Merge pull request #100 from caleb-mabry/webp-support

WebAO Partial Image Fixes

1 files changed, 27 insertions, 0 deletions

diff --git a/webAO/utils/calculateGifLength.js b/webAO/utils/calculateGifLength.js
new file mode 100644
index 0000000..1df0ba9
--- /dev/null
+++ b/webAO/utils/calculateGifLength.js
@@ -0,0 +1,27 @@
+/**
+	 * Adds up the frame delays to find out how long a GIF is
+	 * I totally didn't steal this
+	 * @param {data} gifFile the GIF data
+	 */
+const calculateGifLength = (gifFile) => {
+  const d = new Uint8Array(gifFile);
+  // Thanks to http://justinsomnia.org/2006/10/gif-animation-duration-calculation/
+  // And http://www.w3.org/Graphics/GIF/spec-gif89a.txt
+  let duration = 0;
+  for (let i = 0; i < d.length; i++) {
+    // Find a Graphic Control Extension hex(21F904__ ____ __00)
+    if (d[i] === 0x21
+				&& d[i + 1] === 0xF9
+				&& d[i + 2] === 0x04
+				&& d[i + 7] === 0x00) {
+      // Swap 5th and 6th bytes to get the delay per frame
+      const delay = (d[i + 5] << 8) | (d[i + 4] & 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 * 10;
+};
+export default calculateGifLength;