X4D's Select GPUs widget allows users to specify which graphics cards are used for processing the X-ray and digitally reconstructed radiograph (DRR) images for each view.

Every hardware set-up is different, but we have some general recommendations to help you get the best performance from X4D:

1. If you have two CUDA-capable graphics cards, it is recommended that you use one card for each view.
2. If you have two cards and one is significantly faster than the other, it may make DRR generation (and thus object tracking) more efficient by using the faster card for both views.
3. If you are having problems with DRR generation that you suspect might be related to the graphics card[s], you can also turn off the use of the cards by one or both X-ray views. This will make DRR generation very slow, however.

Note: You cannot assign more than one graphics card to a view.