Question / Help Add Chroma Key to Finished Video?


New Member
I assume this is not possible but figured I'd try to find out anyway. I forgot to add a chroma key to my video game stream. So my camera shows a green background. is there a way in OBS to after the fact edit the MP4 and still make it transparent? I tried but it turned it into a black background when adding a chroma key filter post production.


Active Member
In OBS, your video sources are layered over each other. By using chroma key, you are making the green background transparent, so you see the layer that's "behind" it - that's some other video source.

In your finished mp4, there is nothing layered. OBS renders the frames to output by merging (or "compositing") all layers, then outputs it to stream or recording. This is because no video format or streaming protocol supports such kind of layered video. It would be a waste of space and bandwidth, because usually the consumer will never make use of overlaid and obscured layers.
So if you make some part of your mp4 transparent, you will see what's behind it - it's the black nothingness, since there is no additional layer any more.

If your recording is important to you, you can create an image (or even a video clip) that is better than the plain green or plain black. You can patch this in. Put both your mp4 and this image or clip into OBS as source and put your mp4 on top of the image/clip (the source should appear above the image/clip in the list of sources in OBS). If you now apply the chroma key to your mp4, you will see your image/clip shine through instead of the black nothingness.
Usually, you will make this kind of postprocessing with a video editor, not with OBS, but for such a small change it may be sufficient and overkill to grab a real video editor software.