Capture HTML canvas as GIF/JPG/PNG/PDF?

Priming the canvas for capturing

Before you initiate the snapshot, it's best to prepare the canvas by manipulating its content accordingly. With the HTML5 Canvas API, this is as smooth as using a .toDataURL() method on the canvas. Consider it your mighty morphin' image converter.

Adjusting JPEG quality and alternate formats

For JPEG, the quality level can be specified as such:

canvas.toDataURL('image/jpeg', quality); // quality ranges from 0 - 1 (0.5 being "medium rare")
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Each image format brings along its own showtime features. Some, like PNG, have transparency while others like JPEG strike a balance between quality and file size. Be sure to ensure the selected format is browser-supported.

Easy download link creation

A download link can be dynamically generated as follows:

const dataUrl = canvas.toDataURL('image/png');
const link = document.createElement('a');
link.href = dataUrl;
link.download = 'masterpiece.png'; // Picasso would approve 👍
document.body.appendChild(link);
link.click();
document.body.removeChild(link); // Cleaning up after party 🧹
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Dealing with cross-origin violations

Keep in mind that canvas.toDataURL() might throw a security error if the canvas content has been loaded from cross-origin sources devoid of proper CORS headers. Check your image sources when pulling images from the outside world.

SVG to image conversion

SVG content within a canvas can be properly converted before capture using the canvg library. SVG to the rescue!

Adding GIF animation

To capture canvas as a GIF, use libraries like gif.js. This way, you can combine multiple canvas frames into a single animated GIF, cause static is so 90s.

Advanced techniques, tips and best practices

HTML to PDF conversion using wkhtmltopdf

If the mission is to convert the entire webpage (including the canvas) into a PDF, wkhtmltopdf comes in handy. It uses the Webkit rendering engine, but may require you to dust off your command-line skills.

Keeping the DOM clean

Don't clutter the user view with download links. After the download is triggered, we clean up any dynamical elements we've added. A tidy DOM means a tidy mind.

Prompting users with image/octet-stream

Change the MIME type to "image/octet-stream" in the .toDataURL() method to force a download prompt. This is less common, but effective for user-driven downloads.

Checking for compatibility

Provide fallbacks or alternatives for browsers that do not support certain features. It's a testing party - across different browsers and devices for the best user experience!