A noob question. I have two mp4 video files. The first one is ordinary color video (right side).

The second one is transparency mask video (alpha), it's a mp4 video not an image (left side). This second video has gray-scale colors, and it changes frame by frame. This second video has no alpha channel.

enter image description here

What video editor is able to mask the first color video with the second alpha video, so that the first video is rendered with its transparency changing frame by frame according to the second alpha video? And please a short explanation on how to do it?

I prefer a free editor, but if there isn't one, a proprietary editor doesn't matter. Thank you.

  • Are you using a Mac or a PC?
    – tomh
    Jun 9, 2020 at 13:29
  • @tomh I'm using a PC
    – Lopsi
    Jun 10, 2020 at 0:32

2 Answers 2


If you prefer a free editor, Blender could achieve something like this. You need to look at the luma key nodes.

Blender is free but has a steep learning curve. Adobe Premiere could do this quickly using a track matte, but it's not free.

After Effects is also paid-software, and has a steeper learning curve than Premiere, but can also use track mattes - see the other answer for details.

  • Thank you, I'll give Blender a try.
    – Lopsi
    Jun 10, 2020 at 21:24

This would be easy to do with After-Effects. Do the following:

Create a new composition with the settings of the mp4. You can drag it on the composition symbol to automatically create it.

Then, drag your alpha-mp4 above the color-mp4. Now, set the "track matte" to "Luma-Mask" from the alpha-mp4. This should be all.

  • Thank you, I'll try it out once I have the After Effect.
    – Lopsi
    Jun 10, 2020 at 0:35

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