img: bgr, [0,255]
        heatmap: [0,1]

def colorize(img, heatmap):
    """ img: bgr, [0,255]
        heatmap: [0,1]
    """
    heatmap = viz.intensity_to_rgb(heatmap, cmap='jet')[:, :, ::-1]
    return img * 0.5 + heatmap * 0.5