Export ProRes 4444 with an alpha channel

ProRes 4444 is the format editors ask for when a clip has to carry transparency onto a timeline. Transparency Render can produce one in the browser — no Premiere licence, no command-line ffmpeg, no install.

What the fourth 4 is for

The digits describe the sampling: 4:4:4 means full colour resolution on every channel, with no chroma subsampling to smear the edges. The fourth 4 is the alpha channel — the reason to choose this codec over ProRes 422. Combined with 10-bit depth, it survives repeated re-encoding through a grade or a compositing chain far better than a delivery codec, which is exactly what an intermediate is for.

Bringing it into an editor

The MOV imports directly into After Effects, Premiere Pro, DaVinci Resolve and Final Cut Pro, and the transparency is recognised without any extra step. If a layer appears with a black background instead, the interpretation is wrong rather than the file: check that the footage is set to straight (unmatted) alpha, not premultiplied. Transparency Render writes straight alpha, and guessing wrong is what produces dark fringing around the edges of a subject.

Size is the catch — plan for it

ProRes has no rate control. Unlike H.264 or VP9, it does not target a bitrate and spend fewer bits on simple frames; every frame costs roughly the same, so the file size follows resolution and duration far more than it follows the picture. At 1080p with alpha that is around 6 MB per second — a few hundred megabytes a minute, and up to half again more from busy or grainy footage.

Those limits are the browser's, not the codec's. The finished file has to be assembled in memory before it can be handed to you, and a tab cannot hold several gigabytes to do it. The app estimates the size as you change the settings, and watches the real size as the encode runs — an export that is genuinely heading past what the browser can assemble is stopped in the first few seconds rather than stalling part-way through a long one.

Getting the most out of the quality setting

The quality slider is a qscale, and lower means better: 2 is close to lossless, 9 is a sensible default, and 20 is visibly soft. It buys less room than you would expect — the whole slider only spans about 1.9× in file size, because a fixed quantizer has little left to discard once the source has already been through H.264. Trim to the exact frames you need instead: the in and out points cost nothing and save far more than any quality setting.

Starting from green screen footage

ProRes 4444 preserves whatever alpha you give it, so the key has to be right before you export. Get the edges clean first — how to remove a green screen cleanly covers similarity, edge blend and despill — then pick the ProRes preset and export.