Pulling the 0x80 convention out of the renderer and into gc_formats::gs -- the
side benefit I claimed last commit rather than took -- immediately produced a
contradiction, which is the argument for having done it.
crates/gc-engine/src/level.rs already read it inline:
let a = if c.a < 0x80 { c.a as f32 / 128.0 } else { 1.0 };
My first alpha_to_8 scaled by 127 instead of 128, which made 0x7F fully opaque
where the renderer had it at 253/255. One of them was wrong and it was not the new
one: the disc convention is a 0..0x80 range with 0x80 as the top, so 0x7F is
127/128, not 127/127.
Corrected to match, and the doc now records why -- including that the renderer was
already right, so this is a correction to code I wrote two commits ago rather than
to anyone else's.
One genuine ambiguity remains and it is stated rather than papered over: 0x40 is
exactly the midpoint, and the renderer works in floats where it is exactly 0.5.
Widening to an integer has to round, and this rounds half away from zero giving
128. There is no way to check that against the disc -- it needs a frame, which is
the tfrag morph item's unblock condition -- so it is documented as a choice.
The engine's call site keeps its inline form and now says why: it wants a float,
and the rule has one live site. Converting it would have meant changing working
code for no benefit, which is the opposite of what extracting the rule was for.
Worth noting what actually happened here. Extracting a rule to remove a magic
constant found that two expressions of it disagreed. Neither was obviously wrong
from its own code -- mine looked cleaner, and was not. That is a better argument
for centralising a rule than any amount of saying so.
ReaperOfVeriod
0b660cb0ce