Convert an existing video into a grayscale mask using luminance or auto. Keep continuous tones, apply a threshold from 0 to 255, or invert the result. Useful for animated graphic mattes, title reveals, moving light masks and transitions. Explicit alpha mode is currently rejected by the video backend despite appearing in the input options; auto did not extract alpha in testing. Use an opaque luminance source. This is channel conversion, not object segmentation. Confirm polarity with the downstream tool.