What size are the texture files in SH4?
IIRC the largest textures we've had for U-Boat skins have been 2048x2048 as a TGA file.
Whereas SH4 uses DDS format which works somewhat differently.
A DDS file contains multi-resolution information, so for example, when you are close to the sub you see the highest rez but when far from the sub you see a low rez version. This is why SH4 has been able to maintain performance & FPS while improving the overall graphics.
In SH3, TGA files just don't work this way and regardless of what distance you view a texture it will always use the same resolution. Sure, the TGA files can be made larger but with an exponential effect of performance.
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