Masking out swimming girls

[Ryan Hill]

For shots where the camera is still, you can use difference matte to compare the current frame with a background. You’ll get less trouble from the background that way, but you’ll still get some noise from the water itself. To clean up noise like that, I’ll often apply two simple chokers, one with a positive value, one with a negative.

Multiple layers can be very helpful in this, too. If I’m having a hard time pulling a key, I will make three layers. For one, I will set it up to make sure the foreground figure doesn’t have any holes, and then apply a simple choker until its outline is smaller than my desired outline. Then I do a second layer for a more precise outline, but I don’t have to worry about filling in the holes. If this one has too much noise outside of my foreground figure, I make another layer with a positive simple choker to remove the noise, followed by a negative choker, so its outline is larger than I want. Then I use that as a track matte for my other layer.

Of course, all this depends on you applying a luma key instead of just threshold filter. You can then stick a white layer behind everything.

[Ryan Hill]

Oh, I knew this sounded familiar. It’s the wiener dog guy.

I only feel qualified to give you advice now that in the past month I’ve successfully keyed someone wearing a white shirt against an off-white background using the methods I just described.

But I admit I’ve never tried keying anyone in the water. That will probably have a bunch of its own problems. However, you just want sillhouettes, which means you can be a little more forgiving with a pixel here or there.

I’m not sure I understand what you want to use motion tracking for.