When you apply an effect to video, you change the look of that video, for example by altering its color values or by blending frames from one clip with frames from another. In some circumstances, your Avid editing application is able to calculate these changes and display them in real time.

In other circumstances, the Avid application is unable to do this, so you must render the effect to play it at full speed. When you render an effect, the Avid editing application calculates the changes made to each frame of the effect and stores them in a file, known as a precomputed master clip (or simply a precompute), which it can then use to play back the effect.

Rendering is the merging of effect layers to create one stream of digital video for playback in real time.

It is important to understand when you can use real-time effect playback to keep your effect editing work fast and flexible, and when you must render effects. The following paragraphs provide a brief introduction to the main alternatives available. For complete information on effect playback and rendering, see Main Topics: Playing, Previewing, and Rendering Effects.

Most Avid editing applications can play effects in real time while your work is still in progress. This allows you to preview the look of effects and make adjustments to them without having to render the effects. Your Avid editing application cannot play all motion effects or some third-party plug-in effects in real time; these effects must always be rendered before you can see how they look when playing at full speed.

The Avid editing application's ability to preview effects in real time is dependent on the complexity of the effects in your sequence and on system factors such as processor speed and available memory. For more information on real-time preview of effects, see Real-Time Preview of Video Effects.

If you want to preview effects that your editing application cannot play in real time without rendering them, you can use the Render On-the-Fly option to preview them frame by frame. For more information, see Previewing Effect Frames with Render On-the-Fly.

When you are ready to output a sequence that includes effects, you may have to render some or all of the effects. The exact number of effects that you must render for output depends on the following factors:

For more information, see Rendering Effects.