This Christmas Eve, our family assembled in Philadelphia was thrilled to walk home through the streets of West Philly as a fresh snowfall was drifting down around us. I took the opportunity to snag some stock video as the bright flakes fell around us. This video below was an effort to learn and use a luma key on low resolution footage. It was shot with my Pentax Optio WG-2, a recent acquisition whose 12-meter waterproof rating I am looking forward to testing out in the Red Sea very soon. The original video was shot against a black night sky with falling snowflakes illuminated by a pair of bright floodlights nearby. You'll recognize the background photo from my last post about the Canon XA10. I wanted to get myself some stock snowfall footage that I could layer over other footage in the future. I started by ramping up the contrast in After Effects to separate the snowflakes from the dark, noisy background. Then I used a luma key filter and played around with the gain until I found a setting that got rid of all the background. I placed a blue background behind the snowfall footage so I could see the bright snowflakes as well as the dark edges of the flakes I was trying to clean away. I initially tried using KeyLight but I ended up loosing too many of the flakes in order to get all the background to go away. Then I threw a little Gaussian blur on the whole layer to soften up the edges. Stock snowfall layer: done.
The great thing about a Luma key is that most consumer or prosumer cameras that film at full HD use a 4:2:0 color space. This means (in simple terms) that when the camera compresses the video it groups all pixels into bundles of four and averages the colors of each bundle into a single color. Then it takes each pair of pixels and averages their luminance value. This is done because the eye is twice as sensitive to brightness as it is to color, so you can drop half of the color data (and the associated bandwidth) and your brain won't object. So if you're working with anything less than the beautiful 4:2:2 color space of a 3CCD camera or the 4:4:4 color space of a cinema camera like a RED, your camera footage measures pixel brightness with twice the precision that it measures color. Since a luma key uses brightness (actually, luminance) data to determine the edge of an object, you can get a clean key much easier if your footage it lower resolution or slightly out of focus.