The first step evaluates the Objects candidates checking their bounding box. If the bounding boxes of two, contiguous objects are overlapped, these objects are potentially in contact between them.

This approach does not warrant a perfect contact detection. As you can see here below, the bounding boxes can overlap each other but the related objects are not in contact. It speed-up the next step, the contact refining, allowing to check only the objects potentially in contact.

The second step (optional) converts the shape of the objects candidates in their binary representation. The binary maskes are added and, if a common volume is detected, the contact is confirmed.