image-media.ts

/*
Copyright 2024 New Vector Ltd.
Copyright 2022 The Matrix.org Foundation C.I.C.

SPDX-License-Identifier: AGPL-3.0-only OR GPL-3.0-only
Please see LICENSE files in the repository root for full details.
*/

import { ImageInfo } from "matrix-js-sdk/src/types";

import { BlurhashEncoder } from "../BlurhashEncoder";

type ThumbnailableElement = HTMLImageElement | HTMLVideoElement;

export const BLURHASH_FIELD = "xyz.amorgan.blurhash"; // MSC2448

interface IThumbnail {
    info: ImageInfo;
    thumbnail: Blob;
}

const MAX_WIDTH = 800;
const MAX_HEIGHT = 600;

/**
 * Create a thumbnail for a image DOM element.
 * The image will be smaller than MAX_WIDTH and MAX_HEIGHT.
 * The thumbnail will have the same aspect ratio as the original.
 * Draws the element into a canvas using CanvasRenderingContext2D.drawImage
 * Then calls Canvas.toBlob to get a blob object for the image data.
 *
 * Since it needs to calculate the dimensions of the source image and the
 * thumbnailed image it returns an info object filled out with information
 * about the original image and the thumbnail.
 *
 * @param {HTMLElement} element The element to thumbnail.
 * @param {number} inputWidth The width of the image in the input element.
 * @param {number} inputHeight the width of the image in the input element.
 * @param {string} mimeType The mimeType to save the blob as.
 * @param {boolean} calculateBlurhash Whether to calculate a blurhash of the given image too.
 * @return {Promise} A promise that resolves with an object with an info key
 *  and a thumbnail key.
 */
export async function createThumbnail(
    element: ThumbnailableElement,
    inputWidth: number,
    inputHeight: number,
    mimeType: string,
    calculateBlurhash = true,
): Promise<IThumbnail> {
    let targetWidth = inputWidth;
    let targetHeight = inputHeight;
    if (targetHeight > MAX_HEIGHT) {
        targetWidth = Math.floor(targetWidth * (MAX_HEIGHT / targetHeight));
        targetHeight = MAX_HEIGHT;
    }
    if (targetWidth > MAX_WIDTH) {
        targetHeight = Math.floor(targetHeight * (MAX_WIDTH / targetWidth));
        targetWidth = MAX_WIDTH;
    }

    let canvas: HTMLCanvasElement | OffscreenCanvas;
    let context: CanvasRenderingContext2D | OffscreenCanvasRenderingContext2D;
    try {
        canvas = new window.OffscreenCanvas(targetWidth, targetHeight);
        context = canvas.getContext("2d") as OffscreenCanvasRenderingContext2D;
    } catch {
        // Fallback support for other browsers (Safari and Firefox for now)
        canvas = document.createElement("canvas");
        canvas.width = targetWidth;
        canvas.height = targetHeight;
        context = canvas.getContext("2d")!;
    }

    context.drawImage(element, 0, 0, targetWidth, targetHeight);

    let thumbnailPromise: Promise<Blob>;
    if (window.OffscreenCanvas && canvas instanceof OffscreenCanvas) {
        thumbnailPromise = canvas.convertToBlob({ type: mimeType });
    } else {
        thumbnailPromise = new Promise<Blob>((resolve) =>
            (canvas as HTMLCanvasElement).toBlob(resolve as BlobCallback, mimeType),
        );
    }

    const imageData = context.getImageData(0, 0, targetWidth, targetHeight);
    // thumbnailPromise and blurhash promise are being awaited concurrently
    const blurhash = calculateBlurhash ? await BlurhashEncoder.instance.getBlurhash(imageData) : undefined;
    const thumbnail = await thumbnailPromise;

    return {
        info: {
            thumbnail_info: {
                w: targetWidth,
                h: targetHeight,
                mimetype: thumbnail.type,
                size: thumbnail.size,
            },
            w: inputWidth,
            h: inputHeight,
            [BLURHASH_FIELD]: blurhash,
        },
        thumbnail,
    };
}