VFX 101 Case Study: Greenscreening Without the Green

I’m Herman Wang, and I do all of the VFX work for my web series The Spell Tutor and Illuminagents. I haven’t done one of these VFX 101 posts in a while, but an interesting problem came up recently that I thought would be illustrative to those who do their own special effects.

A friend asked me to work on some footage he’d shot. In this action thriller, he wanted to show a character unlocking a high-tech case by using a handprint scan.

This is a frame from the raw footage:

Since the camera was handheld, the first task was to do some motion tracking so the added elements would blend correctly. It was straightforward to use the shiny rectangle on the bottom right of the case as a tracking target.

Next, I added a simple line graphic with some added glow, and had it track from left to right across the face of the case, using the grooves for placement reference:

Of course the next (and most difficult) step was to make the hands mask out the line, to give the desired appearance. First I drew an outline to limit the working area to where the VFX lives:

Anytime you want to do masking, your go-to effect is usually Chroma Key, more commonly known as greenscreen. So I used the black case as the key colour and tried to make a mask that would leave only the skin tones. This didn’t end up working well, because the difference in colour between black and brown isn’t pronounced enough. The best mask I could create had a lot of bleed and spottiness problems:

As a fallback, I went to Secondary Colour Correction (SCC) to create the mask. My reasoning was this - greenscreen is essentially a pared-down, highly specific application of SCC; so where greenscreen failed, using SCC is like going back to the basics. To put it another way, here are the controls you can use to greenscreen in Vegas Pro:

Whereas these are the controls for SCC:

As you can see, SCC has more elaborate ways to fine-tune the creation of a mask. Without going into too much detail, I used the greater control of SCC to group together a range of dark to light-brownish skin tones, and exclude everything else. This is the mask I was able to achieve using SCC:

This mask is perfectly suitable to isolate the hands from the rest of the elements. I created a top footage layer with just the hands and used it to cover up the VFX scan line. Now the scan line looks like it’s between the hands and the case:

This is the final clip I was able to produce for my friend:

The main lesson here is that in a crunch, SCC can take over when greenscreen plugins just don’t have the power to get the job done. I hope you enjoyed this case study!

Here are my earlier posts about greenscreen and SCC:



All footage used courtesy of Rogue Cell.

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