How to add an alpha channel to a video? (Sounds simple, doesn't it, but Adobe couldn't answer this.)

[Cheesecake185]

Backstory: We're not talk about CREATING a new alpha channel from scratch the usual way in After Effects.

Instead, the question is this:

Does anyone know how to take a simple black and white channel (for example, the blue channel of RGB) and ADD it as an alpha channel to a video that does not already have an alpha channel?

We consulted Adobe tech support and they had no idea how to do this. We're not experts, but it seems like there should be a solution somewhere.

[ToolfarmJP]

Did you shoot the footage on the green or the blue screen background? If so, you do the keying. If not, you convert it to a monotone (greyscale) movie  to use it as a keying plate.  You apply for Effect menu > Channel > Set Channels. You find Set Alpha To Source 4's in the menu and set the greyscale movie as an alpha. This way may work well.

[Cheesecake185]

Hi ToolfarmJP,

Thanks so much for mentioning this.

However, we already have the exact mask we need as a black and white video. We just need to use one of those channels and import it as an alpha channel into a different video.

Is there a way to do this without keying?

Thanks,
David

[Rick Gerard]

Set Matte will allow you to create an alpha channel from any layer's Red, Green, Blue, Luminance, Hue, Lightness, or Saturation. You can throw in some color correction using anything from basic controls to Lumetri's numerous controls to create an alpha channel.

[Cheesecake185]

Hi Rick,

Thanks so much for posting your solution to my issue.

However, could you please explain how to do this in more detail. We have consulted the Adobe tutorials and watched videos, but our After Effects does not seem to work to achieve what you have written here.

Thanks much,
David

[Rick Gerard]

Set Matte uses the values of the Luminance, Red, Green, Blue, and other channels to create an alpha channel. If your image has areas that have a value of .5 in the red channel, then the alpha channel will be .5, and that makes the transparency of the layer where the red is at .5 50%.

Color values are always 0 to 1 because they are calculated in float (32-bit). If you check the Info Panel and it is set to 8-bit, you will see values from 0 to 255, but if you extract the data or type in a value in a color property, you'll get 0 (no color) to 1 (fully saturated) for each of the RGBA values. 

Set matte effectively turns any of the available channels into transparency. It works perfectly. You can modify the opacity by putting a color control above Set Matte, like Curves, so that you can adjust the look.

The one thing that Set Matte cannot do, that a keyer like Keylight can do, is combine colors to create transparency. You cannot create a matte from a blue background, like a sky, that may have red and green mixed in. Check this recording and note the Info Panel. Pay attention to the transparency (alpha channel values in the Info Panel.

The only part of that image that will be 100% opaque or completely transparent is the part of the image with pixels that have a blue value of 0 or 1.

I am not sure what you are trying to do, but if you want to use red, green, or blue, or even luminance to create transparency that matches those channel values, Set Matte is the tool you want to use.