Alpha Channel mapping?

[supermoos]

Hey I'm trying to do alpha transparent video playback. So far I've had very decent results with the chromakey effect - however there's a little bleed in the edges from the chroma key color.

I'm working with a 3d model so I can render the video out with any kind of alpha and edge.

I looked into 2d canvas based solutions like this one: https://github.com/m90/seeThru - they work by doubling the video's height, and then having the alpha channel map in the bottom half of the video.

Would something similar be feasible with seriously.js, or would the double height video have to much of a negative impact on framerate? I'm working with a 940x680 video (which runs fine with chromakey). And which effect should I look into?