Weird scaling with aspect ratio correction on Direct3D9

[Zerowalker]

I have noticed that there's something odd going on when scaling along with using aspect ratio correction,
to confirm it I made a rom that just draws vertical bars of black and white to easier visualize it.

There's a very obvious pattern when using 5:4 with 2x, the others are more subtle and it seems to differ where it take place,
but it's probably just something with the algorithm.

(Overscan set to 6,7,0,0 (up,down,left,right), it does not affect scaling).

Direct3D 9 5:4

OpenGL 5:4

Unaltered

[Zerowalker]

Looking into it it seems to be cause of how Direct3d9 works with half pixel offsets or something.
I don't understand that stuff but the Ares emulator does not have this weird scaling so perhaps something can be taken from there?
https://github.com/ares-emulator/ares/blob/master/ruby/video/direct3d9.cpp#L207

Or perhaps doing it all in a shader.