tools/port/element-residual turns last iteration's one-off suppression into a tool:
pose a screen as verify-capture does, shadow each sprite with a transparent PNG,
rank elements by residual on the pixels they actually paint. Two controls, both
mandatory -- the metric's zero on identity, and a mod that shadows nothing must
move zero pixels, or a footprint is the harness rather than the element.
It generalises. Frames rank 1 and 2 on both screens with captures: ptframe1 9.47x
and ptframe2 5.46x on main_menu, ptframe3 14.23x and ptframe4 10.46x on extras,
all four body-dominated and all four negative. The sign splits, so it is not global
darkness: ptmsg +5.02, ptmsg2 +8.83, pttitle +7.88 and every button positive.
Refuted my own next hypothesis. Premultiplied alpha decoded as straight alpha
predicts error proportional to partial-alpha fraction; the frames are 6.7-7.3 %
partial and the most darkened, while pteff10 is 100 % partial and nearly exact.
What the frames are: neither has a single fully-opaque pixel, against ptbase's
99.1 %. For a wholly semi-transparent overlay the blend equation decides the
result, and alpha-over's shortfall against a background-scaling blend is a*bg. So
the shortfall should track the BACKGROUND, not the frame's own contribution.
Measured on render, frame-suppressed render and capture, needing no placement:
ptframe1 +0.772 vs +0.244, ptframe3 +0.797 vs +0.237.
Not adopting anything. The Decoder has established no blend mode is on the disc
for .t32 and both frames are kind 0, declared identically to elements drawn almost
exactly right, so any blend I pick is authored and that is not mine to take alone.
Recorded as an extension of their negative, not a challenge: the behaviour is real
and replicated, so if nothing in the data selects it, something in the code does --
the draw path they named as untaken.
BLOCKED.md updated: the ask is answered, as a negative.
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