Linefind is a detector for vertical features in images. It tries to find vertical lines using the same algorithm as Calibrate_lens_gui and assigns vertical control points to them. The detection runs on the source images, if the input images are rectilinear images. In the other cases (e. g. fisheye images) the input images are reprojected to an equirectangular projection and the detection works on the reprojected images. It uses the roll value, saved in the pto project file, to determine what is "top" and "bottom". So before running linefind check that your roll values are correct (E. g. when using images straight from the camera, use a roll value of 0 for landscape and a value of 90 or 270 for portrait images. If your camera has an orientation sensor, Hugin can detect this information automatically and sets the roll value accordingly when adding such images into Hugin.
The general usage is
linefind -o output.pto input.pto
If the --output/-o switch is missing then the suffix "_line" is added to the filename.
The maximal number of lines added per image can be given with the --lines switch (or short -l):
linefind --lines=10 -o output.pto input.pto
By default maximal 5 lines per image are added to the project file.
Normally linefind tries to find vertical lines in all images of the project file. If you want to limit the detection to some selected images, you can use the --image/-i switch, e.g.
linefind --image=0 --image=4 -o output.pto input.pto
will only search for vertical lines in image 0 and 4.