Darken emphasizes the darkest parts of each overlapping image. Whites in either image
allow the overlapping image to show through completely. Lighter midrange color values
become increasingly translucent in favor of the overlapping image, while darker
midrange color values below a certain threshold remain solid, retaining more detail.
The order of two clips affected by the Darken composite mode does not matter.
Suggested uses: The Darken composite mode is useful for using one image to add
texture to another selectively, based on its darker areas. You can also use Screen for
variations on this effect.