Setting gamma value for RBF kernel in templateSVM

Question

According to this document (and several others), the RBF kernel is defined by the equation below, and can be adjusted by varying the parameters C and gamma.

The documentation for svmTemplate defines the RBF kernel without a gamma value as shown below.

Why does svmTemplate not use gamma? Is there a way of specifying it?

Answer 1

I'm guessing it is because it is essentially rolled into the "KernelScale" option in the documentation. It indicates that it divides all the values in the data by a constant, which allows you to essentially set the gamma parameter that way. You can see that ||x z-y z|| = ||xy||*z, and so obtains the equivalent impact (so long as you select the KernelScale value appropriately). The documentation says the scale is selected heuristically by default, and there do exist heuristics for the RBF kerne's parameters.

It's not the way I would have coded such a system, but it should work.