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These controls let you crop out other unwanted things in the frame, such as lighting fixtures, boom microphones, tracking patches, etc.
— Matte Shape: You can choose either a Rectangle or Ellipse with which to crop around the subject you’re keying. Choosing None turns off Garbage Matte. When a garbage matte is enabled, the Open FX Overlay mode of the Viewer shows an on-screen control for adjusting the shape and position of the garbage matte.
— Edge Softness: Lets you soften the edges. Defaults to 50; the range is 0 (no softening) to 100 (maximum softening).
— Invert: Inverts the garbage matte.
— Center X and Y: Lets you adjust and keyframe the position of a garbage matte.
— Bounding Width and Height: Lets you adjust the width and height of the garbage matte.
— Rotation: Lets you adjust the angle of the garbage matte.
Output
The Output controls let you choose how the image is output, composited, and displayed in the Viewer.
— Output: This menu offers three options for which channels are output from the effect.
— Alpha Highlight: Displays the transparent part of an alpha channel as gray and the solid part of an alpha channel in full color.
— Alpha Highlight B/W: Displays the black and white alpha channel, which is often helpful when sampling additional areas to key.
— Final Composite: The keyed image is displayed composited over any video track below it.
— Use Alpha: A common control in many Resolve FX and 3rd party Open FX plugins; this checkbox determines if the alpha channel is used when compositing over another video track. It can be used instead of choosing RGB Only (Blank Alpha) from the Output menu.
1 Apply the Luma Keyer to a clip on video track 2 or higher.
2 From the Viewer Overlay menu in the lower-left corner of the Timeline Viewer choose Open FX Overlays.
3 Either click a pixel in a bright or dark part of the image you want to key out, or click and drag across a range of pixels within that subject.
4 The transparency immediately takes effect, showing the keyed subject against whatever clip appears on the video track underneath it in the Timeline as a composite. To see the matte you created for further adjustment, choose Alpha Highlight or Alpha Highlight B/W from the Output drop-down menu.
5 To add or subtract from the matte, click the plus or minus Luma Range buttons, and then click or drag across the portion of the keyed image.
6 To add softness to the outer range of the key you’re creating, click the plus Softness button and then click or drag across the portion of the image you’d like to include as a soft edge.