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Learning OpenCV
book

Learning OpenCV

by Gary Bradski, Adrian Kaehler
September 2008
Beginner to intermediate content levelBeginner to intermediate
580 pages
20h 7m
English
O'Reilly Media, Inc.
Content preview from Learning OpenCV

Stretch, Shrink, Warp, and Rotate

In this section we turn to geometric manipulations of images. [74] Such manipulations include stretching in various ways, which includes both uniform and nonuniform resizing (the latter is known as warping). There are many reasons to perform these operations: for example, warping and rotating an image so that it can be superimposed on a wall in an existing scene, or artificially enlarging a set of training images used for object recognition. [75] The functions that can stretch, shrink, warp, and/or rotate an image are called geometric transforms (for an early exposition, see [Semple79]). For planar areas, there are two flavors of geometric transforms: transforms that use a 2-by-3 matrix, which are called affine transforms; and transforms based on a 3-by-3 matrix, which are called perspective transforms or homographies. You can think of the latter transformation as a method for computing the way in which a plane in three dimensions is perceived by a particular observer, who might not be looking straight on at that plane.

An affine transformation is any transformation that can be expressed in the form of a matrix multiplication followed by a vector addition. In OpenCV the standard style of representing such a transformation is as a 2-by-3 matrix. We define:

image with no caption

It is easily seen that the effect of the affine transformation A · X + B is exactly equivalent ...

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Publisher Resources

ISBN: 9780596516130Supplemental ContentErrata Page