Prioritized Sampling

IoU-Balanced Sampling

Introduced by Pang et al. in Libra R-CNN: Towards Balanced Learning for Object Detection

IoU-Balanced Sampling is hard mining method for object detection. Suppose we need to sample $N$ negative samples from $M$ corresponding candidates. The selected probability for each sample under random sampling is:

$$ p = \frac{N}{M} $$

To raise the selected probability of hard negatives, we evenly split the sampling interval into $K$ bins according to IoU. $N$ demanded negative samples are equally distributed to each bin. Then we select samples from them uniformly. Therefore, we get the selected probability under IoU-balanced sampling:

$$ p_{k} = \frac{N}{K}*\frac{1}{M_{k}}\text{ , } k\in\left[0, K\right)$$

where $M_{k}$ is the number of sampling candidates in the corresponding interval denoted by $k$. $K$ is set to 3 by default in our experiments.

The sampled histogram with IoU-balanced sampling is shown by green color in the Figure to the right. The IoU-balanced sampling can guide the distribution of training samples close to the one of hard negatives.

Source: Libra R-CNN: Towards Balanced Learning for Object Detection

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