Edit Proxies for Browser Video Editors: Proxy Generation API

Why Browser Timelines Lag

Standard video codecs (like H.264/MP4) use temporal compression. To save space, they only store full image frames (Keyframes or I-frames) every few seconds, representing intermediate frames as differences between them (P-frames and B-frames).

When a user scrubs back and forth on a web-based video editing timeline, the browser’s video decoder has to decode multiple preceding frames just to render a single frame. This creates noticeable timeline lag, dropped frames, and a frustrating editing experience.

Standard Compression vs. Editing Proxies

Standard H.264 (Temporal)

Keyframe (I) Diff (P) Diff (P) Keyframe (I)

Laggy scrubbing: Decoders must recalculate differences between I-frames.

Xora Proxy Recipe (gop: "all_intra")

I-Frame I-Frame I-Frame I-Frame

Frame-accurate scrubbing: every frame is a keyframe, so the decoder never has to rebuild it from earlier frames.

Xora's Editing Proxy Recipe

The proxy recipe always re-encodes, and gives you the three settings that matter for editing.

Keyframe interval

The default is one keyframe per second (gop: "per_second"). Set gop: "all_intra" to make every frame a keyframe (-g 1) for frame-accurate seeking. Files get larger.
Size and quality

Set scaleDown with a height such as 480 or 720, then tune crf (default 32) or a target bitrateKbps. Or keep full resolution.
Clip while you encode

Add startSeconds and durationSeconds to build a proxy of just the section your editor needs. Output is always MP4.

FFmpeg Arguments Decoded

The recipe is a shortcut, not a ceiling. If you want full control, send the FFmpeg args yourself in ffmpeg mode — this is roughly what an all-intra 480p proxy looks like.

-g 1 / -keyint_min 1: Forces every frame to be a keyframe (intra-frame).
-vf "scale=-2:480": Downscales to 480p and keeps the aspect ratio.
-pix_fmt yuv420p: Ensures pixel format compatibility with HTML5 canvas and video elements.
-movflags +faststart: Relocates index metadata to the front of the file.

{
  "mode": "ffmpeg",
  "input_files": { "in_1": "https://example.com/master.mov" },
  "output_files": { "out_1": "proxy.mp4" },
  "ffmpeg": {
    "args": [
      "-i", "{{in_1}}",
      "-vf", "scale=-2:480",
      "-c:v", "libx264",
      "-g", "1",
      "-keyint_min", "1",
      "-pix_fmt", "yuv420p",
      "-crf", "26",
      "-movflags", "+faststart",
      "{{out_1}}"
    ]
  }
}

Trigger a proxy from your upload handler

Call the proxy recipe when an upload lands, and take a webhook when the job reaches a terminal state.

const triggerProxyJob = async (videoUrl) => {
  const response = await fetch('https://api.xora.sh/v1/jobs', {
    method: 'POST',
    headers: {
      'Authorization': 'Bearer YOUR_API_KEY',
      'Content-Type': 'application/json'
    },
    body: JSON.stringify({
      mode: 'recipe',
      input: { url: videoUrl },
      output: { format: 'mp4' },
      recipe: {
        name: 'proxy',
        scaleDown: true,
        height: 480,
        gop: 'all_intra'
      },
      webhookUrl: 'https://your-app.com/webhooks/xora'
    })
  });

  return await response.json();
};

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More on video settings: how to resize video with FFmpeg · web-ready video settings · process user-uploaded video · proxy recipe reference

Frequently asked questions

What is a video editing proxy?

A smaller copy of the original video that an editor uses in place of the master while you cut. It has a lower resolution and more keyframes, so the timeline scrubs smoothly. The final export still uses the original file.

Why do proxies need more keyframes?

To show a frame, a decoder must start from the keyframe before it. With one keyframe every few seconds, each jump on the timeline decodes many frames first. With all-intra proxies (every frame a keyframe) or one keyframe per second, seeking is fast.

How do I generate proxies with an API?

Send the master URL to Xora with the proxy recipe, for example scaleDown true, height 480 and gop all_intra. Add a webhookUrl to know when the proxy is ready, and startSeconds and durationSeconds to make a proxy of one part of the file.

Are all-intra proxies large?

Larger than proxies with one keyframe per second at the same quality. Use all_intra for frame-accurate editing and per_second for review and rough cuts. A lower height and a higher CRF keep the size down.

Edit proxies for browser video editors