[Bf-vfx] Keying nodes

[Ton Roosendaal]

Hi Bob,

Thanks for the info!
Do you have reference shots or stills still to illustrate or validate the nodes? Or do you have a group that's well configured for common jobs?

Although I understand each of the technical descriptions well, it's feeling more like a construction kit of functions than as a useful solution to start with.

I know there's cool and well balanced methods that will give good mattes quickly with only a very few clicks... the more specialized nodes then can be used for tweaking mostly.

-Ton-

------------------------------------------------------------------------
Ton Roosendaal  Blender Foundation   ton at blender.org    www.blender.org
Blender Institute   Entrepotdok 57A  1018AD Amsterdam   The Netherlands

On 15 Apr, 2012, at 18:50, Bob Holcomb wrote:

> I think we definitely should look at improving the keying nodes, but I 
> also think there is a bit of misunderstanding about what is already 
> present.  This is largely my fault for not documenting them better-I'll 
> update the wiki after this short novel.  Here's what we got:
> 
> Difference Key:
> This takes the difference between a clean plate and another plate and 
> determines at a per-pixel level if the two pixels are different within 
> an RGB color space (with an adjustable tolerance and falloff).  This 
> node works really well if your clean plate and keying plate are lined up 
> perfectly, such as when using a tripod to shoot the image or using a 
> motion control camera.
> 
> This could be made better by allowing for positional tolerances in 
> addition to account for small movements between the two shots.   This 
> key requires the patch I submitted a couple of weeks ago to fix the 
> sliders from being dependent upon one another, they should be independent.
> 
> Luminance Key:
> This uses the luminance value (in YUV color space) and a high/low slider 
> to determine if a pixels should be keyed or not.  This is useful for 
> anytime the foreground is significantly brighter than the background.  
> Such as  keying special effects like fire/explosions/fireworks/etc for 
> comping over other scenes later.
> 
> Distance Key:
> This keys pixels based on the 3 dimensional distance between colors in a 
> 3D RGB color space.  This key works well when trying to single out a 
> specific color in a background (not necessarily green).  This key 
> requires the patch I submitted a couple of weeks ago to fix the sliders 
> from being dependent upon one another, they should be independent.
> 
> Color Key:
> This is pretty weak.  It keys based off the color in HSV color space.  
> It allows for tolerances for each of the components (H,S, and V) 
> independently.   It was more of an experiment than anything else and 
> probably isn't useful.
> 
> Channel Key:
> This is the (most well known) simple green minus red (or the max of the 
> red/blue) channel keying.  The node can work in other color spaces and 
> allows for any channel to be used to limit the other channels, but I 
> think it has very limited uses outside of the RGB color space.  It does 
> allow for arbitrary channels to be used (e.g.  red minus max of 
> green/blue).  It has some high/low tolerance slider to vary the 
> impact.   This is probably the most useful keying node.
> 
> Chroma Key:
> This node was based on the method described in the book "Video 
> Demistified."  It converts the image to YCbCr color space, then 
> "rotates" the pixels color (in the CbCr plane) to align it with the 
> keying color space.  If the angle between the color and the key (the key 
> laying on the X axis in the rotated color space) is less than the angle 
> tolerance, then the pixel is keyed.  This key node is useful when the 
> background is unevenly lit, but is consistent in chroma.  This is 
> probably the second most useful keying node.  This node usually looks 
> crappy because it works in YCbCr color space, and most digital cameras 
> compress the hell out of the CbCr components.  Separating these channels 
> out and blurring them a little, then recombining them helps pull a 
> better matte.
> 
> 
> None of these keying nodes do spill suppression, so there's a separate 
> node for that.
> 
> I would expect that none of these nodes will produce a good matte on 
> their own.  You'll probably need to combine a garbage matte, keying 
> nodes (more than one) and edge filtering nodes to get a good matte.  
> Each shot is different and will probably require its own node setup.
> 
> I think an advanced keyer such as the bayesian approach would be an 
> awesome addition.
> 
> cheers,
> 
> Bob
> 
> 
> 
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