How to stitch multiple flat virtual images in a grid with Hugin or other FOSSoftware?

Question

I am trying to stitch together screenshots of a map, so that I can get a higher resolution version of it to later vectorize and work with. The screenshots will have to be aligned in a grid of 3x3 or 4x4. They all have the same orientation and zoom level, and all overlap by a fair bit. However I am having trouble doing this job in Hugin.

I tried following the Hugin tutorial on Snitching flat scanned images, but I wasn't able to get the pictures to align correctly. So, how do I do this? Thank you.

Answer 1

After some more studying the program and testing around I finally got it to work! Here is every step I took:

• Launch Hugin (version 2019.0.0.a369cbe55179, on Manjaro Linux);
• Interface menu > Expert;
• In the Photos tab, add images (In my case they are a 3x4 grid, all are 1920x1080, but I have also tried a set of images that don't have a clear shape after stitching). When asked, I set 1 as HFOV and leave Normal (rectilinear) as lens type;
• I set one of the central images as anchor by right clicking in the image and choosing Anchor this image for position/exposure, then proceed to right click on every image except the first one and choose Lens > New lens. I also go in Optimise > Geometric and choose Custom parameters;
• Switch to the Control points tab and let Hugin's engine (default Hugin's CPFind) detect control points between pairs of images. I also make sure that every image has CPs with images overlapping from top/bottom, left/right and angles (is this necessary or overkill?). After every run of CPFind, I check for false positives, a quick way is to compare the distances of every cp, in my case they all match except the false positives;
• In the Optimiser tab, I unselect all Yaw, Pitch and Roll, and select all X and Y (the anchor has nothing active). Under Lens Parameters I select all Hvof (v). Then click on Optimise now! and apply the changes;
• Open the Preview panorama window and choose Straighten, you should be able to see the stitched image at this point;
• Open the Stitcher tab, click on Calculate Optimal Size and Fit Crop to Images, eventually choose your format and other settings, and Stitch!

I couldn't get it to work before because the tutorial that I was following, Stitching flat scanned images, suggested to also activate all Z in the Optimiser tab, or alternatively all Roll and under Lens Parameters all Hfov, d, e.

I hope that this will be useful to someone in the future.

Answer 2

The 'Optimise > Geometric and choose Custom parameters' is very important as otherwise the 'Optimiser' tab is not shown and the menu option is greyed out without explanation.

To set it, go to the 'Photos' tab, then near the bottom is an 'Optimise' section. Set 'Geometric' to 'Custom parameters' and the Optimiser tab should appear.

Answer 3

In more recent versions of Hugin (I'm using 2022.0.0.9bb147ae7161 built by Thomas), having loaded the images you can simply select "Scanned images" when aligning:

This worked flawlessly for me to stitch together several top-down photos of parts of a map into a single "panorama" of the complete map, virtually scanning it.