A user looked at the display and reported three things at once: "there is clearly a gain offset
between latest acquisition and persistent image", "persistent instead of being brighter + opaque is
just gray/white", and "last acquisition does not seem to be affected, it looks like persistent image
+ last acquisition (so for example last acquisition never fade in time mode)". What they add up to is
that this was not a phosphor yet: it was a **density with a solid trace drawn over it**.
Four changes, and the first is the one the others follow from:
- **The current acquisition is deposited like every other, and no solid trace is drawn over the
image.** `Renderer::draw_traces()` returns at once while a density is on: the trace ink *is* the
image, so the newest acquisition is a sweep inside it and fades with it. Drawing the polyline as
well put a solid, unfading stroke over its own faint ghost -- the reported "persistent image + last
acquisition", where the last acquisition never faded in time mode.
- **One grid per trace, in that trace's own colour.** `Plot::density_grid(trace)` is one screen-sized
`f32` grid per trace, deposited from that trace and drawn in `Trace::color`, because a density that
had one hue for the plot could not say *which* trace put the ink there -- two signals agreeing in a
cell would be one blended pixel. The display's memory is therefore `traces * columns * rows * 4`
bytes rather than one plot's worth (a trace with no series, an unbound slot, holds no grid), which
is the one cost this change adds: four signals in one 800x480 row are four grids, 6 MB.
- **The ceiling is the trace's own brightness, not the fade's steady state.** `full_scale` now
defaults to **one sweep**: a cell with one acquisition's ink is drawn in its trace's colour at that
colour's own alpha, and a signal crossing the same place every acquisition settles *at* the colour
its trace would have drawn (the ceiling clamps) rather than a fraction of a brighter steady state --
the gain offset the user reported. `Style::density`'s near-white 0.85 tint is gone with it, because
there is no one tint any more. A caller that wants the fade's ramp on screen instead still sets
`full_scale` to the sum the fade settles at and accepts the offset; that is a legitimate choice, just
not the one a phosphor makes.
The reading is a **framebuffer** one, because neither `ink`/`peak` nor a tint can make a colour or a
fade: `a_phosphor_density_is_the_trace_in_its_own_colour` deposits one sweep, checks the saturated
cell is *exactly* the trace's colour (`full=224`, the trace's `0xE0` green, against `bg=25` -- the old
monochrome tint read ~190 red), then fades it one time constant and checks the same pixel is dimmer
(`faded=115`) and still the trace's hue -- which a stroke over the density could not do. It moves the
trace and draws without depositing to prove no polyline is drawn (`moved` equals the no-density frame
at the moved row), and reads a steady state against a quarter of it (`steady=224`, `quarter=138`), in
the signal's colour throughout. `a_density_holds_one_grid_per_trace` reads two traces' grids apart
(`267 472 cells each, ink 584/584`, in two rows). The grid's own cases are unchanged where they should
be (`a_density_halves_per_acquisition_and_keeps_the_newest_brightest` `peak_steady=2.000`,
`a_density_fades_by_its_time_constant` `exp(-dt/tau)`).
Suite: **397 cases, 0 failed**, 4/4 ctest entries.
`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.
The controller has always tracked its selection; nothing drew it, so a Ctrl or
Shift drag framed a range with no feedback until the release.
The preview is the area the release will change, not what the pointer swept:
Ctrl frames the x range only, so a band across the plot is what it will zoom,
where the sliver a straight horizontal drag sweeps is only the gesture. The
plot carries the rect a host hands it -- it learns nothing about pointers --
and the renderer draws a low-alpha fill under a light border over everything
else inside the axes.
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>
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.
`Style::marker_below_samples` (50 by default, `0` for never) draws a `marker_px` dot on every
*raw* sample when the visible window holds at most that many, in the trace's own colour. Zoomed
in far enough to see individual samples, the polyline between them stops being the shape a reader
is looking at -- a handful of points across a whole panel ARE the picture -- and a dot makes each
one a place rather than a bend in a line. Level 0 only: above it the geometry is pyramid buckets,
which are not points anyone can find.
Test: a_sparse_window_marks_its_samples -- the same flat series at ten samples and at a hundred.
Ten gives exactly ten dots, evenly spaced (their centres are checked against the panel's own
step, so "they are the samples" is asserted rather than "something was painted"); a hundred is
past the threshold and gives none. The dots *group* into runs of columns (a marker is `marker_px`
wide), which is how the test reads one marker per sample -- counting columns read three.
grid_major 0x2A3038 -> 0x39414C, still well under axis_line's 0x8A94A6, so the three-way
hierarchy the eye reads stays minor < major < axis. Already fully opaque, so "more
prominent" meant a brighter colour and nothing else.
The tests that classify grid pixels take their colours from the style, so they followed
the change; the one hardcoded mention (grid_pixel()'s doc comment) is updated, and the
minor's relationship to the major is now stated as an order of magnitude rather than the
fifth the old colours happened to give. 361 cases, 0 failed.
Subdivisions used to be drawn exactly like the labelled lines -- same width, same
opacity -- so a plot dense enough to need them read as two competing sets of ticks.
The minor class is now three things the style states explicitly:
- grid_minor_width 1.0 -> 0.75, a sub-pixel logical width. At 1x that is expressed as
reduced coverage through the anti-aliased ramp; on a HiDPI display the line is
literally narrower.
- grid_minor_dash_on/_off (new, 3/3): dashed by default, off == 0 draws it solid, the
same convention as the crosshair's and a trace's dash. The dash is a fragment-shader
discard, so no geometry is added -- the vertices are the line either way.
- grid_minor carries alpha (0x1C2126 at 50%), and the line shader applies u_color.a,
so it blends into the background instead of sitting on it.
Together that is about a fifth of the ink a major line leaves along its length, which
is what gives the eye one grid to read instead of two. The major class is untouched:
it stays the structure.
Tests: a new pixel test (the_minor_grid_is_thinner_dashed_and_dimmer_than_the_major)
asserts a major column is inked top-to-bottom while a minor one alternates, and that
the minor's ink sits closer to the background than the major's. Three existing tests
counted the minor class as "most of eight rows" of its exact colour; two now take
their floor from the style (minor_line_ink: width x alpha x duty), and the width test
states the dash and alpha away because widths are its subject. 361 cases, 0 failed.
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.
The repository had no licence at all, so every source was all-rights-reserved
by default. Add the verbatim MIT text at LICENSE (Copyright (c) 2026 Martino
Ferrari) and a two-line SPDX header to each of the 113 first-party sources --
48 include/plot, 26 src, 10 adapters, 5 examples, 21 tests, 3 tools -- so the
grant travels with each file rather than relying on the root document alone.
Scripts keep their shebang on line 1; the SPDX lines follow it. Genuinely
vendored code is untouched: third_party/stb_truetype.h carries no header of
ours. src/render/embedded_mono_font.cpp is generated, so tools/fonts/gen_atlas.py
emit() now writes the two lines into its header comment and the file was
regenerated -- gen_atlas.py --check passes and the regeneration adds only those
comment lines to the previously committed file.
README's provenance section now states MIT and links LICENSE.
No code, test or public API change.
Final whole-branch review fixes.
1 (Important). Plot::measurement() cached on (series pointer, sample count)
only, so a saturated StreamSeries -- whose size() stays pinned while x0()
advances every acquire() -- froze its readout on the first saturated frame
until a cursor moved. The cache key now carries the series' x0() (new
MeasureSlot::last_x0), so a sliding window recomputes and a static series
still does not: asserted with a new macro-gated counter hook
(plot/detail/measure_count.hpp, PLOT_COUNT_MEASURE_RECOMPUTES, wired like the
pyramid and ring counters, absent from the release library). New case
saturated_stream_measurement_tracks_the_sliding_window: saturated ring,
stationary cursors, ramp in absolute x -- zero recomputes for repeated reads
with nothing moving, one per acquire while it slides.
2-3, 9. Spec: 4.3 described a GL_LINE_STRIP path that does not exist (there is
one rasteriser -- instanced anti-aliased quads -- and dense_threshold_spp
selects the gather level, not a primitive); 8 named a PLOT_GL_CHECK() macro
that exists nowhere, and stated no exception policy. The table, the field's
comment and 8 now describe the real gather-level threshold, the real
glGetError() drain plus the stores' drop-to-orphan fallbacks, and the real
policy: library failures return Status/bool, allocating entry points propagate
std::bad_alloc.
4. The frame handed back neither GL_UNPACK_ALIGNMENT (init() sets 1 for the
atlas) nor the clear colour (queried once with a new glGetFloatv entry).
end_frame() restores both and renderer.hpp lists them in the residue
contract.
5. draw() kept a host TextBackend for layout while drawing with its own font.
The renderer now installs its font unconditionally -- strips and labels measure
and draw with one backend -- with a debug-build diagnostic if a different one
was set, and Plot::set_text_backend() is documented as the no-renderer path.
New case renderer_measures_the_labels_it_draws asserts layout, measuring and
the drawn label's ink agree.
6-10. draw()'s style overwrite stated at the call site; the load-sensitive
wall-clock bound in test_measure.cpp replaced by a counter assertion (same
interval, same fold count over a 512x length ratio); dump_frame_ppm drops
noexcept and says why it may allocate; error.txt ignored (not this project's,
not generated here) rather than deleted.
Verified: debug and release build clean (-Wall -Wextra -Wpedantic -Wconversion
-Wshadow), 4/4 ctest entries pass, 130 cases pass (was 127), umbrella and
header self-containment checks pass, and no counter symbol in the release
archive.
Martino Ferrari