Using alpha/luma of one video to create mask for another

I can't seem to find this anywhere, but surely it can be done? Essentially I have some videos that are black and white and include an Alpha channel. For example, ink spills or silhouettes. I want to mask another video or image with the output of the alpha channel or could be luma key I guess since the videos are black and white?

I am comfortable with all the standard masking in Fusion, but can't seem to get this. Is it possible? How is this done?

I think if you use a bitmap mask on the footage that you want to 'cut out' and use the alpha channel of your video a the input of the bitmap mask, you should be good to go.

I think if you use a bitmap mask on the footage that you want to 'cut out' and use the alpha channel of your video a the input of the bitmap mask, you should be good to go.

Thank you for this. I have got it to work and now need to refine. Pipe the 'Bitmap' node into the blue (mask) input of the clip i want to mask, pipe the video that i am using for the mask into the Bitmap node. Done!! thanks again...