Based on a linear pinhole camera imaging model,the cross-ratio invariant has been applied to pattern recognition on computer vision for a long time as the most fundamental projective invariant.

By using a planar gridding target calibration points could be achieved,and the precision of these calibration points were improved through revising the distortion coefficients of lens according to the principle of cross ratio invariability.

A new approach for lens distortion calibration is presented based on the cross ratio invariability of perspective projection.

According the cross ratio invariability and perspective reversion,the relation among the vanishing points,the spatial beelines and their projections in image is determined,and the width of the eave can be calculated.

Research on calibration of cameras based on cross ratio invariability;

Firstly,order radial lens distortion parameters are computed by using cross ratio invariability.

In the first method,the camera was first calibrated use the equality of cross ratio,then put the measured object and calibration object on the same plane,put the camera perpendicular to the calibration plane as far as possible,finally 3D reconstruction was transformed to 2D reconstruction,then the calculate becomes simple.

Under the case of first-order radial distortion,the equality of cross ratio in the perspective projection was used to calculate the first-order radial distortion ratio in order to calibrate the camera,then the extending rate of the steel bar can be calculated.