</em> so its digits sit in <code>1..999</code> —
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<code>636.62 mV</code>, not <code>0.64 V</code> — including when the stream declares no
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unit at all, which is always the case for a math channel: there the prefix goes tight
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against the number (<code>636.62m</code>), since two decimals of an unprefixed small
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value is <code>0.00</code>.</p></item>
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<item><p>Cost is bounded by reading the summary pyramid the producer already maintains,
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not the raw window: a bucket's sum is a sum, so the mean, the extremes and a level's own
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mean are exact. <gui>top</gui>, <gui>bottom</gui> and <gui>amplitude</gui> read the
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<em>density</em> of the window out of that same summary — which is what tells a flat level
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from a ripple — so those three cost a second read of the summary and never of a sample, and
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<gui>between</gui>'s second share is one more level in the same summary walk rather than a
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walk of its own.</p></item>
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<item><p><gui>Statistics</gui> adds a second line to every measurement showing
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<code>n</code>, the mean, the standard deviation and the range. Each observation is one
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<em>recompute</em> of the pane, so <code>n</code> counts windows measured, not samples —
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a mean frequency over many windows is what that claims to be. <gui>Reset</gui> clears
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every accumulator and starts fresh. A change of display window, a CONFIG or a
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measurement edit also resets; a pause, a resume, a zoom, a pan and the toggle itself do
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not, so turning the line off to read the plot and back on again keeps the history. On a
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still frame a pan or a zoom changes nothing for the history to record in the first place —
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the values do not move there at all.</p></item>
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</list>
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</section>
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<section id="example">
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<title>Example</title>
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<p>On a 1 Hz ±1 sine: <code>average=0</code>, <code>top=0.957988</code>,
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<code>bottom=-0.957988</code> (the flat levels, near the peaks — a sine has no plateau),
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<code>max=1</code>, <code>min=-1</code>,
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<code>peak to peak=2</code>, <code>amplitude=1.915983</code>, <code>pulse duration=0.5 s</code>
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(duty 0.5), <code>period=1 s</code>, <code>frequency=1 Hz</code>. On the same stream's
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band-limited <code>square</code>, whose edges ring out to ±1.179: <code>top=0.999983</code>,
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<code>bottom=-0.999983</code>, <code>amplitude=1.999966</code> against
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<code>peak to peak=2.358701</code> — the levels are the plateau's and the extremes are the
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edges'. Over a 2 M-sample window those passes read about 0.1% of the samples.</p>
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<p>The mean-over-a-side types on the same two windows: on the sine
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<code>above 50=0.636620</code>, <code>below 50=-0.636620</code> and
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<code>between 50 75=0.255873</code>; on the rippled square <code>above 50=0.986496</code> and
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<code>between 50 75=0.302744</code>, against <code>1.000000</code> for the same
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<code>above 50</code> on an ideal (non-ringing) square. A <code>ramp</code>, whose density is
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uniform, is the shape that names what a band mean <em>is</em>: <code>above 50=0.500000</code>
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and <code>between 50 75=0.250000</code>, each band's own centre.</p>
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</section>
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</page>
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