GTA San Andreas characters can reach out of mirrors "horror movie style" because of a nifty technique to get around a lack of video memory

gamesradar.com:

Grand Theft Auto: San Andreas is an all-time classic, but there's no doubt that its in-game mirrors have always been slightly wobbly in places. Now though, we finally know why, and it centres around a workaround put in place due to a lack of video memory – at least in the PS2 version.

Former Rockstar Games technical director Obbe Vermeij took to Twitter to share that the PS2 version of San Andreas never had enough video memory to render mirrors "the proper way." Vermeij explained that the standard method has developers "render the scene twice", taking the "point of view of the mirrored camera" and using it as a texture for the mirror. However, in the case of San Andreas on PS2, Rockstar "simply didn't have that spare video memory" in order to store this render. 

Instead, Vermeij "rendered a mirrored version of the scene, at the same time as the scene itself." While this was certainly effective, it did have some problems that you'll have likely seen if you've spent a lot of time in the game. For a start, characters and objects can appear to reach out from inside the mirror "horror movie style", which is a bit creepy, to say the least. 

Otherwise, the method in San Andreas also makes back-face culling – a process which determines whether polygons are drawn and displayed when you're not supposed to see them – go "the wrong way round". This can lead to some models in the mirror showing their "back surface" rather than the one you're actually meant to see. Vermeij gave an example of this in a screenshot on Twitter – if you look at the shelf in the background, you can see that the two images don't look the same due to their models displaying differently.

Glitchy mirrors in GTA San Andreas.We didn’t have the video memory to render mirrors the proper way.Instead, I rendered a mirrored version of the scene, at the same time as the scene itself.This method has a few problems:- Mirrors can only be on the outer wall of the room.… pic.twitter.com/0fkt2mtrVaMarch 5, 2024

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