I took hundreds of pictures from a 75mm-300mm lens in Cristo Rei(Lisbon), but since Hugin (panorama tools GUI) canoot distinguish well water from... water(neither i) i took the river pictures with a 18-55mm lens at 18mm(APS-C), and the remaining with 75mm (and some monuments with 300mm). My problem is that since the river-photos are much wider, and lower-res., hugin cannot find any matches nor know how to scale them.
To solve it, i tried to scale them (4 photos) by 376.77% (63.3º / 16.8º * 100) to match 75mm and put them as background, but each picture is so big that i have not enought RAM to process those huge pictures(I only have 3GB). How can i do it? Should i split every scaled photo into about 20 pictures? But then, how will hugin distinguish water from more water?
How do photographers usualy make rivers in gigapixel photos?
If you want to suggest an alternative GUI/program, then it must work in GNU/Linux.