Why does Hugin distort the output image?

Question

I am trying to create a batch process for stitching images together with Hugin, however I end up with distorted images, and can't find out why.

As an example I am using these two images:

And using the process described here with this sequence of commands:

pto_gen -o project.pto image_01.jpg image_02.jpg
cpfind --multirow -o project.pto project.pto

celeste_standalone -i project.pto -o project.pto

cpclean -o project.pto project.pto

linefind -o project.pto project.pto

autooptimiser -v 20 -a -l -s -m -o project.pto project.pto

pano_modify --center --straighten --canvas=AUTO --crop=AUTO -o project.pto project.pto

echo "stitching part 1..."

nona -m TIFF_m -o project project.pto

echo "stitching part 2..."

enblend -o project.tif project0000.tif project0001.tif

rm project0*.tif

However I always end up with an image that's distorted like this:

Why is that? at what stage does the distortion happen, and how can I fix that?

Answer 1

I think there are two or three possible issues: one with the commands and two with your photos.

1. Make sure you (globally) understand the commands you are running. As you have seen for yourself, the resulting picture is a bit disappointing. Check the Hugin components manual for the different commands, their options and their meaning. For example, you use the -s option with the autooptimiser. This automatically tries to pick a suitable projection. I think this is one of the causes for the distorted result. As suggested in the comments, start with a more basic set of commands, and take it from there. For example, this should be a good start:

    # Generate a project file
    pto_gen -o project.pto ./top.jpg ./bottom.jpg
   
    # Auto optimise (note that I don't include the -s option here
    autooptimiser -a -l -m -o ./project.pto ./project.pto
   
    # Set canvas and crop
    pano_modify --canvas=AUTO --crop=AUTO -o ./project.pto ./project.pto
   
    # Process each of the images
    nona -m TIFF_m -o ./project.pto ./project.pto
   
    # Blend the resulting images
    enblend -o project.tif ./project.*.tif
   

2. I suspect you took the pictures while holding the camera in your hand. As you can see below, I managed to blend the images, but the don't stitch very nicely. This is because there must be a minor shift in the point from where the camera took the first and the second picture. Hugin (or any panorama software) can't correct that. Next time, use a tripod to fixate the point from where the panorama pictures are taken from.

3. It really helps if you put exif info in the file for Hugin. This "metadata" is automatically stored in the image file format by your camera. This includes focal length, which is an important part of the info that Hugin needs to process your photos correctly. Always make sure you use "straight from the camera" images for stitching.

Finally: if you have the opportunity, try the Hugin application (Windows/OSX/X). This guides you through the process and gives you a much more friendly user interface.

Answer 2

I walked through your commands and it looks like --straighten is the primary culprit here.

Taking out --straighten produces:

I believe this is a default rectilinear.

Using the interim GUI editor to switch to cylindrical produces:

I'm sure there's much more that can be done, but I haven't played with it before now.

Answer 3

While scripting may be a fun foray, I would use the GUI since there are a lot of tweaks to get a good looking pano.

Here's the script I used on your project:

pto_gen -o zit.pto 1.jpg 2.jpg
cpfind --multirow -o zit.pto zit.pto
cpclean -o zit.pto zit.pto
autooptimiser -a -l -s -m -o zit.pto zit.pto
pano_modify -o zit.pto --projection=6 --center --straighten --canvas=AUTO --crop=AUTO zit.pto
nona -m TIFF_m -o zit --gpu zit.pto
enblend -o zit.tif zit0000.tif zit0001.tif

• I added --projection=6 in pano_modify. This corrects the weird projection you got. You may want to try different projections.

• I added --gpu to nona to speed things up.

• Since you don't have sky in your project, celeste isn't needed.

• The image is bowlegged, so you'll need to figure out how to correct that.

• If you run this through Hugin's GUI interface, things look better.

• Looking at your images in Hugin's preview, it's readily apparent that you did not take care in panning about the lens entrance pupil (where the apparent lens aperture is located). The pillar on the right did not stitch properly due to parallax issues. When hand-holding your camera, use your finger as the point to pan your camera, usually close to the front element on your lens. You'll need to run some experiments to find where your lens nodal point is. Aperture setting could change your nodal point. Enblend does a pretty good job at hiding parallax issues which is why your image still looks decent. More information about pivot point.

• In Hugin GUI, you can place vertical control points and remove the vanishing lines to have a nice architectural projection.