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.
Martino Ferrari
1b8b55d3ee