gimp - Gigapixel panorama problems

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.

Answer

I got it to work. I had two trials.

First, I looked for a similar situation among my existing files, I found three from the same perspective at different focal lengths. They were much closer than your range, though (18mm to 55mm). Hugin could read the EXIF data and adjusted horizontal field of view (hfov, parameter v) accordingly.

Then I simulated your more extreme situation using an existing panorama and "photographed" it at different "focal lengths", i.e. I cropped out some details and scaled them down appropriately.

Here, Hugin choked. First no Exif data and thus no v-parameter to start off with. It could not find any matches. I then went and manually created control points. This is a bit of work (auto-adjustment won't work), and then had it optimized using position and view (view is important!). This worked surprisingly well.

Here is what to try: Load all images, have Hugin find correspondences. There will be two sets (ignoring 300mm for now) of connected images: The 75mm-set and the 18mm-set. Now, you should be able to manually create control points between images from the two sets. Then, optimise.

My Hugin version: 2011.4.0.cf9be9344356

Please let me know how this works out for you. If it doesn't work, maybe you could upload some pictures.