What is the best way to classify data not belonging to set of classes?

Question

I am building a multi-class support vector machine (8 classes to be precise) on an image dataset of pre-defined classes. And then I thought of a question:

What if I have an image that doesn't belong to the set of predefined classes, what would be the outcome?

So I decided to experiment with it and the result was very bad. I got a higher accuracy for images that don't belong to any of the classes. Some images gave 98% accuracy, that they belonged to a particular class, even though my expectation should be that they should should have a very low accuracy.

I also tried using OnceClass SVM to first predict if it's part of the class or not. If yes, then what's the label? (Meaning I have 2 models).

But this doesn't seem to work as the OneClass SVM couldn't classify the "other" images well. Now I am running out of ideas of how to go about it.

How can I approach this problem?

Answer 1

Just a proposal of a method to try out.

Stage $1$: Use one-class SVM to assign those images that do not belong to the set of predefined classes as the $9$-th class.

Stage $2$: For those images that passes through your filter, let the multi-class SVM assign them to one of the $8$ classes.