The Ultimatte Nodes
Ultimatte is bluescreen compositing software for film and video. It was designed by the Ultimatte Corporation, and its functionality is included in Chalice. (You do not require a separate Ultimatte license to utilize this functionality.)
Ultimatte is designed to optimize the bluescreen compositing process by enabling digital artists to seamlessly composite imagery while retaining shadows, fine detail, and transparent objects in the foreground image.
(In actuality, the screen may be blue, green, or red; in fact greenscreen is used quite commonly. However, this manual uses the term "bluescreen" generically.)
As an example of the bluescreen process, a filmmaker might shoot an actor in front of a background (the bluescreen, or "backing") that has been painted entirely blue, using paints formulated specifically for this purpose (e.g., Rosco Labs Ultimatte Blue #5720, or Ultimatte Super Blue #5722, for wire removal).
Later, using the Ultimatte nodes, a digital artist will be able to prepare a processed foreground image (an image of the actor, with the background matted out) and seamlessly composite it over a background clip of a burning building, lunar landscape, or other dramatic scene.
Chalice includes five Ultimatte matte nodes, each of which is optimized to solve a particular type of bluescreen image processing problem:
Ultimatte CSC (Classic Screen Correction)
This node enables you to correct bluescreens (or red or green screens) to create a perfectly even screen area behind a foreground subject through the use of a reference clip.
Ultimatte RSC (Roto Screen Correction)
This node enables you to correct bluescreens in cases when a reference clip is not available.
Ultimatte GK (Grain Killer)
This node enables you to remove film grain (or video noise) from the backing area of a foreground shot.
Ultimatte PFG (Processed Foreground)
This node enables you to create a processed foreground (PFG)--an image of the foreground subject with any spill removed and the backing area suppressed to black, with the Ultimatte matte attached as the alpha channel. You may later composite this PFG image using a node such as Mcomp, Over, or Ultimatte AdvantEdge.
Ultimatte CC (Color Control)
This node enables you to adjust the color balance of one film clip to make it appear natural or appropriate for the color values of a background clip when composited.
Overview of the Ultimatte Process
The following illustration provides an overview of how the Ultimatte nodes are used in a typical compositing process. For detailed information on each node and how it is used, refer to the individual node descriptions that follow this overview. You should obtain the best results when you use the Ultimatte nodes in the sequence illustrated.
Step A: Screen Correction
First, the foreground image--videotaped on a greenscreen stage in this case--is connected to an Ultimatte CSC node to correct any flaws in the backing, or screen area.
In an ideal situation, the screen area against which an actor is filmed would be of perfectly even tone. In reality, the backing almost always shows imperfections due to variations in ambient lighting and surface imperfections. The Ultimatte CSC (Classic Screen Correction) node enables you to perfect the screen area while retaining shadow information.
A clean plate of the stage, shot at the same time as the foreground subject, is connected to the second input of the Ultimatte CSC node to be used as the reference screen for the correction process.
- Note:
- If you do not have a reference clip for the shot, use the Ultimatte RSC node instead of the CSC node to correct the backing.
Step B: Processed Foreground
Next, the output of the Ultimatte CSC node is connected to an Ultimatte PFG node to create a processed foreground that is ready to be composited. The Ultimatte PFG node enables you to remove flare or spill on foreground objects and create a matte that is output as the alpha channel of the image.
Grain Killer Option
For filmed imagery (or for a noisy video clip) you will want to use the Ultimatte GK node to remove grain from the backing area of the foreground image. This will prevent the appearance of a double layer of grain when the foreground is composited over a filmed background.
The Ultimatte GK node should be used after the screen area has been corrected using the Ultimatte CSC node and before the final matte is created in the Ultimatte PFG node.
- Note:
- If you are using the Ultimatte RSC node instead of the CSC node, do not use Ultimatte GK--the RSC node has this grain killing functionality built into it.
Step C: Composite
Finally, the processed foreground image is composited over the background image using a composite node (Mcomp, in this case).
- Note:
- Composite nodes such as Mcomp enable you to specify, for each input, whether the image is premultiplied, not premultiplied, or an Ultimatte input.
Be sure to select the "is Ultimatte" option for the Ultimatte input in the Mcomp node panel.
Color Control Option
When you view the composited image, in some cases you may find that the color balance of the foreground image does not match the background image perfectly. To correct this situation, you would insert an Ultimatte CC node into the network between the Ultimatte PFG and Mcomp nodes and use the Ultimatte CC controls to correct the color balance of the foreground image.
Reviewing the Output
When you use the Ultimatte nodes to prepare bluescreen images, the best way to ensure the optimal result for any particular shot is to review the imagery as it looks when composited over the background in one of the Chalice composite nodes. This is the true test of whether the various node parameter settings need further adjustment.
In summary, the Ultimatte nodes enable you to create perfect bluescreen backings, remove film grain, process the foreground image, and correct the color balance of an image to be composited.
The output of the process will usually be a single processed foreground image suitable for seamless compositing in another Chalice node.
How Ultimatte Generates Matte Data
The Ultimatte process is designed to create highly accurate matte data based on the color values of the foreground image. Most colors occurring in nature have at least some amount of red, green, and blue when represented in RGB colorspace. Given this fact:
- How does Ultimatte determine, based on the RGB value of a pixel, whether it is part of the backing or the foreground?
- And how does Ultimatte determine whether a foreground pixel should be opaque or partially transparent when composited over the background?
Using a blue backing screen as an example, the colors used for the bluescreen backing will have extreme differences between the blue channel value and the red and green channel values.
The Ultimatte process starts by examining the relationships among the values of the three channels of an RGB image. For each pixel, Ultimatte determines which has the greater value, the red channel or the green channel. This value (the max of R or G) is then subtracted from the blue channel value to obtain a difference value for the pixel.
Interpreting Difference Values
Areas where this difference value is pronounced are interpreted as backing. The greatest difference value is called the peak point, and is considered to be totally transparent for purposes of the final composite; that is, 100 percent of the background will show through the foreground wherever a pixel has this value.
In reality, unobstructed areas of the backing will display some variation in color. This means that the pixel with the highest peak point value might not be the most representative blue in the backing, so Ultimatte enables you to specify the ideal blue to use as the peak point.
Otherwise, unobstructed areas of the backing that have slightly lower difference values than the peak point could be interpreted as if they were not entirely transparent (as if they were in a faint shadow, perhaps).
Areas where the difference value is 0 or less are interpreted as foreground and considered to be totally opaque for purposes of the final composite. For example, a pure white pixel has equal values for R, G, and B, so its difference value is 0; and negative numbers will result for colors where B is less than the max of R or G.
Pixels with values that fall between the peak and minimum difference values are assigned varying levels of opacity in a linear distribution. This is what allows Ultimatte images to retain fine details, shadows in the backing area, and soft edges around foreground objects when composited over a background.
However, this also means that blue areas in the foreground subject may be treated as backing and result in print-through in the final composited image. "Print-through" refers to areas where the background is visible, or partially visible, through foreground objects that are supposed to be opaque.
Adjusting Matte Density
Ultimatte can correct print-through problems in a matte by altering the formula used to determine the difference value for each pixel: a matte density value is assigned for the image, and the max of R or G is first multiplied by this matte density value before it is subtracted from blue.
Boosting the value of the channel that is subtracted from blue results in a lower difference value. When the difference values for an image are decreased in this way, pixels which previously had some transparency can become entirely opaque, eliminating print-through.
However, raising matte density too much can harden matte edges and eliminate fine detail, with adverse affects on the realism of the composite.
The Ultimatte nodes include a number of controls that enable you to adjust these values as needed for any particular image.
In the Ultimatte PFG node, for example, adjusting the Matte Controls affects the alpha channel that is output for subsequent compositing. In the CSC node, on the other hand, the matte density parameters affect the matte data that is used in the screen correction operation.
How Ultimatte Mattes Are Displayed
The Ultimatte nodes operate by generating matte data from the RGB values of the image and adjusting various matte parameters, but the specific purpose and display of these mattes differ among the Ultimatte nodes.
The Ultimatte CSC, RSC, and GK nodes preprocess foreground images; they generate temporary mattes that are used only to perform the nodes' screen correction and grain killing functions. These mattes are not part of the node output image.
The Ultimatte PFG node, on the other hand, generates the matte that is used as the alpha channel in the output image. For the Ultimatte CC node, the matte controls where the color correction occurs.
Using the Ultimatte Work Monitors
All of the Ultimatte nodes feature a work monitor, which is accessed by clicking the plus icon on the node panel. You must use the work monitor that is provided for each of the Ultimatte nodes in conjunction with the node parameters.
Work monitors enable you to select the sample color that Ultimatte should use as a reference for the backing color; work monitors also display the node-specific mattes that are created by the Ultimatte CSC, RSC, and GK nodes.
The Ultimatte RSC work monitor provides rotospline tools for drawing and editing shapes (which define the subject areas).
Ultimatte Intelligence
Work monitors are used to activate Ultimatte Intelligence mode, in which you can interactively adjust various parameters by clicking specific areas in the image and letting Ultimatte automatically determine the optimum settings.
For example, if you click on an area of print-through in the matte display of an Ultimatte CSC node, Ultimatte Intelligence will automatically adjust the matte density level based on your selection. In the Ultimatte CC node, however, you click on two different colors you want to match and Ultimatte does the rest.
The Ultimatte Intelligence settings can be adjusted manually, although in most cases it isn't necessary or advised.
The work monitors feature the same toolbar options available in the regular node monitors. The following are especially useful with the Ultimatte work monitors:
- Use the Zoom arrows, or open the Magnify utility monitor, to help identify an individual pixel for selection, as when you want to sample an area of backing without clicking on a foreground detail such as a strand of hair.
- Display the Inspect panel to get RGBA values and other information for the pixel that is currently under the cursor. (The Inspect panel is displayed by selecting the Inspect option from the popup menu that you access by right-clicking anywhere in the image.)
The individual Ultimatte node descriptions that follow provide additional information about how to use the work monitor in conjunction with the parameter controls for each node.
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