Nuke: Rotoscoping

363938477_1280x720What is Rotoscoping?

Rotoscoping is the process of cutting out something from a scene using points and bezier curves as apposed to chrome keying which is faster, but not every shot can use it. The process can be used to remove something from a scene or add something in by creating a mask or ‘matte’ on top of the element. This can be used to create an alpha on a specific part of an image in order to modify it without affecting the rest.

Roto basics:

Before you actually start to Roto, STUDY THE FOOTAGE.
When Rotoing, you do not need to start from the beginning. Examine the footage and see what moves, what will need multiple Beziers, where extremities of movement are etc.

When in Nuke, the first thing you’ll want to do is create a Roto node by pressing tab in the node graph and typing Roto. The input will be the image or sequence you wish to Roto, and the output will be what you set it to (Alpha by default, but you could change it to RBG for a colour output). With the Roto node selected, a few additional options should have come up to the left of the viewer. By using the bottom option (Bezier) you can place points and drag to curve them. Use these to create an outline of an element in the scene. You don’t want to get it perfect first time, and will also want to use as little points are possible. Once you’re back where you started, you can now go back and fine tune it with the select all tool. This will give you a lot more precision than your first time around, as you can move both ends of the bezier independently instead of as one. By using Ctrl you can move only one end of the bezier without affecting the other at all. This will allow you to create hard angles.

Once the initial Roto is done, you could, for example, add another one. When rotoing a humanoid, it is a lot easier to make them up as several different parts and move those as one instead of moving countless individual points by hand if there is more than one frame. To do this, just use the Bezier tool again. DO NOT CREATE A 2ND ROTO NODE, Nuke does not like it when you do that. If you are rotoing a donut for example , you will want to create a mask around the outside, but have it transparent in the middle. For this you will need to create another bezier to cut out the hole. Once you have done this, make sure that bezier is selected in the properties bin and select the little white box in the colour wheel column and change the alpha to 0.

When working with a sequence, try to edit as few keyframes as possible because Nuke will interpolate between them for you. Start by creating a keyframe at every extremity of movement, then fine tune it by creating a keyframe in between where it is most inaccurate.

Once you have completed your roto, you will want to use a merge node with another image to place the roto on top of it. You can use a background, or merge it back onto the original image with any changes you make. However, you will want to use the ‘Premult’ node to multiply the RGB by the alpha. This means you will only get what is inside the alpha.

Feathering is a technique used to smoothen edges, and can be especially useful for simulating motion blur on a moving object. By using the properties bin, you can set an overall feather for your roto, but if you want more control, you can hold Ctrl and drag to create a bezier that controls the feather of that control point. The feather is a linear fade between the feather bezier and the control point.

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