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The Viewport has a button in the top left corner for starting/stopping IPR, creating a crop window, quickly changing to the PxrVizualizer Integrator, restarting (for changes that do not trigger an update), or taking a snapshot.
The crop window tool can be sized/re-sized by drawing on the window or selecting the corners. Mouse dragging from the center moves the window and clicking outside the window negates it by setting it to the viewport size.
The PxrVisualizer Integrator is great for quickly navigating scenes, once deselected it returns to the previously chosen integrator.
IPR Threads
In the threads section of the RenderMan Preferences, the Rendering Threads setting is used for both Preview and IPR renders. The default value of -2 means that all but two CPUs will be used for rendering. It's advised to leave at least one thread to allow for interactivity in Maya while rendering is underway. Zero threads means all processors will be used for rendering. You can also choose a specific number.
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