Keying Issues

[Grant Swanson]

Hi Andrew,

This is best dealt with by using some pre–processing.

Basic steps to follow would go like this:

1)Duplicate layer
2)Remove grain
3)Increase contrast
4)Use Hue/Saturation to change purple to blue (may need to create a mask around the character to keep that from becoming blue)
5)Increase saturation
6)CREATE GARBAGE MATTES
7)Take the key(s) from this and use it as an alpha matte for the original

Hope this helps!

Grant Swanson
Visual Effects Supervisor
Video Apex – Minneapolis, MN
videoapex.net

[Brendan Coots]

“1)Duplicate layer
2)Remove grain
3)Increase contrast
4)Use Hue/Saturation to change purple to blue (may need to create a mask around the character to keep that from becoming blue)
5)Increase saturation
6)CREATE GARBAGE MATTES
7)Take the key(s) from this and use it as an alpha matte for the original “

This technique wouldn’t actually work that well in keylight because it is a color difference keyer. You can push and shove the pixels all you want, but it’s the relationship between them that Keylight pays attention to. Boosting color or saturation to the entire image results in the exact same “difference” calculation.

For this to work and deliver a clean result, you would have to create a detailed, frame-by-frame animated mask around your character to prevent the color corrections from affecting him/her. At that point, you’d have a very nice frame-by-frame rotoscope and keying wouldn’t even be necessary.

Brendan Coots
Splitvision Digital
http://www.splitvisiondigital.com

[Grant Swanson]

Brendan-

The Hue/Saturation effect works like this: it takes the hue of the individual color channels and manipulates them independently from the other channels, thus you can change the relationship that pixels have with other colored pixels.

With almost all keying there is roto work involved, but whatever you can do to separate the character from the background to begin with is worth it to save you time, and if it’s for professional work, you certainly can’t afford to take the time and/or money to create a detailed, frame-by-frame animated mask around your character.

Grant Swanson
Visual Effects Supervisor
Video Apex – Minneapolis, MN
videoapex.net

[Brendan Coots]

“The Hue/Saturation effect works like this: it takes the hue of the individual color channels and manipulates them independently from the other channels, thus you can change the relationship that pixels have with other colored pixels.”

Sure, but boosting the greens in the background boosts the greens in the actor, resulting in a net “nothing” in terms of ability to pull a better key. Trust me, there is no way to color correct your way out of a keying situation.

Brendan Coots
Splitvision Digital
http://www.splitvisiondigital.com

[Andrew Williamson]

Many thanks for this Grant – I owe you an eBeer! I follow the reasoning behind this method and it seems to make sense to me. You’re right in that there certainly isn’t the time to Roto the whole shoot!

I’ll update when I’ve had a chance to try this out.

Many Thanks,

Andy