Two adjacent images, like many of those that Hugin’s users may want to stitch to a bigger picture. Unfortunately, these two won’t stitch well.
Looking at the detail below the relative position of the finger and the library’s vertical support beam has shifted when moving the camera.
This shift is called parallax and it is the most common problem for stitching pictures.
Feel it yourself: Stretch an arm in front of you, like the above picture. Raise a finger and note its position relative to an object in the background. Turn your head left and right and notice how the position of the finger relative to the background changes. Two images taken like that won’t stitch.
Now try a variation on the above experiment: Don’t move your head. Close one eye and move the other eye left to right. Unless wearing glasses or contacts, you will notice that the relative position has not shifted. The nodal point of the rotation is better aligned with the no parallax point (NPP) of your eye.
Your eye focuses all rays of light into one point before expanding them on the retina. Most lenses do the same before expanding the image on the sensor. The NPP is this one theoretical point inside the lens where all the rays of light focus.
Turning a camera around the lens’ NPP is the single most important precaution to achieve a perfect stitch.
To do this, we need to identify the NPP of the camera and constrain its rotation to that point. How to identify the NPP will be topic of a future tutorial.