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Background Rendering

Background rendering causes temporary output to be rendered constantly while the timeline is being modified. The temporary output is displayed during playback whenever possible. This is useful for transitions and previewing effects that are too slow to display in real time. If a Render Farm is enabled, the render farm is used for background rendering. This gives you the potential for real-time effects if enough network bandwidth and CPU nodes exist.

Background rendering is enabled in the Performance tab of the Preferences window. It has one interactive function Settings → Toggle background rendering . This sets the point where background rendering starts up to the position of the insertion point. If any video exists, a red bar appears in the time ruler showing what has been background rendered (figure 7.9). Because this creates a very large number of files, a Shell Command script is available to delete them if in the default location.

Figure 7.9: Settings Background Rendering

It is often useful to insert an effect or a transition and then select Settings → Toggle background rendering right before the effect to preview it in real time and full frame rates (figure 7.10).

Figure 7.10: Timeline with the top red bar

Frames per background rendering job
  This only works if a Render Farm is being used; otherwise, background rendering creates a single job for the entire timeline. The number of frames specified here is scaled to the relative CPU speed of rendering nodes and used in a single render farm job. The optimum number is 10 - 30 since network bandwidth is used to initialize each job.
Frames to preroll background
  This is the number of frames to render ahead of each background rendering job. Background rendering is degraded when preroll is used since the jobs are small. When using background rendering, this number is ideally 0. Some effects may require 3 frames of preroll.
Output for background rendering
  Background rendering generates a sequence of image files in a certain directory. This parameter determines the filename prefix of the image files. It should be accessible to every node in the render farm by the same path. Since hundreds of thousands of image files are usually created, ls commands will not work in the background rendering directory. The browse button for this option normally will not work either, but the configuration button for this option works. The default value will be /tmp/brender . Because using background rendering creates a voluminous number of brender numbered files, a Shell Command script is available to delete them if they are in the default /tmp/brender format.
File format
  The file format for background rendering has to be a sequence of images. The format of the image sequences determines the quality and speed of playback. JPEG generally works well and is the default.

Tip: If you have rendered your whole project with File format set to JPEG and there are no missing numbers in the sequence, you can create a video from that sequence outside of CINELERRA-GG. For example, if using the default output so that your files are named /tmp/brender000000, /tmp/brender000001, ... in a window, you would type:

    ffmpeg -f image2 -i /tmp/brender0%5d -c:v copy brender.mov

which would create the video file brender.mov - be sure to delete existing brender files before creating a new sequence to ensure there are no missing numerical values in the sequence.