def add_heat(heatmap, bbox_list):
    """
    Filter the found windows to combine overlapping detection.
        Parameters:
            heatmap: A zero-like NumPy array with the size of the image.
            bbox_list: A list of bounding boxes.
    """
    for box in bbox_list:
        heatmap[box[0][1]:box[1][1], box[0][0]:box[1][0]] += 1
    return heatmap