fastDrawImage

There are problems with cross browser support of drawImage in different browsers. It works okay for small images, but with big images (~5mb and more) image decoding starts to block the main browser thread and leads to poor UX. Here is an example that shows 2 seconds of interruption in Chrome when we try drawImage without any optimizations

(Results from Chrome Profiler for Macbook M1, with 6x CPU throttling, ~7mb image data)

It is tricky to avoid image decoding on the main thread, because if something works good in one browser it fails in another (check folder examples for my experiments). I haven't looked much in bug trackers, but looks like there are these issues:

Chrome always start decoding image event if it was explicitly decoded before with .decode() method
Firefox always blocks the main thread after calling .createImageBitmap() with Blob as an argument. Surprisingly it blocks the main thread even if the createImageBitmap() is used inside a web worker.

To avoid image decoding on the main thread reliably in Firefox, Chrome and Safari, we need to use .decode() method for Firefox and Safari, and createImageBitmap() with Blob as an argument for Chrome:

function isChromium() {
    return Boolean(window.chrome);
}

function fastDrawImage() {
    const imageUrl = './url_for_your_image.png';
    if (isChromium()) {
        fetch(imageUrl)
            .then(image => image.blob())
            .then(blob => createImageBitmap(blob))
            .then(bitmap => {
                const canvas = document.getElementById('canvas');
                const ctx = canvas.getContext('2d');
                ctx.drawImage(bitmap, 0, 0);
            });
    } else {
        const image = new Image();
        image.decoding = "async";
        image.onload = () => {
            image.decode().then(() => {
                const canvas = document.getElementById('canvas');
                const ctx = canvas.getContext('2d');
                ctx.drawImage(image, 0, 0);
            });
        };
        image.src = imageUrl;
    }
}

The example that shows reduction from 2 sec to 100 ms after applying optimizations:

(Results from Chrome Profiler for Macbook M1, with 6x CPU throttling, ~7mb image data)

License

The MIT License (MIT)

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myshov/fastDrawImage

fastDrawImage

License