How to do hard negative mining for cascade classifier?

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Hi I want to do hard negative mining for my trained cascade classifier. In other words, I want to add false positives to the list of negative images and re-train my cascade to improve accuracy. The question is:

If the cascade detects a large region where a small portion of it is the desired object, then what should I do? The documentation says that negative images must not contain objects. So I shouldn't add it to the negatives. But how to re-train the cascade to ignore such cases?