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Normally, when CINELERRA-GG reads a video frame, it is copied into a Vframe. This frame may also need other actions performed on it, such as a color model change. In addition, ffmpeg and libzmpeg can_scale_input. So the read can be color transformed and scaled just by asking the library to do that. This means that if the compositor is in a good state with no zoom, the VFrame read can be done in the fastest render color model, and already scaled to the correct size for the compositor. In reality, this is not what you need for editing, but quite often the virtualconsole is not used because the render is media only – that is just data. If no data transforms are needed and the input scaling can be done, the vrender program detects this, and tells the codec to transmit the data in a compatible way to the compositor canvas. This is the X11 direct data path.
With the X11 video driver choice, large format files such as 4K, will playback faster than either X11-XV or X11-OpenGL. However, you still have the option to turn off the X11 direct data path if you use Settings → Preferences, Playback A tab set video driver to X11 and uncheck use direct X11 render if possible.