Chromakey Node

The Chromakey node enables you to create an alpha channel matte based on the colors of the input image. This node uses the values of the original image within a narrow range of hue and saturation to create the matte.

The Chromakey node features Setup and Chroma Select parameter tabs. The Setup tab enables you to specify the type of output and access the Chromakey work monitor. The hue and saturation are controlled by using a color wheel on the Chroma Select tab. The default value is the common "bluescreen," with colors centered on a hue of 230, plus or minus 10 (that is, hues 220 through 240) and saturations in the range of 0.5 to 1.0.

The Chromakey node accepts one or two inputs:

  • the primary (top) input image, which must have an alpha channel, and
  • the optional (bottom) input, which is used as a control image for the chromakey process.

Setup 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, which enables you to select hue and saturation parameter values interactively, as described in Using the Chromakey Work Monitor in the Chroma Select parameter tab description.

Output

The Output popup menu enables you to select whether the Chromakey node should output "All Channels" available in the source image, the "Alpha Channel Only," "Inverted Alpha Only," or "All Channels, Invert Alpha."

Control Channel

The Control Channel parameter enables you to specify which channel of the control image to use. The control image is the optional second input to the Chromakey node. The value of each pixel of the selected channel in the control image determines whether the corresponding pixel in the primary input image will be processed.

For example, if a pixel in the control image is black (i.e., has a pixel value of 0), the corresponding pixel in the primary image will not be processed. However, if a pixel in the control image has a value other than 0, the corresponding pixel in the primary image will be processed.

Chroma Select Parameter Tab

The Chroma Select parameter tab enables you to specify the Hue and Saturation, Luminance, and Softness for an alpha channel matte.

Hue and Saturation

The Hue and Saturation parameters enable you to define the range for both hue and saturation by manipulating a color wheel (see below), keying in values, or setting tuners.

Using the Color Wheel

You can specify a color range by using the color wheel, which contains a selection box with borders that you can click and drag. Drag the dashed selection box to adjust the hue. Click the outer borders of the box and drag radially (around the wheel) to adjust the total width of the selection box and thus the hue range. Click the outer borders of the box and drag axially (towards or away from the center of the wheel) to adjust the total length of the selection box and thus the saturation range.

Using the Chromakey Work Monitor

You can also identify a color range by clicking on an area in your source image, as displayed in the Chromakey work monitor. (This monitor is accessed from the Setup parameter tab.)

Click on an area in the image that contains the desired range of color. The color range that you select from the image will automatically be entered as values in the Hue and Saturation fields, and the selection box will shift to the corresponding location in the color wheel.

Clicking and dragging through a range of colors in the monitor image will enter corresponding values for the Hue and Saturation parameters.

Tip:
To include a color range from a different part of the source image, hold down the Shift key while clicking and dragging the mouse.

Using the Tuners and Data Entry Fields

Alternatively, you can key in values or set the appropriate tuner. For the Hue (H) parameter, you can key in a value or set the tuners to a value between 0 and 360, inclusive. For the Saturation (S) parameter, you can key in a value or set the tuners to a value between 0 and 1, inclusive.

Two pairs of tuners are provided for setting the Hue and Saturation ranges: the Hue tuners control the radial boundaries of the selection area (in the Color Wheel); the Saturation tuners control the axial boundaries.

Luminance

This parameter enables you to select a range of luminance to use for the matte. Click and drag the ends of the arrows to set the range. The associated fields will update the high and low range values accordingly. Alternatively, you can key values from 0 to 1 directly into the fields.

You can also use the Chromakey work monitor to select luminance values by clicking in the alpha channel of the source image. Select Alpha from the Channel Display button popup menu in the Monitor Toolbar to display the matte data. The Chromakey node will create a luminance value based on the pixel area you have selected in the monitor.

Softness

The Softness parameter enables you to control the softness of the matte by keying in a numerical value between 0 and 1.0 or using the mouse to set the tuner to the desired value.

A value of 0 represents no softness, i.e., a hard, opaque matte. The closer the value gets to 1, the softer (more transparent) the matte becomes.




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