<script>
function load_multipart_image(src) {
    return new Promise((resolve, reject) => {
        const img = document.createElement('img');
        img.addEventListener('load', () => resolve(img));
        img.addEventListener('error', (e) => reject(new DOMException('load failed', 'NetworkError')));
        img.src = src;
    });
}

function get_image_data(img) {
    const canvas = document.createElement('canvas');
    const context = canvas.getContext('2d');
    context.drawImage(img, 0, 0);
    // When |img.src| is cross origin, this should throw a SecurityError.
    const imageData = context.getImageData(0, 0, 1, 1);
    return imageData;
}
</script>