Chroma is the Key
In the technique called chroma keying, those parts of each frame which exhibit a certain color ("chroma") are rendered transparent, thus allowing some other image to appear as background in the finally assembled scene. This effect can, for instance, be used to place actors/actresses into some virtual environment. To this end, all the actual acting has to happen in front of a uni-colored (usually blue or green) screen -- hence the alternative terms "blue screen" or "green screen" for this technique. During post-production, the blue or green parts of the individual video frames are (nowadays: digitally) replaced with a background (or "matte painting") image depicting the target environment.
Here's our first attempt using a green plastic box as background for a short brick movie and applying Kdenlive's chroma key effect implementation:
As you can see, the keying procedure did not work entirely satisfactory due to uneven illumination of the green background. The remaining greenish areas could be faded out by allowing for a greater deviation from the key shade of color, but then parts of the foreground featuring similar colors would also become transparent.
Also still visible is a thin green fringe around foreground objects, indicating that some enhanced feathering technique would be necessary to clearly distinguish between foreground and background. Unfortunately, no such effect is available in Kdenlive at the time of this writing.
However, another useful post-processing filter is provided:Key spill mop-up! This filter helps to reduce the color tinge intorduced into the foreground by colored light diffusely reflected from the green screen: