Visualizes keypoints (adapted from vis_one_image).
    kps has shape (4, #keypoints) where 4 rows are (x, y, logit, prob).

def vis_keypoints(img, kps, kp_thresh=2, alpha=0.7):
    """Visualizes keypoints (adapted from vis_one_image).
    kps has shape (4, #keypoints) where 4 rows are (x, y, logit, prob).
    """
    dataset_keypoints = PersonKeypoints.NAMES
    kp_lines = PersonKeypoints.CONNECTIONS

    # Convert from plt 0-1 RGBA colors to 0-255 BGR colors for opencv.
    cmap = plt.get_cmap('rainbow')
    colors = [cmap(i) for i in np.linspace(0, 1, len(kp_lines) + 2)]
    colors = [(c[2] * 255, c[1] * 255, c[0] * 255) for c in colors]

    # Perform the drawing on a copy of the image, to allow for blending.
    kp_mask = np.copy(img)

    # Draw mid shoulder / mid hip first for better visualization.
    mid_shoulder = (
        kps[:2, dataset_keypoints.index('right_shoulder')] +
        kps[:2, dataset_keypoints.index('left_shoulder')]) / 2.0
    sc_mid_shoulder = np.minimum(
        kps[2, dataset_keypoints.index('right_shoulder')],
        kps[2, dataset_keypoints.index('left_shoulder')])
    mid_hip = (
        kps[:2, dataset_keypoints.index('right_hip')] +
        kps[:2, dataset_keypoints.index('left_hip')]) / 2.0
    sc_mid_hip = np.minimum(
        kps[2, dataset_keypoints.index('right_hip')],
        kps[2, dataset_keypoints.index('left_hip')])
    nose_idx = dataset_keypoints.index('nose')
    if sc_mid_shoulder > kp_thresh and kps[2, nose_idx] > kp_thresh:
        cv2.line(
            kp_mask, tuple(mid_shoulder), tuple(kps[:2, nose_idx]),
            color=colors[len(kp_lines)], thickness=2, lineType=cv2.LINE_AA)
    if sc_mid_shoulder > kp_thresh and sc_mid_hip > kp_thresh:
        cv2.line(
            kp_mask, tuple(mid_shoulder), tuple(mid_hip),
            color=colors[len(kp_lines) + 1], thickness=2, lineType=cv2.LINE_AA)

    # Draw the keypoints.
    for l in range(len(kp_lines)):
        i1 = kp_lines[l][0]
        i2 = kp_lines[l][1]
        p1 = kps[0, i1], kps[1, i1]
        p2 = kps[0, i2], kps[1, i2]
        if kps[2, i1] > kp_thresh and kps[2, i2] > kp_thresh:
            cv2.line(
                kp_mask, p1, p2,
                color=colors[l], thickness=2, lineType=cv2.LINE_AA)
        if kps[2, i1] > kp_thresh:
            cv2.circle(
                kp_mask, p1,
                radius=3, color=colors[l], thickness=-1, lineType=cv2.LINE_AA)
        if kps[2, i2] > kp_thresh:
            cv2.circle(
                kp_mask, p2,
                radius=3, color=colors[l], thickness=-1, lineType=cv2.LINE_AA)

    # Blend the keypoints.
    return cv2.addWeighted(img, 1.0 - alpha, kp_mask, alpha, 0)