The WebCodecs API is a powerful yet low-level API for handling video and audio without the need for complex libraries. The primary goal of the WebCodecs API is to provide more granular access to video and audio streams, which is valuable for real-time applications like video streaming, broadcasting, or editing. In this article, we'll explore how to decode and encode video using the WebCodecs API in JavaScript.
Getting Started with the WebCodecs API
Using the WebCodecs API requires a basic understanding of streams, buffers, and video codecs. The following example demonstrates how to set up the API for decoding a video. We assume you have a video stream ready to process.
Decoding Video
Decoding video is the process of converting compressed video data into a sequence of images. Here is a step-by-step guide on how to decode a video stream.
First, you need to create a decoder:
const videoDecoder = new VideoDecoder({
output: handleFrame, // Callback to process the decoded frame
error: console.error // Error handler
});
function handleFrame(frame) {
// Process the decoded frame, for example, drawing it on a canvas
const imageBitmap = new VideoFrame(frame).transferToImageBitmap();
context.drawImage(imageBitmap, 0, 0);
frame.close();
}
Next, configure the decoder by setting the desired codec:
videoDecoder.configure({
codec: 'vp8', // Example codec
codedWidth: 1280,
codedHeight: 720
});
Once the decoder is configured, you can start feeding it chunks of video data:
fetch('path/to/video/file').then(response => response.arrayBuffer()).then(buffer => {
let encodedChunks = new EncodedVideoChunk({
type: 'key', // Keyframe or delta frame
timestamp: 0,
data: buffer
});
videoDecoder.decode(encodedChunks);
});
Encoding Video
Encoding video is the opposite process of converting images into a compressed format.
First, create an encoder:
const videoEncoder = new VideoEncoder({
output: handleEncodedChunk, // Callback to process the encoded chunk
error: console.error // Error handler
});
function handleEncodedChunk(chunk) {
// Process the encoded chunk
console.log('Encoded Chunk:', chunk);
}
Similarly, configure your encoder with the codec and encoding parameters:
videoEncoder.configure({
codec: 'vp8',
width: 1280,
height: 720,
bitrate: 1000000, // Example bitrate
});
Now, let's encode a set of image frames:
function encodeVideoFrame(imageBitmap) {
const frame = new VideoFrame(imageBitmap);
videoEncoder.encode(frame);
frame.close();
}
// Assume ctx is a 2D drawing context and canvas holds the frames
async function captureCanvasFrames() {
const imageBitmap = await createImageBitmap(canvas);
encodeVideoFrame(imageBitmap);
}
Putting It Together
The WebCodecs API provides great flexibility, but it's quite low-level, which means careful management of resources like frames and buffers is important. In a real-world application, you would integrate this API with a media handling routine that continuously feeds frames to the decoder or encoder.
Content delivery applications, for example, would decode video streams to provide playback capabilities, while content creation tools might encode video streams for storage or distribution.
Conclusion
The WebCodecs API empowers developers with high-performance video and audio processing capabilities. By allowing access to the underlying codecs directly in the browser, it opens up new possibilities for web-based applications. The API is a solid choice for developers needing precise control over media processing workflows.