Martino Ferrari
e14e1ee069
Two changes in the trigger pane, landed together. **Persistence.** Trigger mode keeps the last N published acquisitions and draws them fading toward the background, N being a panel control (1..8, default 1) with N=1 meaning today's behaviour exactly, so the knob is the switch. The layers are the accumulate path's own -- one layer per (signal, age) in the row's free trace slots, named with the accumulate vocabulary so `layer_base_name()` strips the suffix and `signal_color()` resolves the base signal -- so a layer is not a signal: the signal list, the colours and the measurements see one row per signal. `ScopeModel` retains the outgoing snapshot in seven atomic `shared_ptr` slots (no lock, no copy; the snapshot already exists) and a layer's channel is resolved **by name inside the snapshot**, because the channel list is renumbered by any structural change since that acquisition was cut. `RowSet::sync_layers()` re-plans from the rows and the depth alone, so a latch costs no rebuild, and `event_data_window()` skips layer slots so an older acquisition can never re-pin the axis. The schedule is 0.5^age -- the acquisition on screen at full brightness and each older one half the previous, independent of N, so raising the depth adds dimmer acquisitions rather than rescaling the ones already on screen. Memory is bounded by the ring's own `--buffer-mb` rather than by the depth, which is the bound that cannot be driven into an OOM, and the panel says when either bound bites. This also wired up `set_trace_alpha()`, which the accumulate path was setting and nothing was reading -- so the documented 0.35 translucency of an accumulation was opaque in fact. **Force.** In the trigger pane beside Arm/Disarm, shown only while trigger mode is on: one acquisition now, with the configured window. It is a synthetic edge rather than a second publisher -- the engine opens one capture at the present moment and everything else is the ordinary latch, so a forced acquisition is cut, pinned, framed, recorded and cached exactly like a latched one, and the button owns no window and no publish. The pre-trigger gate is deliberately skipped (capture now is a request for the history that exists, and the snapshot reports its own shorter pre_s), a capture already filling is refused rather than restarted, and the state afterwards is the mode's: Normal stays armed, Single disarms. The request is one atomic flag written under the model mutex with `armed` -- the producer reads `armed` at the top of a packet and the request at the bottom, so a request written between the two would be dropped -- and consumed at the end of a packet, after the channel loop and before the pending publish. **A defect fixed in passing, in the function Force was modelled on:** `on_arm_clicked()` passed `TriggerConfig::channel` (an index into `channels()`) to `set_signal_enabled()` (an index into `signals()`). With one signal acquired, arming switched on the stream's clock signal and left the watched signal untouched -- and since a signal the panel was not showing is a change, it rebuilt the buffer under the click, dropping the very pre-trigger window the arming, and now the Force, was about to capture. Measured before: `0.0100 s of history behind it`; after: `2.5299 s`. `trigger_signal_index()` is now the one resolution, used by both buttons.
247 lines
14 KiB
Plaintext
247 lines
14 KiB
Plaintext
<page xmlns="http://projectmallard.org/1.0/"
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type="topic" style="task"
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id="trigger">
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<info>
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<link type="guide" xref="index#viewing"/>
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<link type="seealso" xref="pause"/>
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<desc>The trigger's four shapes, arming, forcing an acquisition and hold-off, the event cache with its replay,
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compare and accumulate, and searching the buffer for a condition.</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>Trigger</title>
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<p>The trigger watches one signal for a condition and captures a window around it. The
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<gui>Shape</gui> dropdown chooses what counts as an event; the arming, hold-off, pre/post
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window and Normal/Single mode are the same for all four shapes. The captured window is
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frozen on screen until you re-arm.</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>The trigger button in the toolbar (<key>t</key>) reveals the trigger panel: the
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signal, the level, the shape, the edge (rising, falling, either), the mode (Normal or
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Single) and the pre-trigger share.</p>
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</item>
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<item>
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<p><gui>Arm</gui> starts watching; <key>a</key> arms and disarms from the keyboard.
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When the pane is locked with its <gui>[edit]</gui> button — so that it states what the
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trigger is set to instead of showing the controls — <gui>Arm/Disarm</gui> is in its
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heading, and the trigger can be armed or disarmed without unfolding the panel. It is
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offered only while trigger mode is on, because a trigger that is off has no engine to
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arm.</p>
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</item>
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<item>
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<p><gui>Force</gui> takes one acquisition <em>now</em>, with the event window above,
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without waiting for the condition to hold — for when the event you are waiting for is not
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coming and you want to see what the signal is doing this instant. The capture runs out its
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post-trigger share and then freezes, exactly like a latch: the marker sits at the instant
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you pressed. In <gui>Normal</gui> mode the trigger goes back to waiting afterwards; in
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<gui>Single</gui> mode it disarms, because one acquisition is the whole of what that mode
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means either way. It needs a signal to be acquired — the trigger cannot watch one that is
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switched off — and pressing it while a capture is already filling leaves that capture
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alone, since it is the acquisition you asked for.</p>
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</item>
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<item>
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<p><gui>Search the buffer</gui> is at the foot of the same panel.</p>
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</item>
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</steps>
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<figure>
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<title>A held capture</title>
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<desc><gui>event held</gui> in the status bar, the axis pinned to the event, and the
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trigger marker at x = 0.</desc>
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<media type="image" mime="image/png" src="figures/trigger-capture.png" width="1280" height="800">
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<p>The pre-trigger share is drawn to the left of the marker.</p>
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</media>
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</figure>
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</section>
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<section id="shapes">
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<title>The four shapes</title>
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<terms>
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<item>
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<title>Edge</title>
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<p>A crossing of the level in the chosen direction, the classic behaviour.</p>
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</item>
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<item>
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<title>Pulse width</title>
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<p>A crossing and its return, qualified by how long the excursion lasted: less than,
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greater than, within or outside a pair of widths; the event is timed at the closing
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edge.</p>
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</item>
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<item>
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<title>Window</title>
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<p>The signal entering or leaving a band between two levels; the panel keeps the two
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levels at least one increment apart, because a zero-width band is not a band: the
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condition becomes an exact float-equality test against the level, which an ordinary
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signal never satisfies and a signal sitting at the level satisfies on every
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transition.</p>
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</item>
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<item>
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<title>Dropout</title>
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<p>The absence of a crossing for longer than a timeout; one event per outage, re-arms
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when a crossing returns.</p>
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</item>
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</terms>
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<p>The shape's own rows are the only ones shown; the rest are hidden, so the panel never
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offers a pulse width to a window trigger.</p>
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</section>
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<section id="persistence">
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<title>Persistence: keeping the last few acquisitions</title>
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<p><gui>Persistence</gui> decides how many of the most recent triggered acquisitions stay on
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screen at once. It is <em>1</em> by default, which is the event alone — the display you get
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when you ask for no persistence — and it can be raised to 8. Each older acquisition is drawn
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at half the brightness of the one before it (1, 0.5, 0.25, 0.125, …), and they are all aligned
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on their own trigger edge, so the pre-trigger part of the event lines up from acquisition to
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acquisition and whatever changes between them — jitter, ringing, the occasional glitch — shows
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as a faint ghost behind the current trace. That is what a phosphor display does with
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successive sweeps.</p>
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<p>The older acquisitions are drawn <em>behind</em> the current one and nothing else changes
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because of them: the axis still frames the event on screen, the trigger marker still marks
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<em>its</em> edge, and the measurements still describe the current frame. Lowering
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<gui>Persistence</gui> back to 1 remembers nothing and frees the retained acquisitions
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immediately.</p>
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<p>Two things bound how many are drawn, and the panel tells you when one of them bites.</p>
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<list>
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<item><p><em>Room in the plot.</em> Each older acquisition is an extra trace in the row that
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plots its signal, so it needs a free trace slot there. A row holding four signals — the
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layout the app starts with — has room for one older acquisition of each of them; a row per
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signal has room for all seven.</p></item>
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<item><p><em>Memory.</em> A retained acquisition is a whole window copied out of the ring,
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so persistence may hold at most as much again as the buffer (<gui>--buffer-mb</gui>) does.
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On a long window at a high sample rate that bound is reached before the eighth layer.</p></item>
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</list>
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<p>If a depth cannot be shown in full, a notice says how many layers fit, and the panel's
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tooltip carries the same rule. Persistence is a display of what has already been captured: it
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changes neither what is acquired nor what is measured, and it does nothing while the trigger
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is off.</p>
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</section>
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<section id="cache">
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<title>The event cache, replay and compare</title>
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<p><em>Every triggered acquisition is kept.</em> Latches are written to the event cache
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(<file>~/.cache/udpscope/events/<source>/</file>) as a <file>.npy</file> of the
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event, a <file>.txt</file> of what it was, and a <file>.bmp</file> picture drawn from the
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samples. The <gui>Events</gui> pane on the left — opened from the toolbar's
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event-history button, which appears while recording is enabled, and collapsed until you
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ask for it — lists the last 32 newest first (time, duration, signals, rate), each row with
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its own picture beside the numbers.</p>
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<p>Clicking a row <em>replays</em> it: the stored event is loaded exactly like an exported
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file, <em>without changing the layout</em> — the rows, the colours, the window, the
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cursors and the measurements stay where they are and read the stored data instead. A
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replay leaves trigger mode, because a file has no edge to wait for, and the pane stays
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open. Drag the pane's edge to resize it. <gui>Live</gui> goes back to the stream; the
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status bar names the file until you do.</p>
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<p>Select two or more events in the list and the two buttons under the header act on them:
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<gui>Compare</gui> puts two events in two plots side by side, keeping the layout you have
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— one plot per event, the axes synchronised, so zooming a row in one zooms the same row in
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the other — and <gui>Accumulate</gui> overlays up to four of them in the same traces,
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translucent, so where they agree the trace is brighter. Your own traces stay where they
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are in both. They need events with the same signals and the same length. A single click
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selects, a <em>double</em> click replays.</p>
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<p><gui>Record events</gui>, in the trigger panel with the trigger's other options, is
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what decides whether anything is kept. It is <em>off by default</em> — nothing is written
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to disk until you ask — and with it off the history button goes with it.</p>
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</section>
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<section id="search">
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<title>Search the buffer</title>
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<p><gui>Search the buffer</gui>, at the foot of the trigger panel and collapsed until you open
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it, answers the question the trigger answers — <em>where does this condition hold?</em> — over
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the <em>whole</em> retained buffer rather than at the live edge. It runs the trigger's own
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evaluator over the ring in stream order, so a <gui>hit</gui> is not a level you point at: it is
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an <em>instant</em> the trigger would have fired on, timed the way an acquisition would have
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timed it. A pulse-width finding, for instance, is the closing edge of a qualifying excursion,
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not the excursion itself.</p>
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<p>The condition is the one the rest of the panel is set to — the same signal, shape, level and
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options — so there is no second condition to keep in step: change the panel and the next search
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asks the new question. Nothing is searched until you ask, and the status line says
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<gui>not searched</gui>.</p>
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<steps>
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<item>
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<p>Set the condition above, then press <gui>Search</gui>. Covering the whole ring takes a
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moment, and the scan runs in chunks so the stream keeps arriving while it does; the status
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line counts the ground covered — <gui>scanning 1.2 s of 8.0 s, 40 found</gui>.
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<gui>Stop</gui> ends a scan early and keeps the hits found so far.</p>
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</item>
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<item>
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<p>Step through the hits with the arrows either side of the position readout
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(<gui>3 of 40</gui>). The first <gui>next</gui> goes to the first hit, and the ends do not
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wrap — a wrap from the last hit to the first would look exactly like the view not having
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moved. Each step <em>pauses</em> on its hit, centred, exactly as the Pause button does, so
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what you read is the frozen window cut around that instant; press the pause button again to
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go back to the stream.</p>
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</item>
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<item>
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<p>Every plot row marks the hits that fall in the window, up to sixteen of them. When more
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are in view the pane says so and suggests zooming in — <gui>16 of 38 marks shown — zoom in
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to see the rest</gui> — and when the selected hit has scrolled off the window it says that
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instead. A hit the live stream has since overwritten reads <gui>no longer in the buffer</gui>
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beside its position.</p>
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</item>
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</steps>
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<p>How a scan ended is part of the answer: <gui>N found in 8.0 s</gui> when it covered the whole
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ring; <gui>N found; the stream overtook the oldest samples</gui> when the producer overwrote
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samples before the scan reached them, so the count is of what was left; and
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<gui>N found (the limit) in the newest …</gui> when a condition that holds almost everywhere —
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an edge on a noisy signal — reached the ten-thousand-hit ceiling. The last is a stop, not a
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crash: a hit list nobody could navigate is not worth building.</p>
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<p>A result describes the ring it was taken from, so it is <em>dropped</em> when that ring is
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replaced: a new <gui>CONFIG</gui> from the source, or a change to the signal set, re-describes
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the stream and clears the hits. The condition above is untouched — only the answer to it
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is.</p>
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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>Arming <em>enables</em> the signal if it was off, because a trigger on a signal
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nobody acquires can never fire.</p></item>
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<item><p>The pre-trigger share is a share of the <em>display window</em>, not of the
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ring. A 2 s window at 50% captures 1 s before the edge and 1 s after.</p></item>
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<item><p>There are four display states, and the status bar names each: <gui>armed</gui>
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(watching, nothing drawn yet), <gui>capturing</gui> (the edge was accepted and the post
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window is filling <em>in view</em>, with the marker at x = 0), <gui>event held</gui>
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(the frozen capture) and <gui>disarmed</gui>. A held capture is never quietly replaced
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by live data.</p></item>
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<item><p>The first edge after arming is only accepted once the history reaches back a
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full pre-trigger window, so a capture may legitimately be shorter than you asked for.
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The axis is pinned to what the capture actually got. <gui>Force</gui> is the one
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acquisition that skips that gate — it captures the history that exists, because "now"
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is the whole of what you asked for — so on a ring that has just started filling its
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left side is short even though the axis still shows only what it got.</p></item>
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<item><p>Disarming <em>during</em> a capture publishes what the capture got to, so the
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picture on screen at that moment stays on screen. No event fired, so the event count
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does not move.</p></item>
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<item><p>The trigger reads the <em>stored</em> signal — decimated and averaged — not the
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wire signal. A spike narrower than one decimation group is filtered away rather than
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triggering on data the buffer does not contain.</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>A 1 Hz sine, level 0, rising, 2 s window at 50%, Single. The capture spans
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<code>[-1.000 .. 1.000]</code> with <code>n=20001</code> samples at 10 kS/s — the span
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times the real rate, plus one — and the marker sits at x = 0 throughout.</p>
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</section>
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</page>
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