This filter is considered broken/deprecated: see discussion
The purpose of this filter is to progressively darken the top of the image;
for example, to compensate for certain VCRs which make the top of each frame
brighter than the bottom. In practice it only works for NTSC content and it
needs its fields separated.
When applying it to an image with height > 255 pixels, you will see a repetitive
gradient. So it doesn't work for field-separated PAL content.