[Phys-l] chroma key

[John Denker <jsd@av8n.com>]

On 07/17/2007 04:32 PM, Rick Swanson wrote:

> Have you ever
> wondered how those TV announcers who stand in front of a blue screen
> (to get the weather map behind them) can still show realistic skin
> tones even though the camera that captures their image has the blue
> turned off?

I think the question assumes things that aren't true.

What we are talking about here is called the "chroma key" 
process, of which well-known special cases are green-screen 
and blue-screen.

The blue-scree process does not take an RGB vector and project
off the blue component, which would lose roughly 1/3rd of the
information.  Instead it considers the whole color solid as a
three-dimensional solid, and takes a small bite out of it.
Imagine you are in a good-sized room, and somebody marks off
a region the size of a shoe box and says "this is off limits".
As long as the excluded region is far from where you want to
be, it's no inconvenience at all.

Flesh tones sit in the color solid far, far away from where
the blue screen sits.

  At the other extreme, there's an amusing scene in _Groundhog Day_
  where Rita wears her bright blue in front of the blue screen.

BTW, for still photos, chroma key isn't terribly hard to do.
I do it all the time.  It's not trivial, but not beyond the 
ability of anyone on this list.  In photoshop, the "fairy 
godmother wand" will select large contiguous regions that 
have the same color.  You just "erase" the selected region.
This doesn't produce perfect results, but good enough for
many purposes.  I have a big bright green bedcover that I 
often use as the background.

  Hint:  I find it helps to use some fill-in lighting to
  get rid of any shadow the foreground objects would cast
  on the background screen.  That's because the photoshop
  selector wand often cares too much about brightness as
  opposed to chroma per se.  Using fill-in lights is easier
  than arguing with photoshop.

Also BTW, the background doesn't need to be blue-screen or
green-screen.  I shot this picture against a red background:
  http://www.av8n.com/physics/geodesics.htm#fig-darts