Archive:Kdenlive/Manual/Effects/Alpha manipulation
Effects - Alpha manipulation
General Information about Alpha Manipulation
As far as I am aware Alpha Manipulation effects are allways used in combination with a composite transition. This because Alpha manipulation is to do with manipulating the so called "alpha-channel" of images and videos. The Alpha channel is data that describes how two images are to be combined (or composited) into one. So you need a composite transition on the timeline to tell Kdenlive that you are compsiting the videos from two different tracks into one image.
For some background the Wikipedia article in alpha compositing is usefull. See also Porter, Thomas; Tom Duff (1984). "Compositing Digital Images". Computer Graphics 18 (3): p253–259 1984 pdf