1 files changed, 22 insertions, 0 deletions
diff --git a/webAO/utils/calculateWebpLength.js b/webAO/utils/calculateWebpLength.js
new file mode 100644
index 0000000..1b422a0
--- /dev/null
+++ b/webAO/utils/calculateWebpLength.js
@@ -0,0 +1,22 @@
+const calculateWebpLength = (webpFile) => {
+  const d = new Uint8Array(webpFile);
+  // https://developers.google.com/speed/webp/docs/riff_container#animation
+  let duration = 0;
+  for (let i = 0; i < d.length; i++) {
+    // Find ANMF header (41 4E 4D 46)
+    if (d[i] === 0x41
+      && d[i + 1] === 0x4E
+      && d[i + 2] === 0x4D
+      && d[i + 3] === 0x46) {
+      // Swap 5th and 6th bytes to get the delay per frame
+      const delay = (d[i + 21] << 8) | (d[i + 20] & 0xFF);
+
+      // Should be aware browsers have a minimum frame delay
+      // e.g. 6ms for IE, 2ms modern browsers (50fps)
+      duration += delay < 2 ? 10 : delay;
+    }
+  }
+  return duration;
+};
+
+export default calculateWebpLength;