Ultimatte CSC ( Classic Screen Correction) Node

The Ultimatte CSC (Classic Screen Correction) node enables you to correct flaws in the screen area, or backing, of a foreground image. Flaws can be caused by uneven lighting or seams and patches on the surface of the backing. You can use Ultimatte CSC to transform imperfect bluescreen backings into perfectly homogenous fields of blue (or green or red) while retaining shadows and transparency.

You should process imagery with the Ultimatte CSC node before further processing with the Ultimatte GK, PFG, or CC nodes.

The Ultimatte CSC node requires two inputs:

  • foreground imagery (top input)
  • screen correction frame or clip (bottom input)

Clean Plate

The screen correction image, also referred to as a clean plate, is a clip of the lit backing shot in the absence of foreground objects, such as actors or props. The clean plate is used as the reference for the correction.

The screen correction input may be a single frame, or a sequence of frames that matches the number of frames in the foreground image input. The Ultimatte CSC node will use the single reference frame for comparison with each frame of the foreground image.

Locked-Off Shot

For a locked-off shot (where the camera doesn't move, pan, or zoom during the scene), a single reference frame is adequate. Ideally, however, the frame used for the reference image input should be an average of several frames to reduce the interference of film grain as much as possible in the reference image.

Tip:
If you do not have a screen correction clip, you may be able to create a reference frame by combining regions from different frames of the foreground sequence to create a complete reference frame without foreground objects or shadows.

Moving Camera Shot

In a scene where the camera does move, motion control equipment should be used to shoot a correction clip that matches the actual shot exactly. The Ultimatte CSC node will compare each frame of the foreground image to the corresponding reference frame.

Tip:
If you do not have a screen correction clip for a moving camera shot, use the Ultimatte RSC node instead of Ultimatte CSC.

How Classic Screen Correction Works

Essentially, to create a corrected foreground image for output, the Ultimatte CSC node must identify, for each pixel in the original foreground image, whether the pixel represents a totally opaque foreground object, a partially transparent foreground object (a shadow, e.g.), or unobstructed backing.

In the corrected image, all pixels identified as unobstructed backing are assigned an identical RGB value, opaque foreground objects are assigned the same value that they had in the original foreground image, and transparent foreground objects are assigned a value that represents how they would appear over the blue color used for the corrected backing.

To accomplish this screen correction, the Ultimatte CSC node generates temporary mattes for both the foreground image and the reference frame using the process explained in "How Ultimatte Generates Matte Data ."

The Ultimatte CSC node generates a correction frame by subtracting the pixel values of the reference frame from the peak point value.

The value of each pixel in the foreground matte is divided by the corresponding pixel value of the reference matte to generate a correction matte.

Then each pixel of the correction frame is multiplied by the corresponding pixel of the correction matte. The result is added to the corresponding pixel of the foreground image to create a new, corrected foreground image.

Using Classic Screen Correction

There are two steps to follow when using the Ultimatte CSC node: selecting a sample backing color and adjusting the matte.

Selecting a Sample Color

The first step to using Ultimatte CSC is to select a sample color for Ultimatte Intelligence to use as a reference in the screen correction process.

The sample color should be in an unobstructed area of backing (no shadows or strands of hair, e.g.) and should be a representative backing color. See also "How Ultimatte Generates Matte Data " for more information.

Open the CSC work monitor and click on a pixel in the RGB image display that represents the desired color to use for the backing. The RGB values of the selected color are displayed in the Sample Color parameter on the node panel.

The best blue to select from an uneven backing will depend on the image:

  • Choosing too bright a blue can cause darker areas of unobstructed backing to be interpreted as being in shadow when the image is composited.
  • Choosing too dark a blue can cause glowing edges around foreground subjects that are against brighter backing areas, and faint shadows and thin transparencies can be lost.

It is usually advised to select a bright, representative blue that is close to important foreground information, such as near a person's face. Be sure to avoid clicking on fine foreground details, such as smoke, shadows, strands of hair, or mist.

Adjusting the CSC Matte

The second step is to switch to Alpha channel display in the CSC work monitor to view a temporary matte that is generated by the node.

The CSC Alpha matte is used for the screen correction operation only, and is not part of the node output image. It represents how Ultimatte is interpreting the image: what is foreground subject and what is backing. In this matte image, foreground objects are black, and the backing area is white. The Matte Density and Black Gloss parameters enable you to control how the image data is defined for this purpose.

If there are patches of gray or white in foreground objects that should be entirely opaque, they represent pixels with RGB values that cause Ultimatte to interpret them as transparent. You can use Ultimatte Intelligence to adjust these values. Click on the brightest gray or white pixel in a foreground area and Ultimatte Intelligence will automatically adjust the RGB values of these pixels to correct this problem.

To adjust these values manually, refer to the Matte Density & Black Gloss parameter description in the next section.

Viewing the Final Result

To review the result of the screen correction, open a regular node monitor from the local node menu of the CSC node. The backing should now appear even. The final test, of course, is to view the foreground image over the background in a composite node.

Ultimatte CSC Parameter Tab

The Ultimatte CSC parameter tab enables you to select the backing color and adjust the matte.

Work Monitor / At Resolution

First, select a display resolution for the work monitor from the At Resolution popup menu. Then click the plus icon to display the work monitor for this node. By default, the monitor opens in RGB display. To display another channel, click the Channel Display button (cube icon) on the monitor toolbar and select the desired channel from the popup menu:

  • In RGB display, this monitor enables you to select a sample color for the backing by clicking on the appropriate pixel.
  • In Alpha display, this monitor displays a temporary matte image that enables you to adjust the Matte Density and Black Gloss parameters.

Sample Color

The Sample Color parameter displays the RGB values of the sample pixel you select by clicking on an area of backing in the RGB display of the work monitor. You can also key in RGB values for this parameter.

(See "Selecting a Sample Color " in the previous section of this node description for more information about how to choose the best color.)

Matte Density & Black Gloss

The Matte Density and Black Gloss parameters enable you to control the density of the matte channel data that is generated by the node as part of the screen correction process. (Be sure to select Alpha display in the work monitor to view the temporary matte.)

Both parameters function by minimizing patches of gray or white within the black foreground objects of the inverted matte. Click the brightest of these pixels and Ultimatte Intelligence will adjust the parameters as needed.

In most cases, you will want to use the parameter values set by Ultimatte Intelligence, as explained in the previous section, "Adjusting the CSC Matte ." However, you can also set these parameters manually by keying in a numerical value in the range of 0-100 or by using the mouse to set the tuner to the desired value:

  • To increase matte density in light or bright colored foreground objects, use Matte Density (the default value is 50).
  • To increase matte density in dark, reflective foreground objects, use Black Gloss (the default value is 0).

Note:
If you set the Matte Density or Black Gloss parameters too high, the result may include enhanced visual noise or other unwanted effects.
Be sure to check any adjustments you make to these parameters by viewing the result in a composite node.

Reset Parameters to Default Values

Click this button to reset the Matte Density and Black Gloss parameters to their default values. This will not affect the Sample Color parameter values. (To reset all node parameter values to their default settings, select the "Defaults" option from the node's Presets menu.)




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