Friday, March 22, 2013

Quick Tutorial on Using a Vectorscope to Do Colour Correction

All you amateur videographers out there might appreciate this quick introduction to how to use a vectorscope to do color correction. A vectorscope is visual display of the information in a video image. It displays as a colour wheel where one part is red, another green, and another blue and thus around the circle (hue). Intensity (saturation or chrominance) of the signal for each of those colours is then shown by how far from the centre the little dots appear. Here's an example that will make it fairly clear what I'm talking about:

This is what the vectorscope looks like in Final Cut, but pretty much all the video editing software such as Premiere Pro and Avid have the same thing. Notice that besides the patches of red (11.30 on the clock), blue (4 o'clock), and white (in the centre) data being represented, there is another patch extending out in the 11 o'clock position. That's the skin tone. Now, notice how there is a little reference line in "built in" to the scope near that point? That's the standard reference for what will appear as "skin tone." So if your patch of skin data falls on or at least close to that line, it will look like a "natural" skin tone.

Now for the mind blowing part--this reference line is the same for any colour of skin. Any race, black, white, whatever, will fall along the same line. Why? Because what the vectorscope is really picking up in the colour of the blood in the skin! Hidden from our eyes by all the other information our eyes perceive is a big glob of red, and the vectorscope can separate that out easy as kiss-my-hand. This has been observed for a long, long time, which is why virtually all vectorscopes have a reference line in this position. You can quickly correct skin tone just by using this handy tool. Here is a four minute tutorial that shows you exactly how to use a color correction wheel coupled with a vectorscope to quickly do colour correction in Premiere Pro.

See how easy that was? And it goes beyond skin tone. I also have a set of colour reference cards. So if I hold those up in front of someone before video taping them, in post-production I can then look at the vectorscope image and make sure that the "blue" on the card matches the "blue" position on the vectorscope. Easy peasy primary colour correction.

No comments: