it isnt as easy as you think especially when the graphic card used by satrun is way different to the ones we use in pc meaning that it will never show the exact graphics, it will have errors and big glitches.... also if a devs does it then it has to be game specific as he gets one game working especially with filtering, it will break other games
Oh well, I didn't exactly say that it supposed to be easy or anything. I mean, it could have been their choice to code it that way, just like with those PSX emulators which suppose to feel 'like a real thing', etc. It could be either way. At least there was no info (in places I could reach) on why did they choose to go with the DirectDraw and software rendering way.
Actually, isn't the N64's graphic card for instance also a bit different that you use in your PC? I mean, that's the very nature of emulators, right?
Saturn doesn't do 3d, it does 2d sprites deformed in 3d. You can technically draw these quads in opengl or d3d as actual 3d objects, but you'd have a plethora of rendering errors that way, and you'd be unable to reproduce many of the stranger effects the Saturn can do (like the "bowtie quad"). There's a reason for no Saturn emu having hardware acceleration on top of accurate graphics.
The author of SSF is experimenting with using dx11 gpgpu acceleration though, so you may just be able to run a Saturn emu on your GPU in the future. You'd still not get increased resolution though, but your CPU would be freed up completely.