Sounds like existing renderers for PSX emulators are either closed-source, or based on legacy graphics APIs and therefore hacky.
From about halfway down the page:
> This is probably the first OpenGL renderer for PSX that is not based on outdated fixed-function OpenGL code and which is, actually, well, open-source. Because we can assume shaders will be there, we can get reasonably close to emulating texture windows accurately (with Pete’s OGL1 GPU plugin, there were codepaths for doing it with paletted textures – the hacky approach – and the pixel shader approach). Therefore, the renderer is still kept pretty simple and easy to understand.
There are perfectly fine and compatible CPU-based renderers for Playstation 1 in Mednafen or pSX. People don't want compatibility, though, they want their graphics to be better than the original.
So emulator devs make these GPU-based HLE renderers that are always going to be hacky no matter what.
From about halfway down the page:
> This is probably the first OpenGL renderer for PSX that is not based on outdated fixed-function OpenGL code and which is, actually, well, open-source. Because we can assume shaders will be there, we can get reasonably close to emulating texture windows accurately (with Pete’s OGL1 GPU plugin, there were codepaths for doing it with paletted textures – the hacky approach – and the pixel shader approach). Therefore, the renderer is still kept pretty simple and easy to understand.