The Decoder's R2 note: alpha peaks at 80 of 255, so the composite difference is a
small signal either way and the expected magnitude should be set before the test.
Committed here before any capture exists.
Rendering the same pose at peak alpha under both composites: 25 015 px footprint,
mean |additive - alpha-over| 16.92 levels inside it, max 50, RMS over the whole
frame 3.746.
My first prediction was wrong and that is the useful part. The composites differ by
exactly alpha*bg, which with mean background 94.78 predicts 29.73 levels. Measured
16.92. The formula uses the keyframe alpha and ignores that the sprite carries its
own per-pixel alpha channel modulating it -- implied mean sprite alpha about
145/255. Caught only because the prediction was written down and disagreed with the
render.
The pre-registered test: additive is brighter than alpha-over at every footprint
pixel in one direction, so the discriminator is regional SIGN over 25 015 pixels,
not per-pixel magnitude. If the game is additive, rendering alpha-over must leave
the footprint about 17 levels darker than the capture relative to the surrounding
frame, and additive must leave no such regional bias. Neither bias refutes both and
points at the ramp or the pose instead.
Feasibility stated before asking for a run: as a whole-frame RMSE the difference is
3.746, below the title's own +/-5.56 capture-phase term, so verify-capture's
headline number could not settle this even at peak alpha. It is discriminable only
as a signed regional statistic.
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