Sets the terrain material's alpha mode, which is the first draw-path change the
GS work has actually produced.
The material used `..default()`, which is AlphaMode::Opaque: every texel drawn
fully solid regardless of what its alpha said. Given 0.98% of texels on the disc
are fully clear, that means roughly 227,000 texels that should have shown
background are drawn as opaque geometry.
The choice is Blend, not Mask, and the reason is the histogram from 25d73b4:
- Mask(f32) is a *quantising* cutout. Below the threshold -> transparent, at or
above it -> fully opaque. Feeding it the 1.08% partial texels would flatten
every one of them to solid, discarding exactly the soft edges they exist to
produce. 126 distinct values become two.
- Blend handles all three populations at once. A clear texel blends to invisible,
a partial one stays smooth, an opaque one stays opaque.
Which retires the conclusion I gave two commits ago. I said cutout-first, on the
reasoning that the clear texels need discarding and a partial texel drawn opaque
is a failure nobody notices. But Blend does not need a cutout to hide those
texels -- a zero-alpha fragment is invisible by definition. The cutout was
solving a problem Blend solves for free, so the ordering was moot before it was
tested. Three commits on the alpha path and the second conclusion has now also
fallen; the surviving facts are the histogram and the rule that 0x80 is opaque.
One real fidelity gap, recorded rather than papered over: the GS can alpha-test
and blend the same primitive at once. bevy's AlphaMode is a single enum where
Mask and Blend are mutually exclusive, so it cannot express that. Where the
original combines them, this port has to pick one, and the choice will not be
free. That belongs in the Phase 5 frame-for-frame gate rather than being treated
as settled here.
Clippy clean, builds. No test added: this is a material parameter with no
headless assertion available, and asserting a bevy enum value would be a test of
the bevy API rather than of anything in this project.
ReaperOfVeriod
43e508aa21