A standard video cropper applies one fixed rectangular region to every frame of a clip, so the crop does not move during the video in the way a pan or zoom would. When you set X, Y, width, and height in source pixels, those four numbers define a single rectangle that the tool draws into a Canvas on every decoded frame from start to finish. Nothing tracks people or objects, nothing follows motion, and nothing animates the rectangle over time. If a subject walks out of that rectangle, the subject is gone from the output. If the camera pans and your rectangle no longer covers what you wanted, the rectangle stays where you placed it. The trade-off is precision: you get exact, repeatable pixel coordinates and a deterministic output that can be re-created from the same four numbers, which is exactly what platforms require when they specify a known frame size. The Video Cropper works this way, applying one rectangle, to every frame, locally in your browser.
That fixed-rectangle model is the foundation of every part of the tool, from how it reads your inputs to how it produces the final WebM. The rest of this page walks through what that means in practice, how to set the four coordinates, the limits the cropper enforces, the bitrate and container choices the tool makes for the output, and what to confirm before you delete the source file.
How a Fixed-Rectangle Crop Is Applied to Every Frame
The cropper decodes your source once, then re-renders it through an HTML5 Canvas element. On each frame the tool calls the Canvas drawImage operation using your four numbers as the source rectangle and a matching destination rectangle, so only the requested pixels are painted. The WHATWG HTML specification at html.spec.whatwg.org/multipage/canvas.html describes how that drawImage call maps a source rectangle to a destination rectangle, and the cropper uses that exact behavior, sizing the destination to match the source so the output is unscaled within the cropped area. The Canvas stream is then handed to a MediaRecorder, which captures the re-drawn frames and writes them into a WebM container.
Because the same four numbers are used for every frame, the crop window cannot drift, jitter, or shift. The capture runs in real time against the playing source video element, so a one-minute source takes roughly one minute to finish while the tab stays open and the system stays awake. If you want a different region, you change the four numbers and run the crop again; there is no in-place editing of the rectangle after recording has started, and there is no per-frame animation of the rectangle once it is set.
Crop a Video in Your Browser with a Fixed Rectangle
- Open the Video Cropper in your browser and choose a file the browser can decode: MP4, WebM, MOV, M4V, or Ogg. Wait for the decoded dimensions to appear before you touch the coordinate fields, since the tool needs to know the source width and height before it can validate your rectangle.
- Read the reported source width and height. Plan X, Y, width, and height so that X plus width stays within the source width and Y plus height stays within the source height. The defaults select the full frame, which is a safe starting point if you are unsure how much to trim.
- Enter whole, untrimmed, nonnegative pixel values for X and Y (offsets from the top-left corner of the frame), then width and height. Each of width and height must be at least 2 pixels, and the output area must not exceed 3840 by 2160 pixels. Worked check on a 1920 by 1080 source: X = 240, Y = 120, width = 1440, height = 840 gives X plus width = 1680 (at most 1920) and Y plus height = 960 (at most 1080), so the rectangle is inside the frame and the crop will start.
- Confirm the rectangle against your source dimensions. If the rectangle is invalid, for example if X plus width exceeds the source width, the tool rejects it without shifting, clipping, rounding, or producing a partial file, and no recording begins.
- Click Crop video. Keep the tab open during real-time processing; the source plays audibly during capture when audio is capturable, and a one-minute source takes about one minute to finish.
- When the WebM download becomes available, save the file and play it back in your destination player. Confirm duration, crop placement, audio behavior, and compatibility with the platform where you intend to publish. Only delete the original after that full review.
Inputs, Limits, and Pixel Rules the Cropper Enforces
The cropper refuses to start on any input or coordinate combination that could produce a broken, oversized, or out-of-bounds file. These rules come from the audited gates the tool shares with the Video Compressor and from the rectangle validator that runs before recording begins, so the same numbers behave the same way across the two tools.
| Aspect | Accepted value or rule |
|---|---|
| File extensions the picker accepts | MP4, WebM, MOV, M4V, Ogg |
| Maximum source file size | 500 MiB |
| Decoded duration range | Above zero, up to five minutes |
| Maximum source side length | 4096 pixels |
| Maximum source area | 3840 by 2160 pixels |
| Maximum output area | 3840 by 2160 pixels |
| Minimum width and height | 2 pixels each |
| Coordinate origin | Top-left of the source frame, in untrimmed whole pixels |
| Coordinate sign | Nonnegative for X, Y, width, and height |
| Invalid rectangle handling | Rejected without shift, clip, rounding, or partial output |
An accepted extension is not the same as guaranteed decoding. The source has to decode in your current browser before the cropper can do anything with it, and codec support varies. If the file fails to load, the cropper will not attempt a recording.
Bitrate, Frame Rate, and Output Container
The cropper captures the Canvas stream at roughly 30 frames per second and asks the browser for a video bitrate that scales with the cropped pixel area. The formula the tool uses places a lower bound at 180 kbps and an upper bound at 8 Mbps, with the target derived from cropped pixels rather than the original frame size. The output container is WebM, using the first MediaRecorder MIME type the browser reports among VP9, VP8, and generic WebM. When the browser exposes a capturable audio track on the source, that track is appended to the Canvas stream so the WebM has sound; if no capturable track is available, the output is silent.
| Output property | How the cropper sets it |
|---|---|
| Container | WebM with a finite Matroska Duration written in TimestampScale ticks, per the IETF Matroska Media Container Format draft |
| Video codec | VP9, VP8, or generic WebM, whichever the browser reports first |
| Capture frame rate | About 30 fps from the Canvas drawImage loop |
| Video bitrate bounds | 180 kbps floor, 8 Mbps ceiling, derived from cropped area |
| Audio | Appended only when the browser exposes a capturable track; otherwise silent |
| Preserved metadata | None: subtitles, chapters, rotation tags, HDR signaling, and color metadata are not carried over |
| File size guarantee | None: fast motion, noise, source codec, and browser encoder affect size and quality |
Because MediaRecorder output can be missing a finite container duration, the cropper patches the file after recording using the same standards-tested WebM duration writer the Video Compressor uses. It parses the Segment Info and TimestampScale, inserts a big-endian Matroska Duration value in timestamp-scale ticks, and verifies that the result is nonempty and decodes with the requested dimensions. Object URLs, timers, recorder tracks, and playback are released on every exit path so a failed or replaced run does not leak resources.
When a Fixed Crop Solves the Task (and When It Does Not)
A fixed-rectangle crop is the right shape of tool when your edges are static and your region is known. Clips with a logo bar at the bottom, a stray UI element in a corner, a fixed letterbox, or a known black border are exactly the case where pixel-exact coordinates pay off, since you measure once, set the four numbers, and every frame loses the same pixels. The cropper is also a clean fit when a destination platform specifies a frame size, such as short-form vertical feeds, banners, and similar layouts that want a known region.
A fixed crop is the wrong tool when the subject moves and the crop has to follow. The cropper does not track people, faces, or objects and does not keyframe the rectangle over time, so if the rectangle you set does not cover the subject for the whole clip, the subject leaves the output. For a moving subject you would need a tracker or an auto-reframe tool that updates the rectangle frame by frame, and the Video Cropper is not that tool. The cropper also is not a trim-by-time tool, a resize tool, a compress-only tool, a redaction tool, a blur tool, or an object-removal tool, and using it for those jobs will give you the wrong kind of file.
Verify the Output Before Deleting the Source
Treat the WebM as a candidate, not a finished file, until you have reviewed it end to end. Open it in the player where it will actually ship and confirm that the duration matches what you expected, the crop rectangle sits where you intended across the whole clip, audio plays back when the source had sound and is silent when it did not, and the codec plays cleanly without dropped frames. The cropper uses the W3C MediaStream Recording and Media Capture from DOM Elements paths for capture and the Matroska container for storage, so any player that supports those specs is the right place to confirm.
Only after that review should you delete the original. Until then, keep the source so you can re-run the crop with adjusted coordinates if any of those checks fail.