Add interlace example

Author: jeromewu

apps/website/docs/getting-started/usage.md

@@ -359,3 +359,64 @@ function() {
     );
 }
 ```
+
+## Interlaced 2 Videos
+
+```jsx live
+// import { FFmpeg } from '@ffmpeg/ffmpeg';
+// import { fetchFile } from '@ffmpeg/util';
+function() {
+    const [loaded, setLoaded] = useState(false);
+    const ffmpegRef = useRef(new FFmpeg());
+    const videoRef = useRef(null);
+    const messageRef = useRef(null);
+
+    const load = async () => {
+        const baseURL = 'https://unpkg.com/@ffmpeg/[email protected]/dist/umd'
+        const ffmpeg = ffmpegRef.current;
+        ffmpeg.on('log', ({ message }) => {
+            messageRef.current.innerHTML = message;
+            console.log(message);
+        });
+        // toBlobURL is used to bypass CORS issue, urls with the same
+        // domain can be used directly.
+        await ffmpeg.load({
+            coreURL: await toBlobURL(`${baseURL}/ffmpeg-core.js`, 'text/javascript'),
+            wasmURL: await toBlobURL(`${baseURL}/ffmpeg-core.wasm`, 'application/wasm'),
+        });
+        setLoaded(true);
+    }
+
+    const transcode = async () => {
+        const ffmpeg = ffmpegRef.current;
+        await ffmpeg.writeFile('input.webm', await fetchFile('https://raw.githubusercontent.com/ffmpegwasm/testdata/master/Big_Buck_Bunny_180_10s.webm'));
+        await ffmpeg.writeFile('reversed.webm', await fetchFile('https://raw.githubusercontent.com/ffmpegwasm/testdata/master/Big_Buck_Bunny_180_10s_reversed.webm'));
+        await ffmpeg.exec([
+            '-i',
+            'input.webm',
+            '-i',
+            'reversed.webm',
+            '-filter_complex',
+            '[0:v][1:v]blend=all_expr=\'A*(if(eq(0,N/2),1,T))+B*(if(eq(0,N/2),T,1))\'',
+            'output.mp4',
+        ]);
+        const data = await ffmpeg.readFile('output.mp4');
+        videoRef.current.src =
+            URL.createObjectURL(new Blob([data.buffer], {type: 'video/mp4'}));
+    }
+
+    return (loaded
+        ? (
+            <>
+                <video ref={videoRef} controls></video><br/>
+                <button onClick={transcode}>Interlace two webm video to mp4</button>
+                <p ref={messageRef}></p>
+                <p>Open Developer Tools (Ctrl+Shift+I) to View Logs</p>
+            </>
+        )
+        : (
+            <button onClick={load}>Load ffmpeg-core (~31 MB)</button>
+        )
+    );
+}
+```