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cmiVFX – Nuke Advanced Keying And Channel Operations with Expressions

| project files | .mov | 933Mb

In this in-depth training video you will learn, step-by-step, to create perfect mattes from green/blue screen footage (process screens) in Nuke. You will learn to use multiple keying approaches that will prepare you for any shot, and the various challenges that inevitably arise. You will learn proven ways to fix problems, such as preserving hair detail and removing color spill from process screens. Color spill is almost always a problem and dealing with it is one of the most important aspects of integrating your process screen elements into a shot. After learning these techniques Nuke will become an indispensible solution for even the most challenging keying and channel operations.

Simple Color Difference KeySimple Color Difference Key
In a green screen shot, for example, simply subtracting the green from the red channel can create a good starting point for a matte. All commercial keyers will take you some of the way there but, more often than not, your shot will require a multiple step solution. Having access to the component parts of a multiple layer keyer can help you hone in on problem areas and solve your keying issues in less time – freeing up time to refine the composite layers and blending. (see also Modular Keying)

Combining Mattes: Inside, Outside & Edge Approach
Nuke & NukeX come with a range of excellent keyers. All of them are able to produce (and output) an alpha channel and matte. These mattes can be combined for a more refined approach when a single key does not achieve the desired result. One method can create a solid, or hard, ?inside matte? that removes all the unwanted noise from the outside of the matte. Once this hard matte is created, the artist can focus on refining the edges of the subject to extract all the detail required. You will learn which shots are the most appropriate for this more advanced approach to keying. There are several training videos that focus on this approach to keying as well as some free ones in our library.

Combining Mattes: Individual Areas of Interest
A general key might give a good matte, but there are usually areas that need special treatment, i.e. hair, a dark shoulder etc. Utilizing what is known as an ?area of interest? we can focus on specific areas in the matte and produce a key that addresses the specific problems for just this selected area. When several mattes of this kind are combined into one matte it is possible to solve an array of complex keying problems. This method can also be used in conjunction with the “Inside, Outside & Edge Approach”. If you are familiar with “Maynard Keying” or “Blocking Out” techniques (please refer to further cmiVFX keying videos on these subjects) this section of the video will enable you to construct powerful procedural keyers with the utmost control.

Despill
When working with green or blue screen material it is very common for the color of the process screen to reflect onto the subject. This ?spill? will appear as a color cast on the subject when included in your composite. The use of some simple expressions can remove this color cast and enable you to achieve the best possible result. A substantial degree of control over the ?despill? area can be achieved in Nuke by the use of a despill node network. Expressions are non-intuitive to many beginners but just a limited knowledge of some simple expression code can save any artist a ton of valuable time. This portion of the video is a staff favorite!

Changing the Color of the Despill
Simply suppressing the spill color does not always improve the integration of the element into the shot. Usually this method simply introduces a different cast to the subject?s edges. When the ?despilled? color is not present in the background element it will be just as out-of-place as the spill itself! In this chapter you will learn how to change the spill color to integrate with the colors in the background plate. There are several ways to do this using one of several applications. However, one method is faster than the others in most scenarios. We will share the proven method that our research shows is the best and quickest process!

Retrieve Edge Detail
Even when a good, basic key is obtained it is still likely that some of the edge detail will be lost along the way. This is almost always true when one relies on the use of a single commercial keyer. You can make your life so much easier by learning an alternative solution that enables you to retrieve that missing detail. Using a clever method of applying the original foreground element over the background plate (prior to adding our keyed element on top) you can achieve an astounding result in no time ? a technique that you will use over and over again.

Evening / Leveling Green Screen
In order to use the above method to recover edge detail, you will need a consistent, flat process screen with no irregularities. Such a flat process screen would also be handy for other operations. By comparing a clean plate, the actual shot, and a solid green constant we can produce a matte that will enable us to level out any irregularities in the screen as it was shot on set or location. It is true that even the best Visual Effects Supervisor can be forced by time constraints or other circumstances to present you with a less than optimal process screen! This method of achieving a flat green screen is invaluable.

Mocking Up a Clean Plate
Quite often shortcuts in production, as illustrated above, can mean more work for the artist. An adequate clean plate is not always supplied with a set of shots. There are ways an artist can create such a clean plate from the available elements and solve various problems in the shot. One method is to remove the foreground that was shot in front of the process screen and use neighboring colors to fill in the void. This is known as ?color remapping?. Once a clean plate is produced it can be used to help produce a flat process screen that can be used to retrieve the finest edge detail. This technique can even be used to extract a key for an object on a background that was never intended as a keying scenario – without process screens.

Light Wrap
The effect of light coming from the background and engulfing the foreground element can be essential to make a shot look convincingly real. We will show you how to quickly build such a set up that will give you full control of this effect. This will allow you to produce realistic light wraps rather than just a bright fringe that other methods simply add around the foreground object.

This is not your basic keying video. We bring you in-depth, ?software impartial? and intuitive techniques to add to your arsenal – techniques that you will use in your own workflow every day. We are very excited to bring you these tools because we know they will make your job easier and much more fun!

Home page: http://www.cmivfx.com/tutorials/view/271/Nuke+Advanced+Keying+and+Channel+Ops

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