Ultimatte RSC ( Roto Screen Correction) Node

The Ultimatte RSC (Roto Screen Correction) node enables you to correct flaws in bluescreen backings when you do not have a reference clip (a clean plate) for the shot. For scenes with camera motion, you can use Ultimatte RSC to correct the screen area, or backing, even if you were not provided with a motion-control reference clip to match the scene frame for frame.

Note:
If you have an adequate reference clip, use the Ultimatte CSC (Classic Screen Correction) node instead of Ultimatte RSC. It will be faster and easier to produce an even backing area.

How Roto Screen Correction Works

The Ultimatte RSC node enables you to delineate the subject area at every frame by using the rotospline drawing tools provided in the RSC work monitor.

Every pixel located outside the boundaries of the shapes you draw is considered to be backing. Within each shape you have defined, Ultimatte RSC performs essentially the same screen correction operations as the Ultimatte CSC node to remove defects in the backing while retaining detail and transparency in the subject area. For more information about this process, see "How Classic Screen Correction Works ."

However, accurate screen correction requires a reference frame that includes all the seams, smudges, and other flaws in the backing, so that these defects can be distinguished from transparent foreground elements such as shadows.

Ultimatte RSC simulates such a reference frame by extrapolating the characteristics of visible defects in the backing to the areas behind the subject. The transition area values are interpolated from the edge of the spline toward the center of the shape, using guidelines that you specify during the screen reconstruction process.

The RSC node also eliminates grain so that the output can be connected directly to an Ultimatte PFG node. You do not need to use an Ultimatte GK node after RSC, because the functionality of the Ultimatte GK node is included in Ultimatte RSC.

All of the necessary steps for roto screen correction are performed by using the RSC work monitor, in conjunction with the RSC node panel parameters.

RSC Work Monitor

The RSC work monitor provides the tools that enable you to draw and edit shapes that define the subject area. It also enables you to select a sample color to use for the backing, and to use Ultimatte Intelligence to adjust matte density and grain filtering.

Action Modes

This monitor operates in two distinct action modes: Roto Edit and Intelligence. Mode selectors on the monitor panel enable you to select the appropriate mode for the task.

Roto Edit Mode

Roto Edit mode is used for creating and editing the shapes that define the subject areas of the image. When Roto Edit mode is selected, the Shape Tools and Shape List become available for use.

The Roto Shape Tools work in the same way as the corresponding tools in the work monitor of the Rotospline node. (Refer to "Using the Rotospline Work Monitor " for a detailed description of these tools.)

You start by clicking the Draw button to create a shape, and then you use the various point manipulation tools to adjust it.

The Shape button enables you to move or rescale the shape as a whole, and the Keyframer button enables you to access the Keyframe Editor for the shape. The Range parameter is used to specify a frame range for the currently selected shape.

You can create as many shapes as you need and specify on which frames each shape will appear. In some cases it may be easier to outline the backing than to outline the subject. If so, outline the backing and then use the "Invert Roto Screen" option.

A key frame for a shape is created automatically whenever you move to a new frame in a sequence and modify the shape in any way. The node performs a linear interpolation of the shape characteristics for the frames between each key frame. You can click the Keyframer button to bring up the Keyframe Editor, which is explained in the Rotospline Node description.

Intelligence Mode

Intelligence mode is used to select a sample color and to automatically adjust the various matte and Grain Killer filter parameters.

Set Grainkiller Option

Set Grainkiller works by sampling the backing area along the perimeter of all shapes and in the area surrounding the pixel selected as the sample color to characterize the noise and then using this data to filter grain in the transition areas of the subject.

The RSC node automatically invokes Set Grainkiller when you choose a sample color (see step 2 of the next section, "Using Roto Screen Correction "), so in most cases you will not need to use this option. One exception might be if you have drastically revised the outlines of your rotospline shapes.

RSC Monitor Displays

By default, the monitor opens in RGB channel 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.

The Channel Display menu features several displays that are specific to the RSC node: SC Alpha, GK Alpha, and Filter Area. These three options display temporary matte image data that is used in Intelligence mode to adjust various parameters. (The RGB display is used in both Roto Edit and Intelligence modes.)

For screen correction:

  • RGB display enables you to draw shapes to define the subject areas (when Roto Edit is selected) or to select a sample color for the backing by clicking on the appropriate pixel (when Intelligence is selected).
  • SC Alpha display is used in conjunction with the SC Matte Density and SC Black Gloss parameters on the Advanced tab.

For grain killing:

  • GK Alpha display is used in conjunction with the GK Matte Density and GK Black Gloss parameters on the Advanced tab.
  • Filter Area display is used in conjunction with the Screen Filter Offset parameter on the Advanced tab.

Using Roto Screen Correction

The basic steps to using Roto Screen Correction are outlined in this section:

  1. Define the subject areas
  2. Select a sample color
  3. Adjust the SC Alpha matte
  4. Adjust the screen reconstruction
  5. Adjust the GK Alpha matte
  6. Adjust the Filter Area

It is important that they are performed in the order described to achieve the desired result, as each step may use parameter data that you specify in a preceding step.

Step 1: Defining the Subject Area

In the RSC work monitor, select RGB display and Roto Edit mode. Use the tools provided to outline the subject areas.

Be sure to include within the shape any shadows, wisps of hair, or other similar foreground elements that you want to appear in the final composite--all pixels outside of the shapes you draw are treated as backing.

The transition area inside of a shape is where the RSC node will perform the screen correction.

Step 2: Selecting a Sample Color

Once the subject areas have been defined, you need to specify the sample color that the node will use in the screen reconstruction, screen correction, and grain killing processes.

Note:
Be sure to define the subject area before selecting the sample color. The Grain Killer filter parameters are automatically set when the sample color is selected, and these settings are based on the sample color as well as the current shapes. Therefore, if the sample color is set before the shapes are created, the grain filtering parameters will not be set properly.
If you did select the sample color before you created the shapes, either reset the sample color, manually adjust the Filter Area, or click the "Set Grainkiller" button.

To select a sample color--the most representative blue in an unobstructed area of backing--switch to Intelligence mode and click on an appropriate pixel in the image. The RGB values of that pixel will appear in the Sample Color parameter on the RSC tab.

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 "How Ultimatte Generates Matte Data " for more information.)

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 details, such as smoke, shadows, strands of hair, or mist.

Step 3: Adjusting the SC Alpha Matte

First you must switch the monitor display to SC Alpha. This will enable you to review the temporary matte data that is generated by the node for use in the screen correction process.

The matte represents how Ultimatte is interpreting the image: what is foreground subject and what is backing. The SC Alpha display enables you to use Intelligence, if necessary, to control how the image data is defined for this purpose.

In this matte image, foreground objects look black, and the backing area looks white. 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.

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.

SC Alpha Intelligence can be repeated multiple times, if necessary. To adjust these values manually, refer to the "Matte Density & Black Gloss Parameters " description in the Parameters section.

Step 4: Adjusting the Screen Reconstruction

At this stage you will want to evaluate the how the screen area has been reconstructed. Check "Show Screen Reconstruction" on the RSC tab and switch to RGB display.

The Screen Reconstruction display represents the reference frame data that will be used (in lieu of an actual clean plate) to create a corrected foreground image for further processing in other nodes.

The goal, therefore, is to create a representation of what the screen would like if the foreground subject were not present, not to create a homogeneous field of blue.

Note:
The screen only needs to be reconstructed in the transition area which spans the narrow area between the solid parts of the foreground subject and the edge of the shape. When evaluating the reconstruction, you can ignore areas that you are sure fall completely within the solid part of the foreground subject.

If the reconstructed screen areas need adjustment, you can use the tracking parameters on the Reconstruct tab to control how edges, seams, and other discontinuities are extended into transition areas. (The parameters on the Reconstruct tab are explained in the Parameters section later in this node description.)

Tip:
If a dark or discolored smear appears to stain the screen reconstruction, it is usually an indication that the rotospline has accidentally touched the edge of a foreground subject (and the reconstruction process has treated it like a discontinuity and extended it into the transition area).
Return to Roto Edit mode temporarily and move the spline so that it no longer touches the foreground edge detail.

Step 5: Adjusting the GK Alpha Matte

Once you are satisfied with the screen reconstruction, you should turn off "Show Screen Reconstruction" and switch to GK Alpha display to view the temporary matte.

The GK Alpha matte is provided to enable you to use Ultimatte Intelligence, if necessary, to control what portion of the image is defined as solid subject area. This is a preparatory step to grain killing, because the areas defined as totally opaque foreground objects will not be grain-filtered. (Grain killing with Intelligence is performed in the Filter Area display.)

For example, if you boost the density values in GK Alpha to eliminate holes in the matte, the portion of the image that is defined as solid subject, and will therefore not be filtered, is increased accordingly.

The GK Alpha display shows the backing as white and the subject areas as black. If there are patches of gray or white in foreground objects that should be entirely opaque, click on the brightest gray or white pixel and Ultimatte Intelligence will automatically adjust the GK Matte Density or GK Black Gloss parameters.

These parameters, located on the Advanced tab, can also be adjusted manually. (The "Object Area (Green)" parameter on the RSC tab can also be used to manually adjust matte density.)

Step 6: Adjusting the Filter Area

After adjusting the GK Alpha matte, switch to Filter Area display to review how the transition areas inside of shapes will be grain-filtered.

This step is key to a good composite using RSC. If you leave excess grain or noise in the transition areas, you may be able to see the outline of the shape because it will stand out from the homogenous blue of the backing.

The Filter Area display shows the foreground subject in green (green areas are never filtered). The remainder of the area inside a shape is the transition area. White pixels within the transition area will be filtered; black pixels will not.

You can increase the area to be filtered by using Ultimatte Intelligence. If you click on an area of black that looks like grain or noise, Intelligence will adjust the Screen Filter Offset parameters (located in the Advanced tab).

All areas of black that meet similar criteria as the selected pixel will turn white and thus be subject to filtering. You can continue to click on black specks in the transition area until you are satisfied.

If you go too far, however, areas that should remain black will turn white, such as strands of hair and other edge detail. If these areas remain white, they will be filtered and thus blurred. Turn down the Screen Filter Offsets until these areas turn black.

Note:
As you adjust the filter area, be sure to view the RSC node output image by bringing up a regular Chalice monitor from the RSC node menu. You can compare the result against the source image by performing an A/B wipe in the monitor. You should also review the result composited over the background.
This is an essential part of the filter area adjustment process. If there is a problem, as when important edge detail has been accidentally filtered, this is how you will find out. The goal is not to make the Filter Area display look good, but to make the final composite look seamless.

If you are having trouble eliminating grain without also blurring important edge detail, try increasing the foreground subject area (green) to encompass more of the edge detail. (The green area is not affected by Screen Filter Offset adjustments.)

You can switch to GK Alpha display and use Intelligence to increase the green subject area, or you can adjust the GK matte controls manually. Then you can go back to the Filter Area display and try again to remove the grain.

You can also increase the number of filter passes to improve the grain removal. The Filter Passes parameter, located on the Advanced tab, is adjusted manually. However, the more you increase the number of passes, the longer the node will take to process.

Tip:
If necessary, you can turn off filtering temporarily while you are adjusting the filter area by resetting the number of passes to zero. (The black and white areas of the display will still update properly.)
But you must remember to reset the Filter Passes parameter to a nonzero value when you are done, or the output will not be filtered!

RSC Parameter Tab

The RSC parameter tab enables you to access the work monitor and other parameters.

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. For more information, refer to the "RSC Work Monitor " description in the previous section.

Show Screen Reconstruction

Once you have defined the subject areas and chosen a sample color, check this box to display a reconstruction of the current reference frame. If the reconstruction of the backing under any of the subject areas needs adjustment, use the parameters provided in the Reconstruct parameter tab.

Invert Roto Screen

Normally the area inside any shape that you draw is considered foreground and all areas outside the shapes are considered backing. Check this box to invert this definition, such that the areas outside the shapes become foreground and the areas inside the shapes become backing.

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.

Intelligence mode must be selected (not Roto Edit mode) to select a sample color. The mode selector buttons are located in the RSC work monitor panel. You can also key in RGB values for this parameter.

Filter Area & Object Area Parameters

These two parameters can be used to manually adjust the grain filtering process, if necessary. They should be used while viewing the Filter Area display in the RSC work monitor. (For more information about this process, refer to Step 6: Adjusting the Filter Area .")

Note:
Always start by using Ultimatte Intelligence to set the Screen Filter Offset parameter (located on the Advanced tab). Then, if necessary, the "Filter Area (White)" parameter can be used to add small amounts of white.

Filter Area (White)

This parameter adjusts the transition area of the matte used for grain filtering. Increasing the value (the default is 0) will increase the white area of the Filter Area display; that is, the area that will be grain-filtered.

Object Area (Green)

This parameter increases or decreases the density of the matte used for grain filtering. Increasing the value effectively expands the green foreground area in the Filter Area display. (A corresponding expansion will also appear in the black area of the GK Alpha display.)

Reconstruct Parameter Tab

The Reconstruct parameter tab enables you to finetune the screen reconstruction process as needed for a particular scene by using directional tracking.

Directional tracking, in this context, is the process of identifying discontinuities (flaws in the backing area) and extrapolating them into the reconstructed screen area behind the subject. When directional tracking is enabled, the RSC node identifies those discontinuities that follow the same vector as the selected tracking direction and extends them into the screen area. To see the effect, you must turn on Show Screen Reconstruction (the checkbox is located on the RSC tab) and select RGB display in the work monitor.

You can specify how discontinuities are extrapolated by using the four parameters that modify directional tracking: Region Reach, Discontinuity, Thickness, and Angle Span.

Tip:
Select the "No Smoothing" option from the Smoothing menu while adjusting these parameters. This will make it easier to see and follow the extrapolations into the shape area as you change the parameter settings.

"Enable Tracking" Parameters

There are four directional tracking options available:

  • Enable Left-Right Tracking
  • Enable Right-Left Tracking
  • Enable Top-Bottom Tracking
  • Enable Bottom-Top Tracking

You can check as many options as needed for adequate screen reconstruction of the shot. For example, if a horizontal seam in the bluescreen is visible coming in from the left side of the image and going behind the subject, check Enable Left-Right Tracking.

Region Reach

The Region Reach parameter modifies how far into the subject area (inside the splines) the discontinuity will be extrapolated. The range is 0-100, which represents a percentage of the total width of the image.

Discontinuity

The Discontinuity parameter is a threshold that limits what features in the backing are defined as discontinuities that should be extrapolated into the screen reconstruction area.

The RSC node evaluates differences in pixel brightness to identify discontinuities, and only those discontinuities with values that exceed the specified threshold of this parameter are extrapolated.

The value is a function of pixel brightness and the range of 0-100 represents normalized brightness value.

Thickness

The Thickness parameter modifies the width, or spread about the vector, of the discontinuity as it is extended into the subject area. The range is 0-100, which represents a percentage of the total width of the image.

Angle Span (Degrees)

The Angle Span parameter enables you to control whether those discontinuities that deviate from the selected tracking direction will still be interpolated into the subject area.

For example, if Left-Right Tracking is selected and the Angle Span parameter is set at 10 (which is the default), the RSC node will interpolate not only those discontinuities that follow the X axis of the image exactly, but also any discontinuities that describe a slight angle to the horizontal--up to 10 degrees in either direction.

The parameter range is 0-90. A value of 0 specifies no deviation from the tracking angle, while a value of 90 specifies that a discontinuity can deviate up to 45 degrees in either direction (a total span of 90 degrees).

Smoothing

Smoothing refers to the process of filling in the shape areas during screen reconstruction; therefore, you should always use the default setting "Smooth After Combining" for the final output.

However, you will most likely want to turn off smoothing (select "No Smoothing" from the popup menu) while adjusting the Reconstruction parameters.

Advanced Parameter Tab

The Advanced tab provides parameters for making fine adjustments to the temporary matte data that the RSC node creates as part of the screen filtering and grain filtering processes.

In most cases you will want to use Ultimatte Intelligence to adjust these parameters, as described in "Using Roto Screen Correction ." However, these parameters can be adjusted manually.

Filter Orphans

This option, which is on by default, eliminates stray single pixels, or " orphans," from the transition area. Orphans are pixels that were missed in the grain filtering process.

Filter Passes

The Filter Passes parameter enables you to specify the number of times the image data will pass through the grain filter. The default value is 2.

If necessary, you can increase this value; however, the greater the number of passes, the longer the process will take. (You can set this parameter to 0 to turn off filtering temporarily while adjusting the Filter Area.)

Matte Density & Black Gloss Parameters

The SC Matte Density and SC Black Gloss parameters are used in conjunction with the SC Alpha monitor display to enable you to control the density of the matte channel data that is generated by the node as part of the screen correction process. (Refer to Step 3: Adjusting the SC Alpha Matte of the previous section on Using Roto Screen Correction .)

The GK Matte Density and GK Black Gloss parameters are used in conjunction with the GK Alpha monitor display to enable you to control the density of the matte channel data that is generated by the node as part of the grain killing process. (Refer to Step 5: Adjusting the GK Alpha Matte of the previous section on Using Roto Screen Correction .)

All of these parameters function by minimizing patches of gray or white within the black foreground objects of the matte.

In most cases, you will want to use the parameter values set by Ultimatte Intelligence. 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. The associated monitor display will update to show you the result of any manual adjustments.

Try adjusting Matte Density for bright objects, and Black Gloss for dark, or black glossy, objects.

Shadow Noise

The Shadow Noise parameter is used to adjust the SC Alpha matte data to reduce noise in shadows and glare areas.

Screen Filter Offset

This parameter is used in conjunction with Filter Area display to remove grain from transition areas. Each time you click a pixel of grain in the Filter Area display, Ultimatte Intelligence will adjust these values accordingly. If these values get too high, you will begin to filter the edges of the foreground subject, resulting in a loss of detail.

This process is described in Step 6: Adjusting the Filter Area , in the previous section on Using Roto Screen Correction .

Reset Parameters to Default Values

Click this button to reset all of the parameters on the Advanced tab to their default values.

Note:
It will also reset the two Screen Filter parameters (Filter Area and Object Area) on the RSC tab to their default values.



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