The video alpha channel, explained simply
Behind every transparent video there's an alpha channel. Knowing what it holds saves you from most black-background, fringe and format headaches.
What is an alpha channel?
The alpha channel is a fourth value added to each pixel's three colors (red, green, blue). It says how opaque the pixel is, from fully visible to fully transparent.
An image with alpha is RGBA. Without alpha, every pixel is opaque and the empty area has to be filled with some color. In video, alpha changes every frame, just like the colors: it follows the subject as it moves.
Why does partial transparency matter?
Because real edges are never razor sharp: hair, motion blur, soft outlines. Gradual alpha lets a little of the new background through those edge pixels, which makes the composite look natural.
GIF only has on-or-off transparency, hence its jagged edges. See transparent GIFs.
What's the difference between premultiplied and straight alpha?
With straight alpha, colors are stored as is; with premultiplied alpha, they're already multiplied by the transparency. Both work, as long as the app reading the file knows which one it's getting.
When it doesn't, a dark or light fringe appears around the subject. Most editing apps let you change this in the clip properties.
Which video formats keep the alpha channel?
The common ones are ProRes 4444, VP9 WebM with alpha, HEVC with alpha on Apple devices, and PNG sequences. H.264 MP4 never keeps it.
How do you view the alpha channel in an editing app?
Most apps have an alpha view: the subject shows white, the empty area black, the edges grey. It's the best way to judge a key.
In Premiere Pro, Ultra Key's Alpha Channel output shows it; in DaVinci Resolve, the matte view on the Color page. Grey specks in the black mean leftover green, holes in the white mean an eaten-away subject. See Premiere Pro.
How does the alpha channel relate to green screen?
A green background is another way to carry the same information: instead of alpha stored in the file, a flat color marks where the empty area is. Your editor rebuilds the alpha by removing that color.
That's why Keyotte exports an MP4 on flat green #00B140: the file opens everywhere, and alpha is rebuilt in the edit. See transparent background videos and how AI background removal works.
Export price : €1.99
Remove my video backgroundFrequently asked questions
Does an alpha channel make video bigger?
Yes, it adds a value per pixel. ProRes 4444 and PNG sequences are heavy; WebM with alpha stays light.
Can I add an alpha channel to an MP4?
Not in H.264. You have to re-encode to a format that supports it.
Is a black-and-white matte an alpha channel?
It's the same information, stored as a separate video: white for the subject, black for empty. Compositing apps can use it as alpha.
How do I check whether a video has alpha?
Put it in your editor above a colored image: if the image shows through the background, the alpha is there.