Objects detection quiz - Non-max suppression

Course Q&A Deep Learning Specialization Convolutional Neural Networks
1

There’s a NMS-related question that seems controversial. I’ll try my best not to break the honour code with my description:

  • The quiz states that the parameters for NMS include a probability ρ, used for discarding boxes with probability ≤ ρ, and a IoU value of ν for determining if two boxes overlap.
  • In the image, you can observe two objects that belong to the same class κ: object A has been enclosed in two different bounding boxes with probabilty > ρ whereas object B is “contained” by only one bounding box with probability < ρ.
  • The quiz asks if NMS will only keep objects A’s bounding box with the highest probability. This is not an exact transcript of the question, of course.
  • Apparently, the correct answer is “True. Only one of the boxes for an object of class κ is kept because the score S (i.e. probability) is higher, and also because it has a larger bounding box”. Again, not an exact transcript.

I’m convinced that the correct answer is, indeed, “True”, but I don’t agree with the justifications. I believe object B’s bounding box is “immediately” discarded because its probability is less than ρ. In addition, and even though both of object A’s bounding boxes have probability > ρ, only the one with the highest value is kept as a consequence of these boxes having an IoU larger than ν.

I guess my questions are:

  1. What does NMS have to do with the size of a bounding box?
  2. Are my arguments correct?

Thanks in advance,

Joaco.

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2

Yes, your reasoning sounds correct to me. The actual size of the bounding box is irrelevant, other than the contribution it makes to computing the IoU with any overlapping boxes. So it sounds like the wording of the explanation given for why the answer is correct in the quiz is a bit misleading. If you can get a screenshot of the question, you could send me that as a DM without violating any rules. Then I can use that to file a bug after having a look. (Just click my name or avatar and then click “Message” and use the little “Up Arrow” tool to attach the screenshot.) Thanks for pointing this out and for the effort you put into giving a clear description of the scenario, while staying within the honour code!

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3

Ok, @joaco sent me the screen shot and I agree that there is something wrong with the quiz answers as they currently stand. The way the question is worded, you have to pick both the correct answer in terms of “True” or “False”, but also in terms of why the answer comes out that way. The answer that looks like the correct one (as Joaco pointed out) is marked as incorrect by the grader.

I’ll file a bug on this and let you know if I hear anything from the course staff.

Thanks for the report!

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