pronouns - Who or whom? "Figure 1 depicts a surveillance system, detecting pedestrians whom are crossing dangerous regions."

In the following sentence, should I use who or whom? It is highlighted in bold.

For instance, Figure 1.1 depicts an operating smart surveillance system, detecting pedestrians whom are crossing dangerous regions, cars in parking lots, and ships.

I think it should be whom because the "operating smart surveillance system" is the subject, while "pedestrians" are the object. But, my professor corrected it to who and now I'm not sure.

Answer

The system is subject but then the next clause talks about pedestrians who also become subject crossing the regions etc. Removing the pronoun itself will also work.

A quick trick!

Now, cut off the sentence.

Pedestrians whom are crossing [something]

Here only, we get a hint that it seems incorrect.

Now, think that it's about 'you'. If 'I' fits, put 'who' and if 'me' fits, put 'whom'.

me I am crossing [something]

Technically, if the pronoun in question is doing something, prefer putting 'who' (what they call subject) and if it's receiving something, prefer 'whom' (what they call object)!