Convert WMV to Y4M — Free Online Tool
Convert WMV files to Y4M (YUV4MPEG2) format directly in your browser — decoding Microsoft's proprietary MPEG-4-based video stream into fully uncompressed raw frames suitable for lossless video processing pipelines. Y4M is the standard intermediate format used by tools like FFmpeg, x264, and AviSynth when maximum frame fidelity is required.
to
FFmpeg Command
Copy this command to run the same conversion locally with FFmpeg on your desktop. Download FFmpeg
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Settings
Note: Browser-based encoding uses approximate quality targets. For precise CRF compression, copy the FFmpeg command above and run it on your desktop.
Estimated output:
Conversion Complete!
DownloadHow It Works
WMV files store video encoded with Microsoft's MPEG-4 variants (msmpeg4 or msmpeg4v2) inside an Advanced Systems Format (ASF) container. During this conversion, FFmpeg fully decodes every compressed WMV video frame — reversing the lossy DCT-based compression — and writes each frame as raw YUV pixel data into the Y4M container. Y4M adds only a minimal header per frame to carry metadata like resolution, frame rate, and colorspace (typically YUV 4:2:0), but stores no compressed data whatsoever. Because Y4M holds no audio stream, any WMV audio track is dropped. The result is a dramatically larger file that represents each frame at full decoded quality, with no further generation loss introduced by this tool.
What Each Flag Does
| Flag | What it does |
|---|---|
ffmpeg
|
Invokes the FFmpeg command-line tool. In the browser version of this tool, this runs via FFmpeg.wasm compiled to WebAssembly, executing the same decoding and muxing logic as the native desktop binary. |
-i input.wmv
|
Specifies the input WMV file. FFmpeg automatically detects the ASF container and identifies the enclosed video codec (msmpeg4 or msmpeg4v2) and audio codec (typically wmav2) from the file's stream headers. |
-c:v rawvideo
|
Instructs FFmpeg to write the video stream as rawvideo — meaning each decoded YUV frame from the WMV source is written directly to the output with no re-compression. This is the only video codec Y4M supports, as the format is designed exclusively for uncompressed frame storage. |
output.y4m
|
Specifies the output file with the .y4m extension. FFmpeg recognizes this extension and automatically applies the YUV4MPEG2 muxer, which writes the Y4M stream header (encoding resolution, frame rate, colorspace, and interlacing) followed by per-frame markers and raw pixel data. |
Common Use Cases
- Feed decoded WMV footage into a lossless encoding pipeline using x264 or x265 via stdin piping, avoiding intermediate quality loss between decode and re-encode stages
- Analyze individual video frames from legacy WMV archives at the pixel level using frame analysis tools like VirtualDub or custom scripts that require raw YUV input
- Use WMV source footage as input for video quality metric tools (VMAF, SSIM, PSNR) that require uncompressed Y4M streams for accurate reference or distorted-stream comparison
- Preprocess old WMV training or archival videos into Y4M for ingestion into machine learning pipelines or video processing frameworks that consume raw frame data
- Strip away the ASF container and Microsoft codec artifacts to obtain the purest possible decoded representation of WMV content before applying a professional re-encode with a modern codec like AV1 or HEVC
Frequently Asked Questions
The Y4M output is a lossless representation of the decoded WMV frames — meaning no additional quality loss is introduced by this conversion. However, the WMV source itself was encoded with lossy compression (Microsoft's MPEG-4 variant), so any compression artifacts already present in the WMV are preserved in the Y4M output. Y4M simply captures exactly what the WMV decoder produced, with no further degradation.
WMV stores video using lossy DCT-based compression, which can achieve compression ratios of 50:1 or higher. Y4M stores every pixel of every frame as raw, uncompressed YUV data — a 1920x1080 video at 30fps requires roughly 2.8 GB per minute in Y4M format. The size difference is not a problem with the conversion; it reflects the fundamental difference between a compressed format and an uncompressed intermediate format.
No. The Y4M (YUV4MPEG2) format does not support audio streams — it is a video-only container designed exclusively for raw frame transport. Any audio encoded in the WMV file (typically WMA v2 or AAC) is discarded during conversion. If you need the audio separately, you should extract it in a separate FFmpeg operation before or after this conversion.
Yes — this is one of Y4M's primary use cases. You can replace the output filename with a pipe and chain it to another encoder: ffmpeg -i input.wmv -c:v rawvideo -f yuv4mpegpipe - | x264 --demuxer y4m -o output.mp4 -. The -f yuv4mpegpipe flag explicitly tells FFmpeg to write Y4M-formatted data to stdout, and x264's --demuxer y4m flag reads it on the other end, enabling a high-quality transcode of WMV content without writing a temporary intermediate file to disk.
No. Y4M supports only a narrow set of per-stream technical metadata in its header — frame rate, colorspace, interlacing, and pixel aspect ratio. ASF container metadata fields such as title, author, copyright, and description that may be embedded in the WMV file are not carried over, as the Y4M format has no mechanism to store them.
On Linux or macOS, you can loop over files in a directory: for f in *.wmv; do ffmpeg -i "$f" -c:v rawvideo "${f%.wmv}.y4m"; done. On Windows Command Prompt, use: for %f in (*.wmv) do ffmpeg -i "%f" -c:v rawvideo "%~nf.y4m". Be mindful that Y4M files are very large — batch converting multiple WMV files will rapidly consume disk space, so ensure you have sufficient storage before running the loop.
Technical Notes
The WMV-to-Y4M conversion involves decoding Microsoft's proprietary msmpeg4 or msmpeg4v2 codec — MPEG-4 Part 2 variants that predate the ISO standard and are not interchangeable with standard MPEG-4 ASP or H.264. FFmpeg's native decoders handle both variants reliably. The output colorspace defaults to yuv420p, matching the subsampling used by virtually all WMV source material, which means chroma channels are stored at half the luma resolution in both dimensions. Y4M does not support transparency, subtitles, chapters, or multiple audio tracks — none of which WMV commonly uses anyway, except for its optional DRM layer, which FFmpeg cannot strip (DRM-protected WMV files will fail to decode). Interlaced WMV content will be preserved as interlaced frames in Y4M with appropriate ILACE header flags, but downstream tools should be configured to handle interlacing explicitly. Because Y4M files can be several gigabytes even for short clips, browser-based processing of WMV files longer than a few minutes may be memory-constrained; the desktop FFmpeg command is strongly recommended for longer source files.