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"""Operations for [N, height, width] numpy arrays representing masks. |
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Example mask operations that are supported: |
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* Areas: compute mask areas |
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* IOU: pairwise intersection-over-union scores |
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""" |
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import numpy as np |
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EPSILON = 1e-7 |
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def area(masks): |
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"""Computes area of masks. |
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Args: |
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masks: Numpy array with shape [N, height, width] holding N masks. Masks |
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values are of type np.uint8 and values are in {0,1}. |
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Returns: |
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a numpy array with shape [N*1] representing mask areas. |
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Raises: |
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ValueError: If masks.dtype is not np.uint8 |
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""" |
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if masks.dtype != np.uint8: |
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raise ValueError('Masks type should be np.uint8') |
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return np.sum(masks, axis=(1, 2), dtype=np.float32) |
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def intersection(masks1, masks2): |
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"""Compute pairwise intersection areas between masks. |
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Args: |
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masks1: a numpy array with shape [N, height, width] holding N masks. Masks |
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values are of type np.uint8 and values are in {0,1}. |
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masks2: a numpy array with shape [M, height, width] holding M masks. Masks |
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values are of type np.uint8 and values are in {0,1}. |
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Returns: |
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a numpy array with shape [N*M] representing pairwise intersection area. |
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Raises: |
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ValueError: If masks1 and masks2 are not of type np.uint8. |
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""" |
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if masks1.dtype != np.uint8 or masks2.dtype != np.uint8: |
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raise ValueError('masks1 and masks2 should be of type np.uint8') |
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n = masks1.shape[0] |
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m = masks2.shape[0] |
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answer = np.zeros([n, m], dtype=np.float32) |
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for i in np.arange(n): |
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for j in np.arange(m): |
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answer[i, j] = np.sum(np.minimum(masks1[i], masks2[j]), dtype=np.float32) |
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return answer |
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def iou(masks1, masks2): |
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"""Computes pairwise intersection-over-union between mask collections. |
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Args: |
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masks1: a numpy array with shape [N, height, width] holding N masks. Masks |
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values are of type np.uint8 and values are in {0,1}. |
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masks2: a numpy array with shape [M, height, width] holding N masks. Masks |
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values are of type np.uint8 and values are in {0,1}. |
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Returns: |
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a numpy array with shape [N, M] representing pairwise iou scores. |
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Raises: |
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ValueError: If masks1 and masks2 are not of type np.uint8. |
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""" |
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if masks1.dtype != np.uint8 or masks2.dtype != np.uint8: |
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raise ValueError('masks1 and masks2 should be of type np.uint8') |
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intersect = intersection(masks1, masks2) |
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area1 = area(masks1) |
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area2 = area(masks2) |
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union = np.expand_dims(area1, axis=1) + np.expand_dims( |
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area2, axis=0) - intersect |
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return intersect / np.maximum(union, EPSILON) |
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def ioa(masks1, masks2): |
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"""Computes pairwise intersection-over-area between box collections. |
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Intersection-over-area (ioa) between two masks, mask1 and mask2 is defined as |
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their intersection area over mask2's area. Note that ioa is not symmetric, |
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that is, IOA(mask1, mask2) != IOA(mask2, mask1). |
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Args: |
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masks1: a numpy array with shape [N, height, width] holding N masks. Masks |
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values are of type np.uint8 and values are in {0,1}. |
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masks2: a numpy array with shape [M, height, width] holding N masks. Masks |
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values are of type np.uint8 and values are in {0,1}. |
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Returns: |
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a numpy array with shape [N, M] representing pairwise ioa scores. |
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Raises: |
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ValueError: If masks1 and masks2 are not of type np.uint8. |
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""" |
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if masks1.dtype != np.uint8 or masks2.dtype != np.uint8: |
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raise ValueError('masks1 and masks2 should be of type np.uint8') |
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intersect = intersection(masks1, masks2) |
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areas = np.expand_dims(area(masks2), axis=0) |
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return intersect / (areas + EPSILON) |
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