Why the video looks grey
Recent iPhones and Android flagships record HDR by default: 10-bit video with the PQ or HLG transfer curve, usually tagged bt2020. On the phone that shot it the picture is bright and punchy. Open the same file on a desktop, drop it into an editor, or post it somewhere that doesn't understand the tags, and it goes grey, flat and slightly washed — the greens turn sickly, faces go pale, and the contrast collapses.
Nothing is wrong with the file. HDR stores brightness on a completely different scale from SDR, and the tags on the file are what tell a player which scale to use. A player that ignores those tags reads HDR values as if they were ordinary SDR ones. The numbers that were meant to describe a bright highlight get interpreted as mid-grey, and the whole image flattens out.
The fix is to actually convert the brightness scale — not to re-tag the file and hope, and not to crush everything above a threshold to white. That conversion is called tone-mapping.
What tone-mapping actually does
An HDR source can describe brightness far beyond what an SDR screen can show. Tone-mapping decides how to fit that larger range into the smaller one — which highlights to roll off gently, and how much, so the result keeps its contrast instead of turning into a grey smear or a blown-out mess.
This converter does it with a proper tone-mapping curve rather than a straight clamp. The video is linearised, converted to floating-point, run through the Hable curve, then re-encoded as bt709 SDR and explicitly tagged as such, so the next player along has no reason to guess. Saturation is left alone through the curve, because desaturating on top of a tone-map is what gives tone-mapped footage that lifeless look.
The explicit re-tagging matters as much as the curve. A lot of converters tone-map the picture and then write the file with the original HDR tags still attached, or with no colour tags at all. Either way the next player is back to guessing, and you can end up with a file that looks wrong in a different direction.
When it happens automatically
You don't have to find a setting. The converter probes the source's colour primaries, transfer curve, matrix and range before it starts. If it finds an HDR transfer curve — PQ (smpte2084) or HLG (arib-std-b67) — and the output you've chosen is SDR, which MP4/H.264 and WebM both are, it inserts the tone-mapping chain on its own.
If the source is already SDR, nothing is tone-mapped. The converter preserves whatever colour tags were there and re-states them on the output, which is the other half of not silently shifting colour: an SDR file should come out looking identical, not "corrected".
What this is useful for
- iPhone or Pixel clips that look fine on the phone and grey on a laptop.
- Footage headed into an editor that doesn't handle HDR, where it would otherwise need a colour pass to look normal.
- Video for a platform that strips or ignores colour metadata, so the safest thing to hand it is a correctly-tagged SDR file.
- Anything that has to look the same everywhere — SDR is the format with no ambiguity left in it.
The honest caveats
Tone-mapping is lossy in a way that has nothing to do with compression. You are throwing away dynamic range the SDR file cannot hold, and there is no setting that avoids that — only choices about where the loss shows. Keep the original if the footage matters; this is for producing a version that plays correctly, not a replacement.
Tone-mapping also needs a build of ffmpeg with the zscale filter, which this site ships. If a fallback core loads for any reason, the converter tells you so rather than quietly producing a file with the colours mangled — a warning on the job instead of a bad download.
And a tone-mapped HDR source will never quite match the original on a real HDR display. That is the nature of the conversion, not a shortcoming of this particular one.
How it runs
The whole thing happens on your own device, in the browser tab, using a WebAssembly build of ffmpeg. There is no upload step, so there is no queue, no file size ceiling imposed by a server, and nothing about the video is transmitted anywhere. For phone footage — which is often the most personal thing anyone converts — that's the part that matters.