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 compositing the videos from two different tracks into one image.
This image shows two video tracks with no transition.
The project monitor shows how the upper track takes precidence when no transition is in play.
This image shows two video tracks with a composite transition in place but with no Alpha information to tell Kdenlive how to combine the images.
With composite transition but no alpha info to describe how to composite the picture the top track still takes precidence.
This image shows the situation with Alpha info now added via the blue screen method.
The blue screen effect is also known as "Color to alpha" [1] and all the blue screen effect does is say that where the video is blue the alpha value is 0 (ie transparent).
So now that we have alpha information the second tracks image now shows through where the alpha channel says the image should be transparent.
Hint for previewing the alpha manipulation effects
The preview in Project monitor goes all strange when you select the composite transition in the time line or the video 1 in the time line.
What the project monitor looks like when you have just selected the composite transition in the time line. This is not what the rendered video will look like. To get a look at what the video will render as you need to click Video 1 and then Video 2 in the timline.
Compositing Reference Material
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