A browser-based video crop that re-records the cropped region as a new WebM file cannot preserve most metadata fields and only carries the audio track when the browser exposes a capturable one. The tool draws the requested rectangle into a Canvas, records that Canvas as WebM, and writes a fresh container around the encoded stream, so anything that lives in the source container but outside the picture and the audio — subtitles, chapter markers, attachments, rotation tags, HDR signaling, and most color metadata — is dropped. Audio survives only as far as the browser's media-capture surface allows, which varies by browser and by source codec. Because the output is a re-encoded WebM, not a metadata-pass-through copy, the safest expectation is that the picture stays intact, the sound may or may not survive, and the metadata is rebuilt from a small set of Matroska fields. This article explains exactly which properties carry, which ones drop, and how to verify the result before you delete the source.
What "Preserved" Means When the Tool Re-Records to WebM
A video cropper that runs entirely in the browser and re-records the visible rectangle cannot perform a metadata-pass-through operation. The tool takes the source file, decodes it locally, draws the chosen X, Y, width, and height rectangle into a Canvas, and records that Canvas as a brand new WebM. Whatever lives in the source container beyond the picture and any capturable audio is left behind in the source file. The WebM that comes out is a fresh container with its own Matroska headers, a finite Duration field, and the codec the browser selected from VP9, VP8, or generic WebM. None of the source's sidecar data is copied across.
For the reader asking "will audio and metadata be preserved when I use video crop," the practical answer is that the picture is reconstructed, the audio is included only when the browser allows it, and almost everything else is not. This is a direct consequence of how the tool works: re-recording a Canvas produces a new media file, not a copy with edits.
Audio Tracks: When They Carry Over and When They Drop
Audio handling sits at the top of the preservation question, because sound is the one part of a movie that the cropper can sometimes carry across. The Video Cropper appends the audio to the Canvas stream when the browser exposes an audio track through media capture. That condition is not always met: some browsers expose only video tracks for certain source codecs, and capturable audio support varies by browser and source codec. The contract is clear that support varies, the source may play audibly while the tab is open, and the output must be checked for both sound and picture.
Three practical rules follow. First, do not assume the output has audio just because the source did; verify in the destination player. Second, if the browser exposes only a video track, the output is a video-only WebM with no silent audio bed, so the missing-audio case is normal and not a bug. Third, if audio is critical, keep the original file until the destination player confirms the sound matches the source. None of this is a metadata passthrough — audio survives because the canvas stream is captured alongside a media-capture audio track, not because the source container is being preserved.
Metadata: Subtitles, Chapters, Rotation, HDR, and Container Fields
The non-audio side of "will audio and metadata be preserved when I use video crop" is short and direct: most metadata is not preserved. Subtitles, chapters, attachments, rotation tags, HDR signaling, and most color metadata are dropped during re-recording. The cropper's job is to change the visible frame region and re-encode it; it is not a metadata-safe container editor. The output's Matroska headers contain only what the WebM writer needs to play back: a Segment Info block, a TimestampScale, and a Duration patched in timestamp-scale ticks so the file reports a finite length.
| Source property | What survives a browser-side crop |
|---|---|
| Picture pixels in the chosen rectangle | Re-encoded into WebM (VP9, VP8, or generic) |
| Audio track | Only when the browser exposes a capturable track |
| Subtitles and closed captions | Not preserved |
| Chapters | Not preserved |
| Attachments (fonts, images) | Not preserved |
| Rotation tag | Not preserved |
| HDR signaling | Not preserved |
| Color metadata (primaries, transfer, matrix) | Not preserved |
| Source duration | Replaced by a fresh Matroska Duration in timestamp-scale ticks |
If any of the dropped fields matters downstream, the workflow needs a second step. The Video Compressor, which shares the WebM duration writer with the cropper, will not restore them either, because it also re-encodes the stream. Only a container-level remuxer that reads and rewrites the source without re-encoding the picture would keep that data, and that is not what a browser-side crop is doing.
How to Crop a Video Locally With Video Cropper
The crop step itself is straightforward, and every action runs in the same tab. The full procedure is the audited sequence used by the tool, and it is what produces the WebM described above.
- Choose a browser-decodable video from your device. MP4, WebM, MOV, M4V, and Ogg files are accepted by extension, though decoding depends on the current browser, so an accepted extension does not guarantee codec support.
- Wait for the source dimensions to load. The tool reads the decoded width and height before it offers the crop field. Skipping this wait leads to an invalid rectangle.
- Enter whole-pixel X and Y offsets plus a width and height that stay inside every source frame. Coordinates start at the source frame's top-left corner; X moves the crop right and Y moves it down. Every value must be an untrimmed nonnegative whole pixel count, and width and height must each be at least two pixels. The defaults select the full frame.
- Confirm the rectangle passes the gates: X plus width and Y plus height must remain inside the decoded source, and the output area may not exceed 3840 × 2160 pixels. Invalid rectangles fail without shifting, clipping, rounding, or producing a partial file, so the controls will not let you start a partial job.
- Select Crop video. The source plays locally, only the requested region is drawn into the Canvas, and the Canvas stream is recorded as WebM. Keep the tab open during real-time processing — a one-minute source takes about one minute, and five minutes takes about five minutes.
- When the WebM is offered for download, save it, then play it back in the destination player and confirm duration, crop position, picture, and audio before deleting the source.
Open the Video Cropper to run the procedure on your own file. The same gates protect against partial files, so a failed rectangle is a fail-fast signal, not a corrupted output.
What's Inside the Output WebM After Cropping
The output container is intentionally lean. Because the tool re-records the Canvas stream with the first WebM recorder the browser reports, the codec is whichever of VP9, VP8, or generic WebM the browser returns first. The tool inserts a finite Matroska Duration in big-endian timestamp-scale ticks, parsing Segment Info and TimestampScale and patching Duration in place. The resulting file must remain nonempty and decode with the requested dimensions, and the patched duration is what makes a streaming MediaRecorder output report a finite length rather than an open-ended recording.
Bitrate is set as a disclosed single-pass browser setting rather than a quality guarantee. The request derives from the cropped pixel area at 30 frames per second and is bounded from 180 kbps through 8 Mbps. Fast motion, noise, grain, the source codec, and the browser encoder can change both the file size and the visible quality, so the tool does not promise a target size. This matters for the preservation question because it confirms the output is a fresh encode of the rectangle, not a cut of the source.
| Output property | Value or bound |
|---|---|
| Container | WebM with Matroska Duration in TimestampScale ticks |
| Video codec | VP9, VP8, or generic WebM (first reported by the browser) |
| Frame rate | 30 fps for the recorder |
| Bitrate request | Derived from cropped area, bounded 180 kbps to 8 Mbps |
| Max output area | 3840 × 2160 pixels |
| Audio | Included only when the browser exposes a capturable track |
| Source file budget | Up to 500 MiB, duration above zero and no more than five minutes |
For the reader asking "will audio and metadata be preserved when I use video crop," the upper bound tells the story: the picture is preserved as a re-encoded rectangle, the audio is conditional on the browser, and the metadata is whatever the WebM writer writes — a small set of Matroska fields, not the source's full container.
How to Verify the Cropped File Before Deleting the Source
Verification is the safety net for the whole preservation question. Because the contract is "verify both sound and picture" and "keep the original until the entire output has been reviewed," the right workflow treats the WebM as untrusted until the destination player proves otherwise. Play the file in the same player your audience will use, scrub to a known timestamp, and confirm that the picture matches the requested X, Y, width, and height. Listen for audio that lines up with the picture; if the source had a capturable track and the output is silent, that is a known support case, not a corruption.
Use a short checklist before you delete anything:
- Open the WebM in the destination player and confirm it plays to the end without dropping frames or freezing.
- Scrub to mid-clip and to the last frame, and confirm the crop rectangle is identical throughout. The tool applies one fixed rectangle to every frame and does not track a moving subject.
- Confirm audio. If audio is missing, decide whether the source browser was expected to expose a capturable track; if not, re-record on a browser that does, or use a container-level tool.
- Confirm the duration in the player matches the source. The patched Matroska Duration should report a finite length close to the source.
- Only after all four checks pass should the source be deleted.
For broader pre-crop habits, the pre-crop checklist for common video crop mistakes walks through the same review in more detail. The Matroska Duration patching used by the cropper follows the Matroska container specification, which is why a streaming recording still reports a finite length.
Related reading: Audio, Subtitles, Metadata: What Video Compression Keeps.