N64 VI Filter

The N64's RDP chip includes a Video Interface (VI) stage that prepares the video for the final output. From the N64 Programming Manual:

The video interface reads the data out of the framebuffer in main memory and generates the composite, S-video, and RGB signals. The video interface also performs the second pass of the antialias algorithm. The video interface works in either NTSC or PAL mode, and can display 15- or 24-bit color pixels, with or without filtering, at both high and low resolutions. The video interface can also scale up a smaller image to fill the screen.

These functions can make a very big impact on the final image of an N64 game, and the ParaLLEl-N64 libretro core exposes the ability to toggle the postprocessing effects of this stage on and off. Turning it off nets you a few frames per second of speed but also gives us a peek behind the VI curtain:

Filtered

Unfiltered

So, you can see that the filter just barely touches the HUD elements but it does some pretty dramatic stuff to the rest of the image. It applies strong antialiasing to the outside edges of objects, which has a big, noticeable effect (so noticeable, you can see it in the thumbnail images) on Mario's hat and the silhouette of the tree, and it does some blurring that smooths out the dithering that is very visible in the unfiltered shot. On actual hardware, the blurring can be toggled off in some games (Quake II, for example, IIRC) or using Gameshark codes. I believe consoles modded with UltraHDMI or etim's N64RGB boards can also switch it off through the boards' firmwares.