FastVideoPreview

ComfyUI-FastVideoPreview

A fast, disk-free video preview node for ComfyUI. Connect a batch of frames or a video, press Run, and a small looping preview plays right on the node. Nothing is saved to your output or temp folders.

Credit where it is due: thank you, Kijai

This node is a tribute to the Fast Preview node in ComfyUI-KJNodes by kijai. His idea — encode the preview in memory and push it to the browser through ComfyUI's own binary websocket channel, skipping the disk and base64/JSON overhead — is what this project is built on. I read how Fast Preview works and followed the same method, but for video instead of a single still image. If you like this node, please go and star KJNodes: it has hundreds of nodes and a huge amount of care behind it.

What it does

• Plays a looping, muted <video> preview directly on the node (with play/seek controls).
• Works with an images batch (e.g. VAE Decode output) or a video input (e.g. Load Video).
• Shrinks and encodes in RAM (H.264 ultrafast, WebM fallback), so previews are quick and light.
• Streams large video inputs one frame at a time, so even a 4K clip does not fill your RAM.
• Writes nothing to disk.

Install

Option 1 — git (recommended)

cd ComfyUI/custom_nodes
git clone https://github.com/VRH7733/ComfyUI-FastVideoPreview

Option 2 — manual Download the repo as a ZIP, extract it, and put the ComfyUI-FastVideoPreview folder into ComfyUI/custom_nodes/. The folder must directly contain __init__.py (watch out for a doubled ComfyUI-FastVideoPreview/ComfyUI-FastVideoPreview folder after extracting).

Option 3 — ComfyUI-Manager / Comfy Registry Search for FastVideoPreview in the Manager once the node is published to the registry.

Then restart ComfyUI and hard-refresh the browser (Ctrl+F5). Requires PyAV (av), which current ComfyUI already includes. If it is missing: pip install av.

Use it

1. Double-click the canvas, search Fast Video Preview (category video/preview), and add it.
2. Connect images (for example from VAE Decode) or video (for example from Load Video).
3. Run the workflow. The preview appears on the node.

Example workflows are in examples/:

• fast_video_preview_images.json — Empty Image → Fast Video Preview (no model or GPU needed)
• fast_video_preview_video.json — Load Video → Fast Video Preview (pick your own clip in Load Video)

Inputs

| Input | Meaning | |---|---| | images | Frame batch (IMAGE). Optional if video is connected. | | video | VIDEO input. It keeps its own fps. Used only when images is empty. | | fps | Playback speed for images. | | max_size | Longest side of the preview in pixels. Smaller is faster. | | quality | 1–100. | | max_frames | 0 keeps every frame; otherwise frames are evenly sub-sampled to this count. |

How it works

1. Frames are downscaled and encoded in memory (H.264 via PyAV, WebM fallback).
2. The bytes are sent through ComfyUI's built-in binary websocket preview channel (PREVIEW_IMAGE_WITH_METADATA) — the same channel KJNodes' Fast Preview uses for images. A tiny stand-in object lets the already-encoded clip pass through the stock image sender unchanged.
3. A small JavaScript extension (web/fast_video_preview.js) receives the blob (b_preview_with_metadata), matches it to the node by id, re-types it as video by looking at its bytes, and plays it.

Good to know

• Previews live only in the browser tab; reloading the page clears them. Run the node again to bring one back.
• Very long video inputs are sub-sampled automatically to stay under a memory cap (about 800 MB).
• Hosted/cloud ComfyUI (Comfy Cloud, RunningHub, and similar) generally only offers nodes that the provider has pre-installed, so this node will not appear there until the provider adds it.
• Tested with ComfyUI 0.43.1 and frontend 1.57.4. Newer or older frontends may behave differently — please open an issue.

Troubleshooting

| Symptom | Fix | |---|---| | Node missing from the list | The folder is nested one level too deep, or ComfyUI was not restarted. Check the startup log's Import times for custom nodes. | | Blank player | Check the terminal for a traceback and the browser console (F12) for lines containing FastVideoPreview. | | No module named 'av' | pip install av in ComfyUI's Python environment. | | Out-of-memory with a huge video | Lower max_size or set max_frames (for example 48). |

Licence

GPL-3.0, the same licence as ComfyUI-KJNodes, because this node follows the structure and method of KJNodes' Fast Preview. See LICENSE.

Acknowledgements

• kijai — ComfyUI-KJNodes, the Fast Preview method this project is based on.
• The ComfyUI team — for the open, hackable preview channel this builds on.