Video resize troubleshooting means working backward from a broken or unexpected result to identify which control, file property, or step caused it. When a resize goes wrong, the failure usually traces back to one of three things: a source file the browser cannot decode, target dimensions that fall outside what the tool accepts, or a choice between Fit and Stretch that produces a picture you didn't expect. The Video Resizer does its work in the current browser tab using built-in browser codecs, so every step is observable: the file is read locally, frames are drawn onto a resized canvas, and the recording is saved as a WebM file. That visibility makes diagnosing most resize problems straightforward once you know which knob to turn. The troubleshooting path is short: confirm the source is supported, confirm the target dimensions are within the allowed range, pick the right geometry mode, and then keep the tab open while the encoder runs before downloading the result.
What Kind of Problems Show Up During a Video Resize
Symptoms fall into a few predictable groups, and matching the symptom to the group is half the fix.
- The output file never appears, or the downloaded file is empty or unusably short.
- The dimensions of the result don't match the numbers you typed.
- The picture looks squashed, stretched, or shows unexpected black bars.
- Audio is missing, out of sync, or sounds distorted after resize.
- The source file is rejected before processing even starts.
- The encoder reports an unsupported codec, missing track, or invalid dimension.
Each symptom points to a different layer of the pipeline: source validation, geometry calculation, canvas drawing, audio attachment, or recording. Once you group the symptom, the fix is usually obvious from the limits in the section below.
Why Resize Problems Happen in the First Place
Most resize failures come down to a small set of root causes.
- Source codec mismatch. A file whose video or audio codec isn't built into the browser will be rejected outright. Codec support varies even within common containers like MP4 and MOV, so two files with the same extension can behave very differently.
- Source size or length over the limit. Files above 500 MiB, longer than five minutes, wider than 4096 pixels per side, or larger than 3840 × 2160 pixels in total area are refused before encoding starts.
- Target dimensions outside the allowed range. Each target must be a whole number from 2 to 1920 pixels. Anything outside that window is invalid and will fail visibly rather than producing a misleading download.
- Wrong geometry mode for the goal. Fit never crops and never distorts; Stretch always honors both numbers and can intentionally distort the picture. Picking the wrong one for the intended use is the most common cause of "it looks wrong" complaints. A side-by-side comparison of Fit and Stretch can clarify which mode matches your intent before you run the encode.
- Tab closed during recording. Because the encoder runs in real time in the browser tab, interrupts the real-time encode before a usable download is produced.
Confirm the Source File Is Resize-Ready
Before touching the dimension fields, run the source against the limits the tool enforces. The supported containers are MP4, WebM, MOV, M4V, and Ogg. The source must be at most 500 MiB and five minutes long, with no side greater than 4096 pixels and no frame larger than 3840 × 2160 pixels in area. The browser reads metadata locally before any resize starts, so most limit problems show up as a clear refusal rather than a corrupt file later.
| Limit | Allowed value |
|---|---|
| Supported containers | MP4, WebM, MOV, M4V, Ogg |
| Maximum file size | 500 MiB |
| Maximum duration | 5 minutes |
| Maximum source side | 4096 pixels |
| Maximum source area | 3840 × 2160 pixels |
| Target width range | 2 to 1920 pixels, whole number |
| Target height range | 2 to 1920 pixels, whole number |
| Odd target values | Rounded down to the nearest even number |
If the source is within these limits, the next variable to check is the geometry mode and whether the target dimensions are reasonable for the source ratio.
Resize the Video Step by Step
Once the source passes pre-flight, the actual resize is three short steps. Follow them in order and keep the tab open until the download appears.
- Choose one supported local video file and wait for the browser to read its metadata. The status text should confirm the source dimensions before you continue, because the metadata is what the geometry calculation uses.
- Enter a maximum width and a maximum height. Choose Fit to preserve the source aspect ratio inside those dimensions, or Stretch to use the exact width and height independently. Fit treats the numbers as a bounding box; Stretch uses them as the final pixel dimensions.
- Select Resize video and keep the tab open while the encoder draws frames onto a resized canvas at up to 30 fps and records a WebM. When the download appears, save it before closing the tab.
That real-time re-encode is the source of another common confusion: the encode takes about as long as the video plays. A five-minute source takes roughly five minutes of wall-clock time. This is a re-encode, not a metadata edit, so the result is a fresh WebM with new bitrate, frame timing, and color handling — see the MDN reference for canvas captureStream for the underlying capture mechanism the tool relies on.
Read the Result and Spot What Looks Wrong
After the WebM downloads, run a quick sanity pass before assuming the resize failed.
- Check the exact pixel size. Open the file in any player that reports properties, and confirm the width and height match what you asked for, with both values even numbers. If the numbers are one pixel lower than what you typed, that is the documented odd-dimension rounding, not a bug.
- Watch for distortion. If faces look unnaturally tall or wide, the geometry mode was probably wrong. Stretch produces exact dimensions regardless of the source ratio, while Fit scales the picture until the longer side fits the box and leaves letterbox padding on the shorter side.
- Listen for audio. Audio is only attached when the browser can expose one. If the source had multiple tracks or uncommon audio codecs, the WebM may be silent. That isn't a resize bug — it's a track that the browser couldn't expose.
- Compare file size and duration. Because the output is a real-time re-encode at a bounded bitrate, the file size and duration will differ from the source.
When the result still looks wrong after this pass, a follow-up read on fixing a video result that looks wrong after resize walks through the same checks with more detail.
When a Local Browser Resize Isn't the Right Tool
Some resize problems aren't really problems — they're signals that the task has moved past what a local browser re-encode is designed to do. Know these limits before you spend time troubleshooting.
- The output is always WebM, never MP4. If the delivery format must be MP4, a desktop encoder is the appropriate tool, not a local browser re-encode.
- Quality, bitrate, frame timing, audio layout, color metadata, and file size are all decided by the browser's built-in VP9 or VP8 encoder. There is no manual bitrate slider and no promise of matching the source quality.
- The tool does not upscale detail, remove black bars, crop a subject, preserve subtitles or multiple audio tracks, or bypass DRM. Use only media you own or have permission to edit.
- Frame-accurate delivery, long-form footage, color-managed workflows, and exact bitrate control all sit outside the tool's design. In those cases the right troubleshooting move is to switch tools, not to retry the same resize.
The MediaRecorder API that the tool relies on selects whichever VP9 or VP8 WebM configuration the browser supports, which is why the same source can produce slightly different output across browsers. That variability is documented in the MDN reference for MediaRecorder and is the most common reason "it works on my friend's machine" doesn't mean the resize itself is broken.