加载透明视频 / Load Video (Alpha)

You generated a gorgeous transparent video - a walking character, a fire overlay, an intro card with a real alpha channel - and the moment you load it into ComfyUI, the transparency is just… gone. Every frame comes back as a solid rectangle on black. You're not doing anything wrong - the loader is the problem.

This node, 加载透明视频 / Load Video (Alpha) from the Rui-Node pack, fixes exactly that one infuriating case. It's what you reach for when you need RGBA frames out of a video - sprite animation frames, composited overlays, effects to drop onto another shot.

Why the alpha vanishes in the first place

Here's the trap, and it's sneaky. A WebM with transparency (VP8 or VP9) does not store alpha in the main video stream. The main stream is still plain yuv420p; the alpha gets squeezed into a second track, buried in Matroska's BlockAdditional sideband, with a single alpha_mode=1 flag on the container as a hint that it exists.

ffmpeg's built-in vp9 / vp8 decoders don't read that sideband at all. Only the libvpx-vp9 / libvpx family does. The common video loaders (VideoHelperSuite and friends) use the default decoder, so they happily decode every frame and hand you alpha that's pinned at 255. It's a silent failure - nothing errors, nothing looks broken, the transparency just isn't there. The Rui-Node README measured all three paths on the same file: PyAV default and ffmpeg default both lose alpha completely; explicit libvpx-vp9 recovers it (87% fully transparent pixels intact).

So what this node actually does is probe the file - via PyAV metadata when available, falling back to parsing ffmpeg -i output - and when it sees alpha_mode=1 or an alpha-bearing pixel format, it switches to a libvpx-family decoder and reads raw RGBA pixels out of the pipe. Same story for MOV/qtrle and ProRes 4444, where the pixel format itself carries alpha.

The inputs that matter

• video - pick from ComfyUI's input directory (you can also drag-upload). Or fill in 视频路径 with an absolute path and it wins over the dropdown.
• 解码器 (decoder) - leave on 自动 (auto) and it swaps to libvpx only when it detects alpha. If auto still returns opaque frames, flip to 强制 libvpx (force libvpx) and you'll almost certainly get your alpha back.
• 强制帧率 (force fps), 帧数上限 (max frames), 跳过前N帧 (skip first N), 间隔 (every N-th frame) - the usual decimation controls. The author's advice for game sprite work: keep force fps at 0. 自定义宽度/高度 resizes; setting just one side keeps the aspect ratio (leave at 0 for pixel-art, since resizing interpolates and blurs hard edges).

What comes out

• rgba_image - 4-channel frames. Wire this straight into ComfyUI's native Save Image node: it detects the 4 channels and writes RGBA PNGs, alpha intact. No extra save node needed.
• alpha - the transparency as a MASK, handy for previewing or feeding a compositor.
• rgb_image - a 3-channel version for downstream nodes that choke on 4 channels.
• 帧数 and 帧率 - actual frame count and output rate (fps divides by the interval if you're skipping frames).

Installing it

ComfyUI Manager, search RUI-Nodes, install, restart. Or the manual route:

cd ComfyUI/custom_nodes
git clone https://github.com/rui40000/RUI-Nodes
pip install -r requirements.txt

That requirements.txt pulls torch, numpy, Pillow, requests, plus SDMatte's heavier deps (diffusers, transformers, scipy, opencv) - overkill for this one node, but they're all typical ComfyUI env packages. The only real runtime need here is an ffmpeg: it finds one via imageio-ffmpeg, your system PATH, or the copy VideoHelperSuite ships with. If you get a "找不到 ffmpeg" error, pip install imageio-ffmpeg fixes it.

Common gotchas

• Preview shows black. That's not a bug. ComfyUI's preview pane doesn't render transparency, so a black background is normal. Verify with the alpha output or by checking the saved PNG.
• Console prints "alpha 全为不透明" (all opaque). The detector didn't see alpha_mode, or your file really has no alpha. Switch to 强制 libvpx (force libvpx) and retry.
• Video preview gets cropped in the UI. If the node's inline preview only shows part of the frame, that's a known CSS clash with the comfyui-art-venture plugin, not this node. The pack ships a front-end fix - restart the ComfyUI backend and hard-refresh the browser to pick it up.

It's a one-job node, but it's the right one. Core ComfyUI only gained webm alpha saving in v0.25.0 (mid-2026), so the loading half was the broken half for a long time. This closes the loop: transparent video in, RGBA PNGs out.