Encode ndarray to base64 string image data
    
    Parameters
    ----------
    arr: ndarray (rows, cols, depth)
    kwargs: passed directly to matplotlib.image.imsave

def img_encode(arr, **kwargs):
    """Encode ndarray to base64 string image data
    
    Parameters
    ----------
    arr: ndarray (rows, cols, depth)
    kwargs: passed directly to matplotlib.image.imsave
    """
    sio = BytesIO()
    imsave(sio, arr, **kwargs)
    sio.seek(0)
    img_format = kwargs['format'] if kwargs.get('format') else 'png'
    img_str = base64.b64encode(sio.getvalue()).decode()

    return 'data:image/{};base64,{}'.format(img_format, img_str)