Finishes the rule layer. Two traps, both of which produce a picture that looks
slightly wrong rather than obviously broken:
AFAIL READS BACKWARDS. The PS2's alpha-test register holds an enable, a 7-bit
reference, and a bit that looks like "fail if alpha is above the reference". On
the hardware it means the opposite -- AFAIL = 0 discards where alpha is BELOW the
reference. passes_alpha_test takes the side explicitly rather than encoding the
register, and both directions are tested so the convention is pinned in code
rather than in a comment.
THE OPAQUE BIT MUST STAY OUT OF THE COMPARISON. 0x80 is fully opaque, but it is
also numerically 128, so a test that compares the raw byte against a 0x40
reference would have every fully-opaque fragment pass every threshold, and nothing
would ever be discarded. passes_alpha_test clamps the opaque bit off first, and
there is a test that asserts exactly that -- the failure mode here is a scene that
renders correctly with the wrong shading, which is not something anyone notices
by looking.
Blend is the four GS primitive types, decoded from the primitive-type byte and
named by what they do rather than by the documentation's IIIB/IIIA/IIIC labels,
which name two different modes identically. Out-of-range types decode to Off
rather than panicking, because the byte comes from asset data. Addition clamps at
255 rather than rescaling, matching the GS's 7-bit clamp; a caller wanting a true
average blends before widening, and the doc says so.
That completes the four rules the roadmap names for this item:
alpha 0x80 == 1.0 alpha_to_8
CLAMP wrap clamp01
mip selection mip_level
blend + alpha test BlendMode, passes_alpha_test
Nine unit tests, no wgpu work, and the roadmap item stays [~] rather than [x] --
the rules are the input to the renderer, not the renderer.
Worth noting the pattern in both traps: each one produces output that is plausible
and slightly-off, with no error anywhere. That is the same shape as the sign
inversion in mip_level and the byte-order bug in the anchor search. The tests are
the only thing that catches this class, which is the argument for having them even
though nothing here needs a GPU.
ReaperOfVeriod
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