Okay, spent a few hours tonight trying to figure out great settings for my set. Now, your mileage may vary, but this should probably be universal. The only difference is that my monitor is a 4:3 set, so I have to enter some funky aspect ratios to get it to show up properly. Anyway, I concluded that my 640x480 resolution setting sucked, as none of the shaders or overlays worked properly other than the basic Wii scanline overlay, which sucked for this purpose.
First, let me explain the difference between a shader and an overlay in overly basic terms. A shader is a filter, or combination of filters, which on a powerful system can lead to stellar results. We are not dealing with a powerful system here, so I'm trying to avoid shaders. Overlays, on the other hand, are literally an image placed on top of the emulator screen, and uses little if any processor power. Ideal for our uses. The CRT-Caligri shader looked absolutely badass, but the SNES emulator was dropping a few frames. Most people wouldn't notice, but I did.
Second, BACK UP YOUR CONFIG FILE BEFORE YOU MESS WITH ANYTHING. It's possible to make an unrecoverable change to the config, or you just changed so many options you don't know how to go back. Make a copy first, then you can restore it later without having to reinstall the emulator.
I figured out that the best settings for me (and probably a lot of people) is to set Retroarch to display at 720p. You do this by starting a game and hitting a button to go into the display menu. From here you can select which emulator starts the rom, and you can also configure both the Output resolution (the resolution that will display on the screen) and the Render Resolution (the resolution that will be drawn internally.) You want them to match so that Retroarch doesn't do the upscaling, so I set Output to 720p and Render Resolution to Match Output Resolution. Launch the rom, and this leads to a crazy aspect ratio on mine, since it's displaying a 16x9 image on my 4:3 screen. That's okay for now, we're not done.
Open up the Retroarch configuration (Select and X with a SNES style pad) and mosey on over to the Settings and Overlay settings. When you select overlays it shows several folders, there is one called "rpi" or something like that that contains Raspberry Pi specific overlays. If you don't have them, I think there is an update script you can run in the Retropie configuration or something like that. Anyway, select that folder, and it has some scanlines for different resolutions, I picked the one that was for 1280x720 resolution. Go back into the menu and there is an option for "Opacity" which controls just how dark those scanlines are. I set it for .35, which is pretty light, you might like darker scanlines.
Now we need to adjust the video settings. Go to settings-Video. There are a ton of settings, but there are only a few we're interested in. First set Integer Scaling to ON. It's worth noting that most of your games will not be full screen now, you will have a border around the screen. Deal with it, or live with a blurry picture. I can just stretch my screen horizontally and vertically a tad because CRT's rule. Next is set Bilinear Filter to OFF. If this was On, changing it to Off will not have an effect until you exit out of the game and restart. There is an option for "crop overscan." Whether you want this ON or OFF depends on the emulator you are using, so you may need to experiment. Like the Bilinear Filter, it will not take effect until you exit out of the game and restart.
With those set, there's one final thing to change, and that's the aspect ratio. On mine with the SNES and Genesis emulators, I actually had to set this to 16:9 to get a 4:3 image on my monitor due to the odd (for my screen) resolution. On the 32x emulator, I just flipped through the options until I found one that made the image the proper 4:3. Flip through it to find one that looks right. Retroarch was designed with widescreen monitors in mind, so you probably won't have as much trouble as I did. Exit out of the menu, see if your shit looks good, and if it does make sure you save the configuration.
One thing to note, I don't know firsthand but I read on several sites that said that although the Pi can output 1080p, the thing isn't powerful enough to render most of the emulators at 1080p and still get a smooth 60fps. You also don't want Retroarch to render at a lower resolution and upscale, that looks horrid. That's why most seemed to recommend 720p, the emus can run perfectly, and most 1080p displays do a nice job upscaling 720p content. Still, it doesn't hurt to try, and the rpi folder does have a 1080p scanline overlay, so give it a shot if you want.
