Any time Final Cut Pro must perform more calculations than your computer can handle in real time, you need to render. Rendering is the process of creating temporary video and audio render files for segments of your sequence that Final Cut Pro cannot play in real time. When you render a segment of your sequence, Final Cut Pro substitutes a render file for the segment during playback. Render status bars above the ruler in the Timeline indicate which sections can play back in real time and which segments require rendering.
Because rendering takes time away from the editing process, the goal is to render as little as possible. For more information about real-time playback, see Using RT Extreme.
Rendering is generally required for:
The use of filters, transitions, generators, or any combination of effects that exceeds your computer’s real-time playback capabilities
High-quality final output. Real-time effects that play back at preview quality must ultimately be rendered for high-quality video output.
Video clips using codecs that Final Cut Pro can’t play in real time
Multiple audio tracks that exceed your real-time playback limit
Clips with audio effects that require too much processing power
Some nested sequences, which can include layered Photoshop files