I have a problem in understanding the sentence

Alpha indicates the IoU (Intersection over Union) for neighbouring candidate boxes; step sizes of the sliding windows are determined such that one step results in an IoU of Alpha.

What I understand is: The value of Alpha (which is step size) is actually the IoU (Intersection over Union) for neighbouring candidate boxes ;(further explanation of this previous sentence.) step sizes of the sliding windows can be calculated as one step produce in an IoU of Alpha.

I didn't understand the meaning of "in an IoU of Alpha."

Please help me to explain this in simple words or with examples

Orignal Context

  • I think I get it. α=N÷U, and "sliding windows" (whatever they are) are selected such that the value of α for each is always the same. But without context, and maybe the diagram or three from the source of the original statement, it's really difficult to plug yourself into some random subject matter.
    – Andrew Leach
    Jan 8, 2017 at 9:37
  • sorry, what is N÷U? Jan 8, 2017 at 9:42
  • The number in the intersection divided by the number in the union. Set theory. If your statement has nothing to do with set theory, that shows just why it's necessary to give complete context.
    – Andrew Leach
    Jan 8, 2017 at 9:43
  • In my area, IoU is defined as "dividing the area of overlap between the bounding boxes by the area of union" . So, 'in an IoU of Alpha' means values of Alpha is always same as IoU? Jan 8, 2017 at 9:48
  • OK, so it's area of intersection rather than number. I was nearly right on that. Without complete context though, I'm not going to be able to get any further. If you think that what you have given is enough, then you might be able to get someone on Mathematics or Cross Validated to interpret it -- but it's not even clear what your "area" is and whether those sites will know instinctively what you're referring to.
    – Andrew Leach
    Jan 8, 2017 at 9:52

1 Answer 1


Alpha is not step size. Alpha is a measure of how well the blue boxes match -- it's a ratio calculated as (area of intersection)÷(area of union), and varies between zero [no match at all] to one [exact overlap].

This site doesn't have MathJax, and alpha looks like α.

A particular value of α is selected (say 0.7), and then the bounding box is tested, adjusting its position, scale and aspect ratio by step to produce the "neighbouring boxes", but always keeping α constant.

  • let me explain in step wise, please correct me if I miss understood anything From some algorithm we have candidate bounding boxes. Then we search the bounding boxes using sliding window search. Then the step size for each is determined using a single parameter α. So does it mean α is step size? Jan 8, 2017 at 10:53
  • No, as I understand it the step size is arbitrary. It doesn't matter what the step size is as long as the ratio α is constant.
    – Andrew Leach
    Jan 8, 2017 at 13:05
  • Is it the correct rephrasing of the above highlighted text, "Alpha indicates the IoU of neighboring candidate boxes. That is, the step sizes of the sliding windows are shaped in such a way that each step produces an IoU of alpha. " Jan 8, 2017 at 22:38

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