Short answer: ordinary H.264 MP4 cannot store an alpha channel. Use WebM (VP9 alpha) for the web and OBS, ProRes 4444 for editing software, and HEVC with alpha for Apple devices.

Why H.264 MP4 has no alpha

A video stores colour for every pixel. Transparency needs a fourth value per pixel—the alpha channel—saying how opaque that pixel is. The H.264 profiles used in everyday MP4 files have no place for it, so an exporter must fill the empty area with something: usually black, sometimes green or white.

That fill is baked into the pixels. No player or converter can turn it back into transparency, because the information about which pixels were empty is gone.

Formats that do keep alpha

  • WebM with VP9 alpha — small files for websites and OBS. Plays transparently in Chrome, Firefox and Edge.
  • ProRes 4444 (MOV) — large editing masters for Premiere Pro, After Effects, DaVinci Resolve and Final Cut Pro.
  • HEVC with alpha (MOV/MP4) — Apple’s format for Safari and iOS, encoded with Apple’s tools.
  • PNG sequence — one image per frame, for game engines and animation tools.

Note the last one: an MP4 container can hold HEVC with alpha, but only when it was encoded that way. Renaming a file never adds transparency.

How to check whether a file really has alpha

With FFmpeg, look at the pixel format and, for WebM, the alpha tag:

ffprobe -v error -show_entries stream=codec_name,pix_fmt:stream_tags=alpha_mode -of compact input.webm

ProRes with alpha reports a pixel format starting with yuva. A transparent WebM is trickier: FFprobe shows its pixel format as plain yuv420p, because the alpha is stored separately—the sign to look for is the tag ALPHA_MODE=1. Then look at a frame: decoding a WebM needs -c:v libvpx-vp9 before -i, or FFmpeg’s default decoder silently drops the alpha.

ffmpeg -c:v libvpx-vp9 -i input.webm -vf alphaextract -frames:v 1 alpha.png

In alpha.png, white is opaque and black is transparent. A completely white image means there is no usable transparency.

Already have a black-background MP4?

Then the transparency was lost at export. Either export again from the original project as WebM or ProRes 4444, or remove the background from the footage with AI: our video background remover produces a transparent WebM, and a free ProRes 4444 export for editors.