Image stitching

Stitching (English stitch " sewing ", " attach " ) used in photography to create a great photography from various smaller individual shots. Stitching is mainly used for taking pictures of objects which can not be captured with a single exposure because of their size. These individual recordings of excerpts are combined to form a larger image. Application examples are panoramic image ( for example, from landscapes) or total viewing of large objects (for example, architectural shots ). The technique is also suitable for photographing an object in individual parts, to obtain a high resolution image of it. In this way, the limited resolution of a camera can be extended by subsequent processing in the computer to build large- format photographs.

• 4.1 Stitch software ( selection)

Projection method

Photography is a two-dimensional image of the three-dimensional environment. The reduction from three to two dimensions takes place when photographing. Stitching requires, firstly, that all the sub-images are made ​​with the same reduction ( ie from the same recording stand point), on the other hand it is not possible to change this reduction (that is, one can not preclude the host state point or vanishing point subsequently change ). You can change the type of map from the two-dimensional ( declination, right ascension ) on the Cartesian coordinate system of the monitor or paper ( x, y).

Stitching is done in two steps. First, the conversion of the output images is done in the same Cartesian coordinate system, followed by the assembly of the sub-images ( with a usually necessary exposure compensation).

For the conversion of the 2D map in a Cartesian coordinate system, there are several options with different strengths and weaknesses:

• Length linear (square ): Both axes are mapped length linear.
• Polar angle linear (? ): ...
• Cartesian angle linear plus linear length (cylinder): An axis is mapped length linear, the orthogonal axis is mapped linearly angle. Although the length of linear axis is usually vertical, but also can be horizontal or inclined.
• Cartesian linear angle (ball, sphere ): Both axes are mapped linearly angle. Type of mapping depends here but just as with the cylindrical projection of the position of the reference axes from.
• Projection on the sides of a cube. It is about six length linear projections.

These projections, but is not the type of the 3D 2D reduction changed, i.e.

• No change of occlusions
• No changes in the relative size between objects of different distances
• Not change the position of different objects
• No change of the vanishing point.

For heavily equalized projections that leads to unusual images.

Representation method

In addition to the direct representation of the image there (especially at 360 ° projections ) the possibility to convert the representation in the representation static or interactive. In the static display there are always distortions of the originally spherical image when converted to a planar image.

Cubic representation

In the cubic display, the image is projected to the inner sides of a cube. So that an image without horizontal and vertical limitation is also possible in this illustration. The edges are, however, highly distorted when juxtaposition of images, making visible seams occur.

In the interactive display in particular a QTVR technique is utilized. In this case, all the images are aggregated into a single image, and the viewer can select the angle and thus the stressed and distorted space itself.

Spherical representation

In the spherical representation of the image is projected to the inside of a sphere, whereby the space is additionally extends to the cylindrical projection and in the vertical direction. This projection is also called equirectangular format. The curvature in the vertical plane the object in the center of the image is emphasized even more.

Cylindrical representation or projection

The panoramic image is curved, and the imaged object is represented realistically. The top and bottom of the picture recorded barrel-shaped. The cylindrical conversion is particularly suitable for subjects in the image center.

Stitchingfehler

• Is not the center of the entrance pupil ( perspective center ) on the rotation axis about which the camera between the individual images is rotated, there is an incorrect composition of the images. In the adjacent picture was in the middle of the square, a church, around which the pictures were taken.
• In moving scenes changing objects in the individual images are taken at different times, these objects appear to be partially transparent in the image ( ghosting ) or multiple times ( so-called clones in the picture, some passers-by are partially transparent ).
• Exposure differences can occur due to different camera settings, but also by changes in the natural lighting and lead to unsightly streaks in the images, if these differences are not compensated by the software.
• Also changes the white balance between the individual shots lead to non-uniformities in the pictures.

Software

On the Internet there are a large number of free stitching software. Also is included with digital cameras often a corresponding software. The scope and performance of stitching software is very different. The functionality includes the following features ( not exhaustive ):

• Support of file formats ( import / export)
• Conversion process
• Exposure compensation
• Correction method
• User interface and application support
• Assistant function

Furthermore, conventional image editing programs such as Photoshop (version CS3), also Stitchingfunktionen.

Stitch software ( selection)

• Adobe Photoshop
• Autopano giga ( as autopano pro, but with some additional features, including support for different recording items, such as aerial photographs )
• Autopano pro ( paid software with many editing options, including export to Photoshop format; unlimited trial edition but only with stamp)
• AutoStitch ( free, but with obligation to state of the program at publication)
• Hugin (software) (free software, for Windows, Linux, Mac OS X)
• Microsoft Research ICE Image Composite Editor v1.4.3 (7 April 2011), for free, apparently by the same author as Autostitch, as 32 ​​- and 64- bit version
• Panorama Tools (free)
• PTGui ( paid software with many editing options, including export to Photoshop format, trial edition here but only with stamp)