Picked Phase 3's GS item and did the part that is checkable without a GPU -- the
rule layer the renderer will have to feed wgpu. New gc_formats::gs with three of
the four rules the roadmap names, each unit-tested.
alpha_to_8: 0x80 IS OPAQUE. The obvious implementation of this is
let v = value & 0x7F;
and it is wrong, because 0x80 & 0x7F is 0 -- which is precisely the mechanism
that makes a 7-bit alpha read as "half" to anything treating the byte as 8-bit
unsigned. The first version of this function did exactly that and the test caught
it. Below 0x80 the value is genuinely 7-bit and scales onto 1..255, so 0x7F is
opaque too and no non-zero alpha vanishes.
clamp01: CLAMP wrap, with 1.0 kept AT the far edge rather than wrapping to 0.
Under repeat sampling an exact 1.0 is a discontinuity; under clamp it is the
edge texel, and the game loads its textures that way.
mip_level: round(log2(image / on_screen)) clamped to the chain. The ratio is
image-over-screen and not the reverse -- a SMALLER on-screen size selects a
HIGHER mip level. I had this inverted in the first version, which made
mip_level(1.0, 64.0, 8) return 0 where it must return 6, and the test caught
that too. Both bugs were the same kind: a plausible implementation of a rule
whose sign convention is not obvious, and the only thing standing between them
and a commit was the test.
Not started: wgpu itself, and blend/alpha-test behaviour. This is the rule layer
only, and the roadmap item stays [~] rather than being ticked, because three of
four rules and zero of the GPU work is not "done".
Two of the four rules the item names were already half-present in the renderer --
moby alpha and the 1/4096 UVs -- so making them explicit here rather than scattered
is worth something on its own: 0x80's meaning was previously implicit in however
each call site happened to read the byte.
ReaperOfVeriod
81ec17857a