improving doc

docs/USER-GUIDE.md

@@ -0,0 +1,201 @@
+# udpscope — user guide
+
+A viewer for a UDPStreamer source: it connects, decodes the CONFIG, plots the signals you enable,
+and lets you freeze, measure and derive from them. This is the *user* guide; `README.md` documents
+what was built and why, and `SPECS.md` is the requirement document.
+
+![the main window](img/main-window.png)
+
+*The main window with a source streaming: the signal panel on the right, the plot in the middle, the
+status line directly under it, and the toolbar above. The window control — `2 s` — is the first thing
+on the left.*
+
+## 1. Connecting
+
+```sh
+udpscope                                              # asks which source to open
+udpscope --server 10.0.0.5 --port 8000                # connect straight away
+udpscope --server 239.1.2.3 --port 8000 --multicast 239.1.2.3 --interface 10.0.0.9
+```
+
+With no `--server` the app shows a chooser: the sessions it has stored, each naming its source, plus
+a form for a new host. A source opened from the list reconnects to the multicast group and interface
+it was stored with.
+
+**A unicast source serves one client at a time.** A second viewer's `CONNECT` displaces the first, and
+they take turns — the status line says so when it happens. Watching one source from several instances
+needs `--multicast <group>` with `--interface <local address>`.
+
+Every launch is its own process, so running two scopes side by side is fine.
+
+## 2. The status line
+
+Directly under the plot, it reads `● <state> · <rate> · <window>`, and the ⓘ beside it opens the
+detail: packets, frames, gaps, per-channel sample counts, the buffer's span and its decimation.
+
+- **The state word is one word** from a fixed set: `live`, `paused`, `armed`, `capturing`,
+  `event held`, `disarmed`. `no data` appears in the rate's place when nothing is arriving.
+- **The rate is the *stored* rate**, which is the wire rate only while the whole requested window
+  fits in the buffer. A window wider than the budget allows is bought by averaging groups of samples,
+  so the rate falls and the status line says so — that is the anti-alias filter doing its job.
+- **`connected, but no data is arriving`** appears after three seconds in which no frame arrived while
+  connected: the usual cause is the unicast point above. It clears the moment data returns.
+- **`N reconnects`** is appended when the client has had to re-establish the session.
+
+## 3. Signals
+
+The signal panel lists every signal the CONFIG described, with a switch each. **Signals start
+disabled — enabling one is what starts acquiring it**, and a signal nobody enables costs nothing.
+Drag a signal from the panel onto a plot row to add a trace; drag it again to another row to plot the
+same signal twice. The colour is per *signal*, so the same signal is the same colour wherever it
+appears.
+
+The panel also holds the per-signal calibration: gain and offset, applied at ingest, so the
+measurements and the export carry the same numbers the plot shows.
+
+## 4. The window and navigation
+
+The header's window control (`2 s`) sets the time span on screen; the popover takes a number and a
+unit, with a decade stepper either side. The trigger's event window is a share of *this* span, so a
+window change moves the trigger with it.
+
+- **Wheel** zooms about the pointer; **Shift+wheel** on x only, **Ctrl+wheel** y only.
+- **Ctrl `+` / `-`** zooms x, **Alt `+` / `-`** y, about the plot centre.
+- **Drag** pans, **right-drag** too (the right button also grabs a cursor at the pointer).
+- **Ctrl+drag** frames x to a box, **Alt+drag** y.
+- **Fit to data** (menu) auto-scales both axes and frames the whole buffered range.
+
+The view is bounded by the **time window**, not by the buffer: zooming out stops at the span you set,
+and panning stops where the data starts. When the buffer holds *less* than the window — right after a
+signal is enabled — the window is left empty on its left, which is what a scope that has just started
+should look like.
+
+## 5. Trigger
+
+![a capture held](img/trigger-capture.png)
+
+*Single mode with a capture held: the axis is the event's own `[-pre, +post]`, the trigger marker
+sits at the edge, and the status line reads `event held`.*
+
+The trigger panel has the edge, the pre/post split (as a share of the event span), Normal or Single,
+and Arm/Disarm. Selecting a signal for the trigger enables it.
+
+- **While a capture fills** the plot keeps drawing live data on the event's clock, so the post side
+  fills in view; the axis does not move.
+- **A held event is navigable within its own data** — zooming in works, panning stops at the capture's
+  edges — and it is never quietly replaced by live data.
+- **Disarming mid-capture** publishes what the capture got and holds that, so the picture on screen
+  when you disarm is the picture you keep.
+- The event window is counted in **stream seconds** end to end, so a source whose clock advances in
+  jumps still gets the window it asked for.
+
+## 6. Pause
+
+Pause freezes the *picture*, not the acquisition: the producer keeps filling the buffer behind it, and
+the frozen view is a snapshot of the window that was on screen. Resuming shows the data that arrived
+meanwhile — there is no hole. A paused view is held like a held event, so zooming in works and panning
+does not walk it off its snapshot.
+
+## 7. Cursors
+
+The cursors toolbar toggle switches on two instruments per row:
+
+- **vertical** — two times, shared by every row;
+- **horizontal** — two rulers per row;
+- **both** — the two together;
+- **mix** — the two times, read off each *trace* where they cross it.
+
+The pane beside it reports each trace's values at the markers, and **Reset** puts them back at the
+window's edges. Values read `--` when a marker is past the newest sample and `partial` when only one
+marker is over data.
+
+## 8. Measurements
+
+The measurements pane computes statistics of the **visible window** of a signal: average, top,
+bottom, min, max, peak to peak, amplitude, pulse duration, period and frequency. `+` adds one, the
+row's pencil edits it, and **show on plot** draws its result as a ruler (levels) or markers (times).
+
+`top` and `bottom` take a *split* as a share of the window's own range — the percentage travels
+between signals where a threshold in volts would not. `period` and `frequency` deliberately have no
+split: they count rising crossings of the mid level, one edge per cycle, and a split there is a
+setting you can only get wrong.
+
+The values are refreshed on a cadence — at most every 100 ms while the window is scrolling, once when
+it stops, and once per mode change — so a live view cannot cost you frames. The pane's cost is
+reported in the exit dump with the app's self-check.
+
+## 9. Math channels
+
+![the math channel editor](img/math-editor.png)
+
+*The compact node editor: each node is a box, its inputs the squares on the left edge and its output
+the square on the right. The orange dot marks the node that carries the channel's value.*
+
+A math channel is derived from the others and is then an ordinary channel: enable it and it is
+acquired, plotted, measured and exported like a streamed signal, and another math channel can use it
+as a source.
+
+Operations: `channel` (a source), `gain + offset`, `abs`, `sqrt`, `clamp`, `low pass`, `high pass`,
+`band pass` (biquads, corner in Hz against the stored rate), `sum`, `difference`, `product`, `ratio`.
+
+- **Click** a node to select it; its parameters appear in the form beside the canvas.
+- **Click an output square, then an input square**, to wire them. An input is only offered the nodes
+  above it — the graph is a list walked in order, so a cycle cannot be drawn.
+- **Drag** a box to move it. **add node**, **delete node** and **set as output** are below the form.
+- The **output is the last node** of a graph you build top to bottom, and the orange dot says which;
+  *set as output* changes it. A graph whose output is the source is valid and just passes the input
+  through — that is the usual reason an operation "does nothing".
+- **Save** refuses an incomplete graph and says why: an unconnected input, no output, or a source that
+  is not a channel the scope is acquiring.
+
+The samples are produced at ingest, once per stored slot, so the filters carry real history — a low
+pass is the filter you asked for rather than a window recomputed every frame.
+
+## 10. Sessions
+
+Everything above is saved per source in `~/.config/udpscope/<host>_<port>.toml`: the window, the rows
+and which signal each slot holds, the calibration, **which signals were enabled**, **the math channels
+with their graphs** and **the measurements**. Reopening the same source brings the scope back as you
+left it.
+
+The file is plain TOML and safe to edit — a malformed line is skipped rather than refusing the whole
+file. Deleting it starts from the defaults.
+
+## 11. Export
+
+**Export visible range…** writes the plotted signals over the visible x range at the *stored* rate to
+`.npy` or `.csv`, picked by the file name's extension. The table is collected when you click, not when
+the chooser closes, so a live view cannot scroll under the dialog. The `.npy` is a 2-D array: one row
+per sample, `time`, `stream_time`, then one column per signal.
+
+## 12. Keyboard
+
+| key | action |
+|---|---|
+| `space` | pause / resume |
+| `a` | arm / disarm the trigger |
+| `t` | the trigger panel |
+| `l` | the layout editor |
+| `g` | go live |
+| `f` | fit to data |
+| `d` | add the first free signal to the focused row |
+| `c` | cursors on/off |
+| `m` | measurements pane |
+| `e` | export |
+| `?` | shortcuts |
+
+`Ctrl+E` exports, `Ctrl+Q` quits, `Ctrl+plus`/`Ctrl+minus` zoom x and `Alt+plus`/`Alt+minus` zoom y.
+A keystroke is ignored while a text field has focus, so typing in the window control never arms the
+trigger.
+
+## 13. Troubleshooting
+
+| symptom | cause |
+|---|---|
+| `connected, but no data is arriving` | another client holds a unicast source — use `--multicast` |
+| the plot is empty, the status reads `no data` | nothing is enabled: enable a signal in the panel |
+| the trace is blank while the trigger is armed | `Waiting` — armed and nothing latched yet |
+| the axis has a margin at the right | a capture is filling: the post side arrives as it is acquired |
+| a math channel equals its input | its output node is the source — see §9 |
+| a very wide window shows a low rate | the ingest's anti-alias filter, not a fault |
+| a level reads `--` | the measurement is invalid in this window (e.g. a period with one edge) |