Trying to get SSAA with downsampling to native PSP resolution on PC

[LunaMoo]

SSAA is supersampling antialiasing, maybe you're confusing it with SMAA or some other form of AA, since when downsampling to extremely low res then streatching it up to display res you don't gain anything from supersampling.

Even if you speak about sprites being broken by higher rendering resolution(since increased render res does introduce glitches in many games) they would still be broken by supersampling even if the output was set to low resolution you will see that by simply running the game with SSAA in x1 window, glitches will still be there even if they can be harder to see in small window.

What you probably want is any antialiasing which does NOT work by supersampling, but AA will NOT increase the resolution of 3D graphics, it will just blurr/smooth things out. Also don't use pixelate shaders either, that's dumb, there's absolutely NO point in using increased rendering resolution at any point in time if you want to output to low res then stretch it or pixelate it to simulate similar results afterwards, both things would KILL the rendering resolution making it pointless and needlessly heavy, it's like taking out the supersampling word from supersampling antialiasing and SSAA actually uses very poor smoothing effect since it doesn't need any compex math or detection thanks to rendering at much higher res. All supersampling shaders(which means just one example shader in official PPSSPP) use a supersampling flag which is a multiplier, it can be used to set much higher render res(than the options provided in the emulator), but it can also be disabled or removed to disable supersampling and make it a standard smoothing effect which doesn't change your render resolution and would do what you want, but again that's not what you want, it will not make anything pretty, you're just confusing things probably based on results from Wii emulator.

Wii should not be compared to PSP in any way, especially with textures since it doesn't suffer from such extremely low resolution and it's graphics typically scale much better to higher res displays, but even there best results are archieved by texture replacement which is a feature also existing in PPSSPP.

Best solution is to just use texture replacement(requires tons of manual work, but the results can be shared by community so if game is popular sooner or later someone might create a texture pack for it) or at least abuse texture scaling(poor, and doesn't scale everything and might look bad, but at least it's automatic) to increase the resolution and level of detail of the sprites.

If by sprites looking bad you mean some effects, you might try "lower resolution for effects" hack instead, this keeps some effects rendered at x1 res making them look properly without affecting other graphics, however it's just a very simple hack and cannot detect everything, it was made mostly for bloom which some popular PSP games create in similar way.