I got interested in this topic so I tried it myself. What I've found you can pretty easily enable antialiasing on dx9.
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int CGraphicDevice::Create
Now scroll down where the parameters of ms_d3dPresentParameter are set.
Add those two lines
ms_d3dPresentParameter.MultiSampleType = D3DMULTISAMPLE_8_SAMPLES;
ms_d3dPresentParameter.MultiSampleQuality = 0;
And now that's a riddle. I haven't got too much time for a research, but ms_lpd3d->CheckDeviceMultiSampleType tells that multiSampling is indeed available, and it sets the quality to 1. But I can't get it working with it, so I left the quality set to 0, it still looks good enough, IDK what would be the difference.
Also there's one more thing to set in the parameters, and I don't know the long term effects of this change, but it looks like it works just fine, like I said I don't have too much time for doing research atm.
You'll be having a "D3DERR_INVALIDCALL" error if ms_d3dPresentParameter.Flags are set to D3DPRESENTFLAG_LOCKABLE_BACKBUFFER, and I don't know the answer for now, maybe I'm missing something, but setting the flag to 0 does the job.
After the device is created with success you must write last line to make it work:
ms_lpd3dDevice->SetRenderState(D3DRS_MULTISAMPLEANTIALIAS, TRUE);
And now you can enjoy those SMOOTH EDGES:
