How to remove black background in video

Dear experts,

I created a transparent video.
(ticked transparent in Film, Format - ffmpeg, Encoding - Quicktime, Codec QT QT)
After rendering I got a black background.

Thank you for any suggestions.

Afaik there are no movie formats with alpha so the output turned black. Instead you can save images sequence and convert them to gif if its only small animations.
The transparent film option is for use with the compositor for mixing.

What might work if you render a 2nd black white mask movie version and use it as mask in for your video editing. You can use the alpha channel for that in the compositor and output it to the composite node. That will give an black / white movie to use as mask.

The QTRLE (also know as Quicktime Animation) codec supports transparency. Make sure that you set the Color in the Output panel to be RGBA after you choose that codec.

Also, not all programs that play video will recognize the alpha channel, so they may often render black. I tend to check by loading the video back in Blender using either the Sequencer or the Movie Clip Editor. But you can also use DJV or Mr. Viewer to check as well.

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Add one-by-one in Sequencer? Is the length of each image 1 frame?

Yes, one image is one frame.
You dont need to add one-by-one, you can select all the images at once with shift first and last.