Learning OpenCV

Affine and Perspective Transformations

Two transformations that come up often in the OpenCV routines we have discussed—as well as in other applications you might write yourself—are the affine and perspective transformations. We first encountered these in Chapter 6. As implemented in OpenCV, these routines affect either lists of points or entire images, and they map points on one location in the image to a different location, often performing subpixel interpolation along the way. You may recall that an affine transform can produce any parallelogram from a rectangle; the perspective transform is more general and can produce any trapezoid from a rectangle.

The perspective transformation is closely related to the perspective projection. Recall that the perspective projection maps points in the three-dimensional physical world onto points on the two-dimensional image plane along a set of projection lines that all meet at a single point called the center of projection. The perspective transformation, which is a specific kind of homography, ^[192] relates two different images that are alternative projections of the same three-dimensional object onto two different projective planes (and thus, for nondegenerate configurations such as the plane physically intersecting the 3D object, typically to two different centers of projection).

These projective transformation-related functions were discussed in detail in Chapter 6; for convenience, we summarize them here in Table 12-1.

Table 12-1. Affine ...