Ultimatte GK ( Grain Killer) Node

The Ultimatte GK (Grain Killer) node enables you to remove film grain from the screen areas, or backing, of a foreground clip. (It is also effective at reducing visual noise, such as that introduced by video cameras under low-light conditions.)

One of the greatest strengths of Ultimatte is its ability to retain fine details in a composite. However, this means that film grain (or video noise) in the backing area of the foreground image may be composited onto the background and result in a double-grain effect in those areas.

The Ultimatte GK node is designed to eliminate this problem by recognizing and filtering only the backing area of the foreground image. This enables you to avoid softening the foreground subject and eliminating detail. (See "How Ultimatte Generates Matte Data " for an explanation of the process Ultimatte uses to recognize backing areas of a foreground image.)

Ultimatte GK filters grain pixels in the screen area by averaging the values of surrounding pixels to create replacement values for the pixel currently being processed.

The Ultimatte GK node accepts one input.

Using Grain Killer

The first step to using the Ultimatte GK node is to specify a sample color of the backing area that represents the ideal color for Ultimatte Intelligence to use as a reference.

Sample Color

If the input to the GK node is an Ultimatte CSC node--and this is highly advised, unless you have a foreground image with perfectly even backing--use the same sample color value in the GK node that you used for the Ultimatte CSC node. You can

  • copy and paste the values from the Sample Color parameter entry fields.
  • use the STED to export the Ultimatte CSC sample color values and then import them into Ultimatte GK.
  • check Expressions and use the ch function to access the sample color data.

If your shot does not require the use of the CSC node, select a sample color by opening the GK work monitor to RGB display and clicking the appropriate pixel. See "Selecting a Sample Color " in the Ultimatte CSC node description for guidelines on selecting the best color.

Alpha Matte

The next step is preparatory to the actual grain filtering process. Switch the monitor to Alpha display, which shows foreground subjects as black and the backing as white. You can then use Ultimatte Intelligence to adjust the matte density, if necessary.

Solid foreground subject areas are not filtered by the GK node, and this process enables you to control what is defined as the subject by adjusting the Matte Density and Black Gloss parameters on the Advanced tab.

Filter Area Matte

The final step is to switch to Filter Area display in the GK monitor. This display shows foreground subjects in green, which indicates that they will never be filtered. White areas will be filtered and black areas will not; however you can change black pixels to white by using Ultimatte Intelligence.

Click on a black area that looks like grain and Ultimatte will adjust the Filter Area matte accordingly by setting the Screen Filter parameters on the Advanced tab (you can also adjust these parameters manually).

You can repeat this process as many times as necessary to achieve the best result. Be sure to review the GK node output as it will be composited. If you begin to blur important edge detail, you can increase the matte density to turn these areas green, so that they will not be filtered. Increasing the number of Filter Passes may improve the effectiveness of the grain removal, although it will also increase processing time.

Ultimatte GK Parameter Tab

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 mode. 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 following displays enable you to use Ultimatte Intelligence to make the indicated parameter adjustments:

  • RGB display enables you to select a sample color for the backing by clicking on the appropriate pixel.
  • Alpha display enables you to view a temporary matte image that is used to adjust the Matte Density and Black Gloss parameters on the Advanced tab.
  • Filter Area display enables you to view a temporary matte image that is used to adjust the Screen Filter parameter on the Advanced tab.

Filter

The Filter parameter displays the type of filtering process used by the node, which is Average. This filter averages the color values of surrounding pixels to create replacement values for the grain pixels being filtered.

Pixels are averaged 25 at a time, in a 5 by 5 matrix, as long as the matrix does not encroach on transition or foreground areas. For pixels too close to a transition edge for an unobstructed 5 by 5 square to be defined, 9 pixels (in a 3 by 3 matrix) are averaged.

Filter Orphans

Check this box to eliminate stray single pixels, or " orphans," from the screen area. These orphans are pixels that failed to be filtered by the node.

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 work monitor. You can also key in RGB values for this parameter.

(Refer to "Using Grain Killer " in this node description for more information about using this parameter.)

Reset Parameters to Default Values

Click this button to reset the parameters on the Advanced tab to their default values. This will not affect the parameters on the Ultimatte GK tab. (To reset all node parameter values to their default settings, select the "Defaults" option from the node's Presets menu.)

Advanced Parameter Tab

Filter Passes

The Filter Passes parameter enables you to specify the number of times the image data will pass through the filter. The default value is 1; however, multiple passes may be necessary. If the backing still appears mottled after the initial pass, try increasing the setting. The greater the number of passes, the longer the operation will take.

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. (Select Alpha display in the work monitor to view this matte.)

Control of matte density means control of what portion of the image is defined as solid subject area. This is important because the areas defined as totally opaque foreground objects will not be grain-filtered.

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.

In most cases, you will want to use the parameter values set by Ultimatte Intelligence, as explained in the "Using Grain Killer " section. 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:

  • Use Matte Density to adjust light or bright colored foreground objects.
  • Use Black Gloss to adjust dark, reflective foreground objects.

Screen Filter

This parameter displays the values that Ultimatte intelligence sets each time you click a pixel of grain in the Filter Area matte display. The higher the values, the greater the area of backing that will be filtered.

If these values get too high, you will begin to filter the edges of the foreground subject, resulting in a loss of detail. To ensure that these values are set properly for your shot, be sure to review the resulting image in another monitor and judge for yourself.




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