A user looked at the display and reported "the colour fading is not applied, it looks white or
full color, no opacity gradient" -- and the *numbers* were right: ink decayed, `step-time` was
1/e, `step-acquisitions` was 1 + 0.5 x 1. It was the mapping from a cell's value to a pixel.
**Neither the shader nor the blend was wrong** (the answer to the question that was asked): the
fragment write is already the weighted one (`vec4(u_color.rgb, u_color.a * ink)`) with the frame's
straight-alpha blend, and a cell at half ink over a grid line rendered 160 where the arithmetic
predicts 160. What was wrong was the **scale**:
- `full_scale` defaulted to **1.0**, so the *newest* acquisition already sat at the ceiling and
everything above it clamped to the same pixel -- a display of one sweep and a display of five
were one white. It is now **1/(1 - 0.5) = 2.0**, the steady state of the default per-acquisition
fade, so one sweep reads half ink and white is reached only where acquisitions agree. A caller
whose fade settles elsewhere states its own sum (udpscope's is `1/(1 - factor)`, in its commit).
- `gamma` defaulted to **0.45**, the value `Style::intensity_gamma` wants where the measure is a
*dwell fraction*; on a *count* it spends most of the ramp on the last sweep (a quarter of the
scale mapped to 0.54). It is now **1.0 -- linear in sweeps**, because half as many sweeps must
read as half the ink.
The case that would have caught this is a **framebuffer** reading, not the grid's, and
`a_phosphor_density_is_drawn_under_the_traces` now makes it: it fades the grid to a quarter
through the time rule (`dt = tau ln 4`, asserted at 0.25 off the ink) and reads the same pixel
back, so the ramp is asserted where the eye sees it -- **`empty=21 full=83 quarter=54`** (red
channel), against the 199 / 199 the ceiling and a single sweep both read at the old defaults. A
full cell must be measurably above the background and above a quarter cell, and a quarter cell
above the background: three assertions on pixels, none of which the grid's own numbers can make.
Suite: **396 cases, 0 failed**, 4/4 ctest entries.
Martino Ferrari
738205293e