Use two-pass with ffmpeg

.gitignore

@@ -1,2 +1,3 @@
 public/assets/scripts/ffmpeg/
+public/assets/scripts/core/
 

public/assets/scripts/main.js

@@ -8,18 +8,45 @@ async function compress() {
 	const filesizeUnit = document.getElementById("filesize-unit").value;
 	const file = document.getElementById("file-input").files[0];
 
+	const videoLength = document.getElementById("uploaded-video").duration;
+	let targetFilesize; // Stored in kBit
+	switch (filesizeUnit) {
+		case "K": targetFilesize = filesize * 8.192; break;
+		case "M": targetFilesize = filesize * 8388.608; break;
+	}
+
 	updateProgress(0);
 
-	ffmpeg.on("progress", data => {
+	ffmpeg.on("log", event => {
-		console.log(data);
+		console.log("[ffmpeg]", event.type, event.message);
-		updateProgress(data.progress);
+	});
+
+	let pass;
+
+	ffmpeg.on("progress", event => {
+		updateProgress(event.progress, pass);
 	});
 
 	await ffmpeg.load({ coreURL: "/assets/scripts/core/package/dist/umd/ffmpeg-core.js" });
 
 	await ffmpeg.writeFile(file.name, await readFromBlob(file));
 
-	await ffmpeg.exec(["-i", file.name, "-preset", "veryfast", "-fs", filesize + filesizeUnit, "compressed.mp4"]);
+	// Use Two-Pass to create a video file with desired file size
+	// https://trac.ffmpeg.org/wiki/Encode/H.264#twopass
+
+	const bitrate = Math.round(targetFilesize / videoLength) - 128; // Subtract audio bitrate
+
+	console.debug("Target bitrate:", bitrate);
+
+	const options = ["-i", file.name, "-preset", "veryfast", "-c:v", "libx264", "-b:v", bitrate + "k"];
+
+	pass = 1;
+
+	await ffmpeg.exec(["-y", ...options, "-pass", "1", "-vsync", "cfr", "-f", "null", "/dev/null"]);
+
+	pass = 2;
+
+	await ffmpeg.exec([...options, "-pass", "2", "-c:a", "aac", "-b:a", "128k", "compressed.mp4"]);
 
 	const video = await ffmpeg.readFile("compressed.mp4");
 
@@ -32,9 +59,10 @@ function cancel() {
 	showSection("file-picker");
 }
 
-function updateProgress(progress) {
+function updateProgress(progress, pass) {
 	const percent = (progress * 100).toFixed(1) + "%";
 
+	document.getElementById("progress-step-value").innerText = pass;
 	document.getElementById("progress-percentage").innerText = percent;
 	document.getElementById("progress-indicator").style.clipPath = `rect(0 ${percent} 100% 0)`;
 }

public/assets/style/main.css

@@ -157,7 +157,7 @@ input:focus, select:focus {
 
 #loading-title {
 	color: #757575;
-	font-size: 3rem;
+	font-size: 2.5rem;
 	font-weight: bold;
 	margin-top: 4rem;
 	margin-bottom: 0;
@@ -175,7 +175,6 @@ input:focus, select:focus {
 	margin: auto;
 	background-color: #E0E0E0;
 	position: relative;
-	margin-top: 3rem;
 }
 
 #progress-indicator {
@@ -192,3 +191,8 @@ input:focus, select:focus {
 	font-size: 1.2rem;
 }
 
+#progress-step {
+	font-weight: bold;
+	margin-top: 3rem;
+	margin-bottom: 0.5rem;
+}

public/index.html

@@ -29,7 +29,6 @@
 			--><select id="filesize-unit">
 				<option value="K">KB</option>
 				<option value="M" selected>MB</option>
-				<option value="G">GB</option>
 			</select>
 
 			<button id="compress" onclick="compress()" class="primary">Go!</button>
@@ -39,6 +38,7 @@
 			<h3 id="loading-title">Please wait...</h3>
 			<p id="loading-description">This may take a while</p>
 
+			<p id="progress-step">Step <span id="progress-step-value">1</span>/2</p>
 			<div id="progress-container">
 				<div id="progress-indicator"></div>
 			</div>