NvDsOpticalFlowMeta

class pyds.NvDsOpticalFlowMeta

Holds optical flow metadata about a frame.

Variables:

• rows – int, Number of rows present in the frame for given block size e.g. if block size is 4 and frame height is 720, then number of rows = (720 / 4) = 180

• cols – int, Number of columns present in the frame for given block size e.g. if block size is 4 and frame width is 1280, then number of columns = (1280 / 4) = 320

• mv_size – int, Size of motion vector. Refer NvOFFlowVector.

• frame_num – int, Current frame number of the source.

• data – Holds the motion vector.

• priv – Reserved field, for internal purpose only.

• reserved – Reserved field, for internal purpose only.

Example usage:

def visualize_optical_flowvectors(flow):
    """
    Converts the flow u, v vectors into visualization by mapping them into
    grey color space
    :param flow: flow vectors
    :return: bgr image
    """
    shape_visual = (flow.shape[0], flow.shape[1], 3)
    mag, ang = cv2.cartToPolar(flow[..., 0], flow[..., 1])
    hsv = np.full(shape_visual, 255, dtype=np.uint8)
    hsv[..., 1] = 255
    hsv[..., 0] = ang * 180 / np.pi / 2
    hsv[..., 2] = cv2.normalize(mag, None, 0, 255, cv2.NORM_MINMAX)
    bgr = cv2.cvtColor(hsv, cv2.COLOR_HSV2BGR)
    bgr = 255 - bgr
    return bgr

...

l_user = frame_meta.frame_user_meta_list #Retrieve glist containing NvDsUserMeta objects from given NvDsFrameMeta object
while l_user is not None:
    try:
        # Casting l_user.data to pyds.NvDsUserMeta
        of_user_meta = pyds.NvDsUserMeta.cast(l_user.data)
        except StopIteration:
            break
        try:
            # Casting of_user_meta.user_meta_data to pyds.NvDsOpticalFlowMeta
            of_meta = pyds.NvDsOpticalFlowMeta.cast(of_user_meta.user_meta_data)
            # Get Flow vectors as np array
            flow_vectors = pyds.get_optical_flow_vectors(of_meta)
            # Reshape the obtained flow vectors into proper shape
            flow_vectors = flow_vectors.reshape(of_meta.rows, of_meta.cols, 2)
            # map the flow vectors in HSV color space for visualization
            flow_visual = visualize_optical_flowvectors(flow_vectors)
            got_visual = True
        except StopIteration:
            break
        try:
            l_user = l_user.next
        except StopIteration:
            break

cast(self: capsule) → pyds.NvDsOpticalFlowMeta

casts to NvDsOpticalFlowMeta, call pyds.NvDsOpticalFlowMeta(data)