why only directx 9 ?

[[Unknown]]

OpenGL is largely better, unless you have bad OpenGL driver support. It is by far the most heavily tested, and for newer video cards, supports more features than Direct3D 9.

Direct3D 9 was implemented because, at the time, the person implementing it was interested in porting PPSSPP to Xbox which only supported Direct3D 9. This was a ton of work but he really got a lot working - and that effort also helped improve the OpenGL backend.

Direct3D 11 or 12 aren't implemented because no one passionate about implementing them has come along and done it (PPSSPP is open source after all.)

Vulkan support is still looking fairly promising across the various drivers available on Windows devices, and that translates also to Linux and Android devices. From my perspective, Vulkan support is much more interesting than Direct3D 12 support - especially since I don't have or really want Windows 10. But that's just my personal opinion.

I'm sure if someone made a solid effort at implementing Direct3D 12 we'd be very interested in the pull request. This is probably true of Direct3D 11 as well. We might be less interested in 10, since there doesn't seem to be a lot of advantage of 10 over, say, 11 or 12.

-[Unknown]