def create_ogblob(ogheader):
    """create <meta> tags from Open Graph dict

    if ogheader is empty dict, then return empty string.
    if ogheader has some values but not the minimum, then raise ValueError.

    if og:image is a path, not a URL of http or https scheme, then
    'https://help.rerobots.net/' is prepended to it.
    """
    if len(ogheader) == 0:
        return ''
    if not ('title' in ogheader and 'url' in ogheader and 'image' in ogheader):
        raise ValueError('some but not all required metadata provided for Open Graph')

    if not (ogheader['image'].startswith('http://') or ogheader['image'].startswith('https://')):
        ogheader['image'] = 'https://help.rerobots.net/' + ogheader['image']

    blob = """
<meta property="og:type" content="website" />
<meta property="og:url" content="{URL}" />
<meta property="og:title" content="{TITLE}" />
<meta property="og:image" content="{IMAGE}" />
""".format(URL=ogheader['url'], TITLE=ogheader['title'], IMAGE=ogheader['image'])

    return blob