def polys_to_mask_wrt_box(polygons, box, M):
    """Convert from the COCO polygon segmentation format to a binary mask
    encoded as a 2D array of data type numpy.float32. The polygon segmentation
    is understood to be enclosed in the given box and rasterized to an M x M
    mask. The resulting mask is therefore of shape (M, M).
    """
    w = box[2] - box[0]
    h = box[3] - box[1]

    w = np.maximum(w, 1)
    h = np.maximum(h, 1)

    polygons_norm = []
    for poly in polygons:
        p = np.array(poly, dtype=np.float32)
        p[0::2] = (p[0::2] - box[0]) * M / w
        p[1::2] = (p[1::2] - box[1]) * M / h
        polygons_norm.append(p)

    rle = mask_util.frPyObjects(polygons_norm, M, M)
    mask = np.array(mask_util.decode(rle), dtype=np.float32)
    # Flatten in case polygons was a list
    mask = np.sum(mask, axis=2)
    mask = np.array(mask > 0, dtype=np.float32)
    return mask
Convert from the COCO polygon segmentation format to a binary mask
    encoded as a 2D array of data type numpy.float32. The polygon segmentation
    is understood to be enclosed in the given box and rasterized to an M x M
    mask. The resulting mask is therefore of shape (M, M).
    
detectron/utils/segms.py