The fdp uses a strange system choose alpha values. each half-pixel has a "alpha palette" of two alpha values that can be used for it.
You can think of it like this: a color (i.e. as contributed by a layer, and stored in the priority table) consists of 14 bits of data: a 13-bit index into the color palette, and a 1-bit index into the "alpha palette", which we call "alpha select". alpha values range from 0% to 100%, in steps of 12.5%.
Lineram Register 2.1 "6200"
B1:4,B0:4,A1:4,A0:4confirm the order of these
A0
half-pixel A, alpha value 0 alpha = (15-value)/8, clamped to [0,1]
$0-$7
100%
$8
87.5%
$9
75%
$A
62.5%
$B
50%
$C
37.5%
$D
25%
$E
12.5%
$F
0%
A1
half-pixel A, alpha value 1
B0
half-pixel B, alpha value 0
B1
half-pixel B, alpha value 1
direct reader to per-layer alpha select pages? also idk if this needs to be a separate page or should just be merged with priority and blending. but it's like, a separate concept i think.