This is something I've always wanted to see. The ability to create an HD version of favorite games simply by replacing the tiles, or textures with a higher resolution version. It could include things like making a folder to dump the tiles/textures in, and creating a high res folder to load the higher resolution ones.
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Did that make sense?
If you are talking like one of those feature in PSX emulator where texture/sprite being enlarged using 2xSaI, i don't think it's possible for 2D engine.
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PSX uses 3D accelarator where the texture/sprite being rendered in 3D so the texture resolution doesn't matter,
while 2D engine like GBA rendering method might be affected by the texture resolution (some games might be only rendering it partially if the resolution is bigger that what the game "thinks") and it might be varies on different games (not like PSX where 3D accelerator is the one that renders it on all games)
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2D engine usually renders by one-on-one copy for each pixel (so the width and height will be the same), while 3D engine renders based on the triangle/polygon (stretched/shrinked automatically depend on the triangle/polygon size)
many GBA games also use paletted textures