Starting IPR

There are a good number of different ways for you to fire off an IPR session in Blender. 

Viewport

For rendering inside of the Blender viewport, you can change the viewport shading to render

or, you can select the RenderMan IPR icon and choose whether to send your IPR session to Blender's Viewport or RenderMan's image tool, it

One other way to begin an IPR session is to right-click the context menu, and select Render → IPR or IPR to it

IPR Operators

Once IPR has started, you'll see a row of operators in the viewport that look like this:

Note, that not all of these operators are available if you started IPR to it.

 
Stop IPR /
This button stops the current IPR session.

 
Integrator /
Allows you to quickly change the current integrator. Note, this does not actually affect the integrator chosen for the scene.



Interactive Refinement /
Changes the current interactive refinement value.

In RIS, the value determines how much refinement (in a dither pattern) will be applied to the image during interactive rendering. 0 means full refinement up to a value of 6 which is the least refinement per iteration.

In XPU, the interactive refinement mode is called Progressive Pixels (PP). The visual impact in an IPR setting is similar to RIS and primarily aimed at providing artists with a fluid interactive experience at the cost of integrating a fraction of screen pixels at a time. When PP is enabled, the results of pixels within a screen bucket are splatted progressively starting from a large screen region all the way down to a single pixel. When no edits are received after resolving at the single pixel, XPU continues working with pixels at the finest level. Only when new edits are received does XPU revert back to using large progressive splats.

Scale Resolution /
Allows you to scale down the resolution of the render in the viewport. Scaling down the resolution can help speed up interactivity.

Channel Select /
If you are rendering multiple AOVs, this allows you to quickly switch between your AOVs in the viewport. This currently does not work in XPU.

Crop Window /
Allows you to draw a crop window so that subsequent edits are only visible within that crop window. This is the equivalent of using Blender's Render Region. 

Snapshot /
Saves a snapshot of the current render in the viewport and saves the image into Blender's render window.

Enhance /
This only works with perspective cameras or the PxrCamera projection plugin.

Embiggens the region around a pixel (X,Y) by a zoom factor for troubleshooting.  
The zoom factor can be changed in the RenderMan for Blender preferences.
The magnified pixel will remain anchored in place relative to the image.  Camera effects such as vignetting will be scaled accordingly.  

Intentionally does not affect level-of-detail, dicing, displacement, or MIP map levels.
Left-click on the region you want to embiggen. Right-click to reset the zoom. Enter to simply exit out of the operator, and keep the current zoom. Esc to exit and reset the zoom.

Texture Cache /
this clears the RenderMan texture cache. This should only be needed if the .tex file on disk has changed, outside of the Texture Manager.

IPR Sampling

You can tweak your Min / Max Samples, Pixel Variance, and Interactive Refinement settings whilst you are currently running a live IPR session.