Re: Luminance key

There are three main types of keyers... luminance keyers, color keyers, and difference keyers. From my understanding, the keyers you mentioned are color keyers. Even if they are using 3D space, what they are talking about is RGB defined in three dimensions. This is a pretty complex process and I'm not an enginner, so here is the short answer to what's going on... let me think back to Puffin's whiteboard after we saw Discreet's 3D keyer explaining the "golden nugget" theory...

Normal keyers simply plot the RGB values linearly. If you were to click a pixel to define for your key and then display it in 3D space with red on one axis, green on another and blue on the last, you would have a single pixel sitting inside this three dimensional space. When you add "tolerance" to this, you are spreading the RGB values in every direction resulting in a cube. Adding softness to the key, you are rounding the edges of the cube. Due to abnormalities in most blue/green-screens after they are lit, instead of a rounded cube what you really want is something like a nugget of gold. In order to produce this golden nugget, you need to do the key in 3D space from the get-go. This is what keyers like Primatte and Discreet's Master Keyer, Modular Keyer, and 3D Keyer are essentially doing.

Many times, you can combine multiple keys to create your final matte. If you find that primatte does a great job but leaves edges bad and a few holes, you can then use another color keyer (or primatte again) to fill in the holes, then use a luminance key to get the edges. You can then use lighten or darken tansfer modes to combine these mattes together. Your goal is to create a high-contrast matte, so you gotta do what you gotta to to get to this end.

If you had a big budget, you can get a big soundstage and cover the walls in green... icheck out the Yahoo Critters spot on my site (www.phoenixeditorial.com). We needed to have a soccer match against green, and set up a 60ft high screen by 60ftx60ft. It was huge. If you don't have this luxury, then your best bet will probably be some sort of luminance extraction and/or rotosplines.

-Matt