Using a Roto node a vector shape may be drawn (like the pen tool in Photoshop or Illustrator). There are two main uses for this node:
- To make a mask: In a situation where a node such as Multiply is being used to darken an image, a Roto shape may be used as a mask: to limit the effects of that node.
- To make an alpha channel: In a situation when the alpha of an image needs editing, a Roto shape may be used to add to, remove from or replace that alpha.
Added to this is a host of other functionality. It may, for example, be used to hide or revel one layer from another. This functionality can be used to hide glitches in an image using something called a 'clean plate' (a process I shall describe in class).
Nuke's Roto node is surprising advanced, much of which is served by the various parameters of the tabs. However, most day to day usage is covered effectively by the first two tabs.
The tabs of the Roto node
Knowing how to use the Roto, it is necessary to get to know what its different tabs do. Excluding the ubiquitous 'Node' tab, the Roto features seven tabs.
These attributes only become effective when a roto shape has first been selected from the roto list. This list is available in each tab. The icons to the right of each roto shape denote various attributes of that shape including its color, visibility etc.
- The Roto tab
- Here the opacity of the roto shapes can be defined. Also, which channels they impact. The default is alpha only, which might or might not be what you want. When animating the roto shape, this tab gives access to the animation of each point in the roto. CAUTION: animation of points should always be done after the animation of shape (accessed via the Transform tab).
- The Transform tab
- Here the entire shape of the roto may be edited and animated. In the Viewer a widget appears using which the entire shape of the roto may be edited and animated. This tab is identical in form and function to the Transform node.
- The Motion Blur tab
- Animated movement in Nuke usually requires motion blurring before it looks natural. The two parameters are 'motionblur' (which controls the quality) and 'shutter' (which controls the amount). If the amount is increased, then the quality should be increased. Howvere, this will impact upon the rendering time.
- The Shape tab
- Here the color of the roto may be defined. The default is white. From the drop-down menu other options are available (foreground, background1, background2 etc) which only become available if images have been fed into the node. This can be used for selectively laying one image over another. It is useful for selectively painting regions of one image over another. This only works if the 'channels' value of the Roto' tab has been set to RGB or RGBA.
- Here also its blending mode may be specified, which is defined according to other roto shapes. This is very usually for making complex roto shapes by using other roto shapes in interaction with each other. The Max and Min blend mode are particularly useful in this respect.
- The Clone tab
- This only becomes active when the 'source' parameter in the Shape tab is set to anything other than its default 'color'. It is useful for selectively painting unwanted glitches out of footage (e.g. wires, dirty marks etc).
- The Lifetime tab
- Here the lifetime of the roto may be defined. The default is 'all frames', but arbitrary frames values can be defined. Again, this is useful for painting out glitches that only last a few frames.
- The Tracker tab
- Right clicking on any roto shape will offer the option to 'planar-track this shape'. Selecting it will turn the shape purple and place it into a purple folder. A planar track (lightly described here) is a special sort of tracker that tracks regions, as opposed to features. There is effectively no difference between a PlanarTracker node and a Roto node, the former being little more than a pre-set of the later. I shall not cover this functionality in this class unless called upon to do so.
The Viewer controls
The vertical controls on the left are used for adding and editing the shapes. Various flavours of Roto are available. The most common is 'Bezier'. 'Cusped Bezier' is simply a bezier with sharp terminations to its points. A point can be cusped or de-cusped by selecting the point and selecting 'smooth' or 'cusp/de-smooth' from the contextual menu. Other roto shapes include an 'B-Spline' and 'Open Spline', which I shall demonstrate in class.
During the creation of a roto shape, the top bar offers the ability to edit such things as its color, blend mode and opacity. These controls double for controls found in the tabs of the Roto properties window. They are there for super-users who wish all their controls to be available in the Viewer whilst they make their rotos.
Upon creating a roto, the top controls change. Using the controls on the far right of the bar, the animation properties of the roto shape can be defined. Most importantly, animation keys in the timeline can be added or removed. Using the buttons on the left of the bar, the visibility of the on-screen controls can also be edited. This is required when many roto shapes are being edited or animated.
Rotoscoping with the Roto
Making and editing roto shapes
I shall elaborate this section later. For now, refer to this tutorial from The Foundry.
Below are some random, yet important things to know about the Roto:
- When using the Roto to make an alpha channel, a Roto may be fed into the alpha of an image by using a Copy or ShuffleCopy node.
- It may also be fed into an image by simply placing it under an RGB image in the Node tree. If the RGB image does not have an existing alpha then the Roto's alpha will be used. Caution... turn off the RGB in the Roto so that they are not used as well. If the image already has an alpha that you don't need, then you may tick the 'replace' in the 'Roto' tab.
- The Roto will inherit the format of the project, as defined in the Project Settings. This can be troublesome if you are using the Roto to mask something of a different format (its just plain confusing to be working between two formats). To fix this, simply place a Reformat node before the Roto that has been set to whatever format you wish it to match.
A nice little tutorial, describing some of the ways that a Roto might be used: here.