`Plot::set_density()` gives a plot a **phosphor density**: a screen-sized intensity image behind
its traces, into which each acquisition is deposited and which fades by a time constant or by a
per-acquisition factor. It is the library's and not a host's because the library is the side that
knows a trace's device geometry -- a host would be re-rasterising traces the renderer has already
staged, at a second derivation of the same mapping.
It is an **image and not a set of retained traces**, and that is the whole of its cost: one `f32`
cell per device pixel of the axes rect (`DensityGrid`), a cell's value a *number of sweeps*, so a
display of a thousand acquisitions costs what one costs and there is no depth to get wrong.
Measured on a 640x360 viewport's axes: **573x313 = 179 349 cells, 717 396 bytes** of grid, and one
byte per cell uploaded on the frame the grid moved.
- `Plot::accumulate_density()` deposits the acquisition on screen: every visible trace's window is
mapped to sample indices and gathered exactly as `Renderer::draw_traces()` maps it, then walked
column by column -- so the deposit is bounded by the axes' columns and not by the samples behind
them -- and a cell takes **1.0 once per trace per sweep**, however many of that trace's own
samples crossed it. Counting the hits instead put a single dense sweep at the ceiling of the
brightness scale and hid the sweeps behind it, and it made a cell mean "how much the beam moved
here" rather than "how many acquisitions were here"; the dedupe is a stamp per grid *row* (one
`u64`, keyed by trace and column) so two sweeps agreeing on a cell still count twice. Dwell is a
different question the library already answers (`Style::intensity_grading`).
- `decay_density(dt_s)` fades by the clock, `clear_density()` and switching the density off drop
the image at once. `DensityConfig` carries `full_scale` and `gamma`, applied on the CPU when the
grid changed: the value a host reads back through `ink()`/`peak()` and the picture it sees are
one derivation, and a frame whose grid did not move skips the conversion and the upload entirely
(`FrameStats::density_cells`/`density_upload_bytes`, the latter the *frame's* total over every
row, which one row's assignment used to overwrite).
- The renderer draws it as one attribute-less quad over the axes rect (`kDensityVS`/`kDensityFS`,
`DensityProgram`), over the grid and the invalid-span bands and **under the traces**, inside the
axes' clip: it reserves no layout space and nothing on it can be picked, exactly as
`Plot::set_invalid_spans()` does. `Style::density` is its tint.
- The two fades are the two questions a persistence display can be asked -- "still visible a second
later" and "the N-th back is gone" -- so the choice is the caller's.
Six cases, all in the suite: four plot-level ones (the grid's resolution and memory, the
per-acquisition schedule **584 -> 876 -> 1022** ink with `peak_steady=2.000`, `exp(-dt/tau)` off
the ink, and the deposit's own geometry including a NaN as a gap) and one on the real driver
(`a_phosphor_density_is_drawn_under_the_traces`: `cells=179349 uploaded=179349` on the moved frame
and `uploaded=0` on the identical one after it, with the trace's stroke keeping its own colour over
the ink). The suite is **396 cases, 0 failed** and all four ctest entries pass.
Intensity grading was gated on the column reduction, which is gated on a
density whose only job is killing the moire beat. Dwell has nothing to do
with that, so narrowing a window past the threshold turned the whole
phosphor model off and a discontinuity went from a third of a flat top's
ink to all of it in one pixel of zoom.
stage_trace_slopes() grades each segment by its own slope instead, with a
measure that agrees with the dense path at the boundary, and reports
itself as FrameStats::slope_traces -- disjoint from column_traces, so a
host asserting graded_traces == column_traces reads the same as before.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
A pixel column reduced from a dense envelope is honest about where the signal
went and silent about how long it was there: its min and max are identical
whether its samples fill the span or pile at its two ends, so a ramp and a pulse
are one band. Style::intensity_grading grades it instead -- bright where the
signal sits, faint where it only passes through -- by asking the gather for a
level finer than the pixel grid and reading the distribution from those
sub-buckets. Off by default: a change of look, not a correction of one.
The measure is dwell per row, each segment depositing one unit spread over the
rows it spans; counting +1 per row instead grades a sine's fast middle as
brightly as its rails, which is the pattern inverted. The accumulator is one
column wide (a full w*h density image is 33 MB at 4K) with each span written as
two delta writes, and normalisation is against the column's own peak -- a
frame-wide scale fades the trace out exactly where it moves fastest. Alpha is
quantised into tiers, one LineBatch and one draw call each, because the line
pipeline's colour is a uniform and a per-segment alpha is inexpressible.
The finer ask is floored at level 1 whenever the flat path would have used the
pyramid: level_for() answers 0 -- raw samples -- as soon as a sub-column falls
under one bucket, so an unfloored ask turns a moderately dense frame into an
O(visible) gather (measured at 172 samples a pixel: raw=1154 lod=0 and 6.4 ms a
frame against raw=710 lod=453 and 2.3 ms). Grading is a way of drawing a
reduction and may not undo it.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
An LOD envelope staged as a polyline moirés. level_for() sizes a bucket at one to
two pixel columns and Envelope emits two points per bucket, so a column carries
three or four near-vertical hairlines at as many sub-pixel phases; under src-alpha
compositing its brightness is then a non-linear function of those phases, the
pattern repeats at the beat between the bucket pitch and the pixel pitch, and a pan
sweeps the beat so the banding crawls.
draw_traces now reduces such a window onto the pixel grid: one exactly vertical
segment per integer column of the axes, from the lowest to the highest screen y any
gathered point put in it, flushed in framebuffer pixels with the snap on. One phase
for the whole trace, so there is nothing left to beat -- and the geometry becomes
O(axes.w) rather than O(buckets).
Gated on the gathered density rather than on the level (the raw path beats the same
way), and on Envelope alone. A hole stays a hole, disjoint neighbours are joined so
a fast edge keeps it, and a span under the stroke is drawn as the stroke since the
shader caps nothing. It is the one carve-out in u_snap's data/chrome rule: a column
is a pixel column, and the y bounds it reports are untouched.
Measured by the new test as the number of distinct coverage levels in the ink --
4 against the polyline's 126, with 627 segments against 60 054. A count of
partially covered pixels was tried first and separates nothing (577 against 652):
a 1.5 px stroke's fringe always spills into the next column.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
The mark was placed at the *pointer's* x with the quoted sample's y, so with the
nearest-sample readout it sat off the datum horizontally -- and further off the deeper
the zoom. `CrosshairRow` now carries the data `x` the value belongs to and the marker is
placed from it, so in `NearestSample` the mark lands on the sample the panel is quoting.
`Style::crosshair_marker_px` (9) and `crosshair_marker_width` (1.5) replace the
renderer's hard-coded 3 px arm and 1 px stroke: a snapped mark lands among a trace's own
sample dots (`marker_px`, 3) and at that size would read as one more sample rather than
as the answer to the question the hover asks.
`CrosshairValue` makes the previous behaviour reachable rather than deleted. The two
modes answer different questions -- `NearestSample` "which datum is under the pointer",
`Interpolated` "what is the signal at this instant" -- and only the caller knows which it
is asking, so the plot offers both with the sample as the default.
`crosshair_interpolate()` is restored for it.
`Plot::crosshair_point()` is the lookup with both coordinates, public because a host that
draws instruments of its own needs the same snap and a second rule for it would drift
from this one; `crosshair_value()` delegates to it.
The readout's y was a linear interpolation between the two samples adjacent to
the pointer, which answers "what is the signal at this instant" -- a number the
series does not contain. On a source whose samples are already averaged or
decimated at ingest that is a value nothing measured, and at a zoom past the
sample spacing it is visibly not the datum under the pointer.
`crosshair_span()`'s `t` becomes a selector rather than a blend factor: the
nearer end wins, a tie reading the earlier sample (the bias the span already
takes for a degenerate pair). `t` is measured on the samples' own x through
`neighbours()`/`x_at()`, so "nearer" is exact on a non-uniform clock too, and
the lookup still reads level 0, so the value is a stored sample and never an
LOD bucket's aggregate. `crosshair_interpolate()` had no other caller and is
gone.
`CrosshairRow::marker` is placed from the same y, so the dot follows the
reported sample: past the sample spacing it sits beside the polyline rather
than on it, deliberately -- one y feeding both keeps the mark and the number
from ever disagreeing.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Breaking: Series::gather emits x relative to x0() and takes a GatherMode;
consumers reconstruct absolute x in f64 before mapping (series.hpp documents
the contract).
Bugs, each pinned by a fail-before regression test:
- render: f32 staging quantised epoch-time x into 128 px columns; every
data-space draw now stages to_space(x) - to_space(origin) with the origin
folded into the offset uniform in f64 (epoch/zero-origin twins rasterise
identically)
- ticks: major test was ULP-fragile and blanked epoch axes; majors are an
integer-index test and are never truncated by the minor cap
- gather: LOD envelope emitted buckets straddling [begin,end); ceil/floor
bucket edges + raw lead/trail spans
- live follow: log x axes held a linear span and zoomed every frame; span and
offset now live in axis space (plot + both figure link-group paths)
- input: a zero-notch wheel disarmed auto_scale on every axis; dy==0 pans x,
factor == 1 / inf refused in wheel and drag zoom
- render: the single text flush painted legend names over the crosshair
readout panel; two batches, under flushed before the panel
- data: NaN dropped from min/max unless it seeded the fold; non-finite
samples are now excluded from every aggregate and from cached bounds
Robustness: viewport normalisation (no clamp UB), non-finite range refusal,
pyramid level cap at 57 (no shift/div UB through public gather), rounded ring
capacity documented + pre-sized, GL blend func and UNPACK_ALIGNMENT
save/restore, Renderer::release_gl() teardown, u64 buffer epochs, annotation
captions clamped, button-aware GLFW row capture, cached figure row rects (no
per-event layout), ruler-pair measurement refused, span-nonfinite guards in
axis_ends.
Features: XY series (sorted x, binary-search neighbours/snap, LOD over sample
index); StreamSeries::freeze(); ArraySeries::push(); per-trace dash and
markers (scatter at width 0) with auto or literal stride; GatherMode
Envelope/Nearest/Mean via bucket_sum/sumsq; Style::line_aa_px; per-glyph text
colour (readout values in trace colour); wheel-dx x pan; arrow-key pan;
Escape cancels a drag and restores captured ranges; modifier-masked box zoom
(Shift both, Ctrl x, Alt y); double-click reset via PointerEvent::time_s;
ImGui key repeat unified; pinch gestures on Qt and GTK; pixelDelta wheel path
and DevicePixelRatioChange handling; GNUInstallDirs, SOVERSION,
SameMinorVersion, generated plot/version.hpp, relocatable plot.pc, licence
install, FILES_MATCHING, GitHub Actions CI (debug/coverage/asan).
Suite 283 -> 350 cases, 0 failed; coverage 93.9% (3730/3971) over 24 files,
floor 80/90 held; ASAN preset suite green; all four demos run headless (the
GTK scripted-resize check needs a window manager and fails identically on the
parent commit); readme_main.cpp committed and compiles with the README
command; pkg-config and find_package consumption verified from a relocated
install prefix.
Martino Ferrari