Z64 is a low level emulation(LLE) video plugin for N64 emulators, it is built off of video work from MAME/MESS though the amount of code left from that is minimal. Unlike other video plugins, it does not emulate the RSP, thus requiring a separate plugin to emulate the RSP for the video plugin to produce any results. To do this it provides both a RSP emulator based off Project 64 1.4 code and a slower solution that uses MAME/MESS code with the Project 64 one being the faster of the two solutions it can run into a few problems.
Emulating the RDP separate to the RSP allows the Video plugin to much more direct and more accurate. The project never aimed to beat those such as Glide 64 and Rice Video but provide another video plugin though slower, would be able to successfully emulate games where the ucode was never reversed engineer. Sadly the developer for Z64 disappeared stopping all development on the plugin despite it making some fairly significant progresses in emulating undocumented ucodes.