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Homography

From Wikipedia, the free encyclopedia

Homography is a concept in the mathematical science of geometry. It is defined as a relation between two figures, such that to any point in one figure corresponds one and only one point in the other, and vise versa.

[edit] Computer Vision Applications

In the field of Computer Vision, a homography is defined in 2 dimensional space as a mapping between a point on a ground plane from as seen from one camera, to the same point on the ground plane as seen from a second camera. This has many practical applications, most notably it provides a method for compositing 2D or 3D objects into an image or video with the correct pose.


[edit] 3d plane to plane equation

Having two cameras.

Passing from a point p1 to a point p2.

p_2 = K_2 \cdot H_{12} \cdot K_1^{-1} \cdot p_1

where H12 is

H_{12} = (R + (t \cdot \frac{n^T}{d}))

R is the rotation matrix. t is the translation vector. n and d are the normal vector of the plane and the distance to the plane respectively.

K1 and K2 are the cameras intrisic parameters matrices.

[edit] Mathematical definition

p_{a} = \begin{bmatrix} x_{a}\\y_{a}\\1\end{bmatrix}, p_{b} = \begin{bmatrix} x_{b}\\y_{b}\\1\end{bmatrix}, \mathbf{H}_{ab} = \begin{bmatrix} h_{11}&h_{12}&h_{13}\\h_{21}&h_{22}&h_{23}\\h_{31}&h_{32}&h_{33} \end{bmatrix}

p_{b} = \mathbf{H}_{ab}p_{a}

And:

p_{a} = \mathbf{H}_{ba}p_{b}

Where:

\mathbf{H}_{ba} = \mathbf{H}_{ab}^{-1}

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