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## Scalable rendering

VisIt typically uses graphics hardware on the local computer to very quickly draw plots once they have been generated by the compute engine. This becomes impractical for very large databases because the amount of memory needed to store the graphics commands that draw the plots quickly exceeds the amount of memory in the graphics hardware. Large sets of graphics commands can also degrade performance when they must be shipped over slow networks from the compute engine to the VisIt's viewer. VisIt provides a scalable rendering option that can improve both of these situations by creating the actual plot images, in parallel, on the compute engine, compressing them, and then transmitting only an image to the viewer where the image can be displayed.

Scalable rendering can be orders of magnitude faster for large databases than VisIt's conventional image drawing strategy because large databases are typically processed using a parallel compute engine. When using scalable rendering with a parallel compute engine, VisIt is able to draw small pieces of the plot on each processor in parallel and then glue the image together before sending it to the viewer to be displayed. Not only has the image likely been created faster, but the size of the image is usually on the order of a megabyte instead of the tens or hundreds of megabytes needed to transmit graphics commands, which results in faster transmission of the image to the viewer. The drawback of scalable rendering is that it is usually not as interactive as graphics hardware because each time you want to change the view or interact with the plots, round trip communication with the compute engine is required.

VisIt can automatically determine when to stop sending geometry to the viewer in favor of sending scalably rendered images. You can help VisIt decide when to use scalable rendering by either setting the scalable rendering threshold, which is the number of polygons that must exist in the set of geometry to be rendered on the viewer before the compute engine will kick into scalable rendering mode. You can set the scalable rendering threshold by entering a new number of polygons into the When polygon count exceeds spin box in the Rendering Options Window. The number that you enter is measured in thousands of polygons.

If you want VisIt to always use scalable rendering, you can click the Always radio button under the Use scalable rendering options. If you don't want VisIt to ever use scalable rendering no matter how many polygons are in the set of geometry that your workstation will have to draw, click the Never check box under the Use scalable rendering options. Note that if you disable scalable rendering and then you decide to plot a database with millions or hundreds of millions of cells, you risk running out of memory on your workstation or having to wait a very long time for the image to appear. In general, it is much faster to draw images of large databases with scalable rendering.