These settings are sent to the fda and control blending and blur etc.
It's likely that the fda mode setting also affects some behavior in the fdp but idk exactly.
Line Register 2.2 "6400"
fda:2,blur:2,#:12
blur
Fda: Horizontal Blur
In practice:
0,1
blur disabled
2,3
blur enabled
Technically, this controls an input to the fda. This signal is normally high, and goes low during visible half-pixels, as configured by this field:
0
low never
1
low during A half-pixels
2
low during B half-pixels
3
low during both half-pixels
The level during half-pixel A doesn't seem to matter.
In fda modes 2 and 3, the level for half-pixel B is sent for both.
fda
Fda: Mode
0
legacy color mode (15-bit rgb)
1
normal mode
2
no blending, display half-pixels separately (debug mode?)
3
invalid? first pixel's color repeats all the way across (but alpha still functions)
Modes 2 and 3 are not used by any games, and may have various strange effects that we haven't tested much