Image stitching

Image stitching or photo stitching is the process of combining multiple photographic images with overlapping fields of view to produce a segmented panorama or high-resolution image. Commonly performed through the use of computer software, most approaches to image stitching require nearly exact overlaps between images and identical exposures to produce seamless results.

Stages of the stitching process

Image calibration (perspective correction, vignetting correction, chromatic aberration correction). Images are processed in this stage to improve results.
Image registration (analysis for translation, rotation, and focal length). Direct or feature-based image alignment methods may be used. Direct aligment methods search for image orientations that minimize the sum of absolute differences between overlapping pixels. Feature-based methods determine proper image orientations by identifying features that appear in multiple images and overlapping them.
Image blending; combining the sections, possibly involving:

Color correction (matching the adjoining areas of the component images for color, contrast and brightness to avoid visibility of the seams).
Dynamic range extension.
Motion compensation / deghosting / deblurring, to compensate for moving objects.

Not of all these tasks may be performed. In their omission, artifacts may result.

Projection geometry

For image segments that have been taken from the same point in space, stitched images can be arranged using one of these graphical projections:

Rectilinear projection, where the stitched image is viewed on a 2D plane.
Cylindrical projection, where the stitched image shows a 360° horizontal field of view and a limited vertical field of view. Panoramas in this projection are meant to be viewed as though the image is wrapped into a cylinder and viewed from within. When viewed on a 2D plane, horizontal lines appear curved while vertical lines remain staight.
Spherical projection, where the stitched image shows a 360° horizontal by 180° vertical field of view. Panoramas in this projection are meant to be viewed as though the image is wrapped into a sphere and viewed from within. When viewed on a 2D plane, horizontal lines appear curved as in a cylindrical projection, while vertical lines curve as they get closer to the poles of the sphere.

Challenges of image stitching

Compensation for images not taken from the same place (on a pivot about the entrance pupil of the camera), which can introduce parallax errors between images.
Coping with dynamic scenes, such as voluntary or wind-induced motion. Dynamic scenes can have ghosting or blurring artifacts as a result of time differences between the image segments.
Recognition of component images belonging to a particular scene from an unsorted set of images ("blind" stitching) through feature-based alignment methods; see autostitch.

Image stitching applications

Dedicated programs include Autostitch, Hugin, Ptgui, Panorama Tools, Photostitch, and CleVR.

Many other programs can also stitch together multiple images. A popular example is Adobe Photoshop, which includes a tool known as "Photomerge" and the new "Auto-Blend" in CS3. Also, Photoshop has a "Nudge" option, which makes it easy to manually stitch two pictures together if there's enough contrast along their edges.