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Chapter 11 Realtime FX
Optimizing Realtime Preview
Some tips on enhancing your system‘s realtime performance.
First CPU, then GPU (in effect editing)
You can apply all realtime effects in all combinations imaginable to a clip. But: The best performance is
achieved when CPU effects are applied first, and GPU effects last.
The processing order of effects applied to a clip can be viewed and changed in the clip’s FX Properties:
The list reflects the order in which effects were initially applied. The first effect in the list is processed
first, the last effect is the final. (Right-click a clip and select FX Properties from the shortcut menu.)
The explanation lies in the way CPU and GPU cooperate. The CPU mainly does basic video processing,
e.g., it decodes a frame, and works on certain effects. It then passes on the data to the GPU, which takes
care of additional effects and displaying the image, among other things. But if the GPU has to pass data
back to the CPU, there‘s a bottleneck because bandwidth in the direction from GPU to CPU is much
smaller than vice versa.Therefore it is recommended to process all CPU effects before data is passed to
the GPU. And, as a rule of thumb if maximum performance is called for, use GPU effects - the CPU is
busy with a host of other tasks.
Reduce Preview Quality
The Timeline Properties include an option for reducing FX Quality in the preview. This serves to reduce
system utilization slightly and increase the frame rate at which effects are played back. Final quality for
the master is not compromised.
Turn Off Audio Playback Temporarily
In certain borderline cases, deactivating timeline audio (Playback Off) can help prevent “dropped
frames”, i.e. frames omitted during playback. This is always the case when your system is operating
near or beyond the limits of its performance capability. Deactivating audio takes off a little of the
strain.