What does an alpha channel in video editing primarily control?

An alpha channel primarily controls transparency in video editing. This channel allows editors to define the visibility of different parts of a video frame, determining which areas are fully opaque, partially transparent, or entirely transparent. By manipulating the alpha channel, you can create effects such as overlays, cutouts, and compositional layering, enabling complex visual storytelling and seamless integration of various graphical elements.

For instance, when layering multiple video clips or images, the alpha channel lets you see through the upper layers to the layers below, thereby creating a composite image. Its functionality is essential for keying effects, like green screen technology, where the background can be made transparent to reveal the subject in the foreground.

The other options focus on different aspects of video properties. Color value pertains to hue and saturation, brightness affects the overall lightness or darkness of an image, and contrast refers to the difference in luminance or color that makes an object distinguishable. While these elements are crucial in video production, they do not relate to the transparency control that the alpha channel provides.