<mediaobjectco> and calspair unit coords

Hello,

I would like to have some precision about how the  
mediaobjectco/areaspec/area/@coords should be interpreted for calspair  
units.

I understand it is a percentage (for x and y) of the total size in the  
related direction, but which size?

The question I have: is the total imagedata size to use is the imagedata  
viewport size (width and depth) even when the viewport size is different  
 from the image size itself (because a scale or contentwidth attribute is  
specified, or because scalefit=0)? Or is it the image size (maybe scaled)  
to use, where the coords (0,0) are the left corner of the image itself,  
and not the left corner of the viewport?

I hope that my questions are clear enough.

Thanks,
BG