how I can belive that it is impossible to use same senzor to capture smaller resolution ?... It is like you say for cannon sx20is, that is capable to make movies in HD 720p format (1280x720) but cannot make VGA movies...
If chdk have not support to enable 16:9 format for low resolution, this is it. But, this, do not proove that is impossible... To have 640x360 instead 640x480 it is a matter of software. Hardware (senzor + cpu) that is capable to compress 640x480 can also do 640x360. Maybe I'm wrong, but nobody try to explain why...
CCD readout is not like RAM where you can read the value of arbitrary pixels. The readout process also things also depend on various clock signals etc. that may or may not be software programmable in a given implementation.
Maybe canon firmware could be hacked to discard some lines while making the video file, but this is unknown, and if possible would probably require a lot of work.
