ffmpeg 0.11.x how to achieve a linear moving crop or moving focus? Advanced

[feelart]

RE: ffmpeg 0.11.x
Hi,

A fix crop, can be achieved by:

ffmpeg -t 60 -i "M4H00773.mp4" -vf crop=820:660:240:20 -acodec copy -sameq outCrop.mp4

Now I want my 820x660 cropping rectangle to start at time 0 sec at (0,0) and finish at (240:20) after 60 sec, this in linear translation, i.e.

see http://en.wikipedia.org/wiki/Slope

How can this be achieved?
Some evaluation, select command ???

Thanks