Compressor lets you control and refine the encoding process by setting up a series of two or more jobs that use the output from a preceding job as their source media. This is known as job chaining.
For example, an H.264 encoding job with multi-pass engaged could require three or four passes over the media. If any frame controls (see Chapter 10) are enabled, they will process along with each pass. Depending on the settings, this situation can lengthen the encoding time considerably. So, instead of encoding the frame controls during each pass, you can use a job chain to encode them once and then pass the output of that job to the H.264 job.
To create a job chain, import source media, apply a target, and make any necessary changes to the settings. In the following example, an HD clip is being downconverted to a 640 x 360 resolution using frame controls. An Apple ProRes 422 high quality setting was used as the intermediate codec. You'll find it in the Settings tab of the Presets window by following this path: Apple > Other Workflows > Advanced Format Conversions > Apple Codecs.
In the above example, the 640 x 360 Apple ProRes 422 movie output from the first job passes to the next job in the chain where an H.264 encoder is applied. Since the previous job already set the output frame size using frame controls, the Geometry parameters (Chapter 11) can be set to 100% of the source. This process retains all of the image quality but realizes a benefit in efficiency because the time-consuming frame controls processes are encoded only once.
Think of job chaining as a form of assembly-line encoding that lets you control the stacking order of encoding tasks based on necessity or personal preference. For example, you may want to encode all frame rate conversions before processing any frame size conversions. Using job chaining, you can establish the task order by controlling the encoding order within each of the steps. In essence, each job in the chain becomes a part of the larger job of outputting the final movie.