Ceres-Solver学习笔记(3)
Ceres的一个主要目的是解决大尺度bundle adjustment 问题。多说无益，直接上代码：
// file bal_problem.h
#ifndef CERES_EXAMPLES_BAL_PROBLEM_H_
#define CERES_EXAMPLES_BAL_PROBLEM_H_
#include // file bundle_adjuster.cc
#include // file snavely_reprojection_error.h
#include "ceres/rotation.h"
namespace ceres {
namespace examples {
// 小孔相机模型. camera用9个参数表示， 3 个表示旋转, 3 个表示平移, 1个焦距，2个径向畸变
// 中心点没有建模，假设为图片中心
struct SnavelyReprojectionError {
SnavelyReprojectionError(double observed_x, double observed_y)
: observed_x(observed_x), observed_y(observed_y) {}
template <typename T>
bool operator()(const T* const camera,
const T* const point,
T* residuals) const {
// camera[0,1,2] 是angle-axis 旋转.
T p[3];
AngleAxisRotatePoint(camera, point, p);
// camera[3,4,5] are the translation.
// 点转换到相机坐标系
p[0] += camera[3];
p[1] += camera[4];
p[2] += camera[5];
// Compute the center of distortion. The sign change comes from
// the camera model that Noah Snavely's Bundler assumes, whereby
// the camera coordinate system has a negative z axis.
const T xp = - p[0] / p[2];
const T yp = - p[1] / p[2];
// Apply second and fourth order radial distortion.
const T& l1 = camera[7];
const T& l2 = camera[8];
const T r2 = xp*xp + yp*yp;
const T distortion = T(1.0) + r2 * (l1 + l2 * r2);
// Compute final projected point position.
const T& focal = camera[6];
// 得到3D点投影到图像上的位置
const T predicted_x = focal * distortion * xp;
const T predicted_y = focal * distortion * yp;
// 特征位置与3D点计算位置的差
residuals[0] = predicted_x - T(observed_x);
residuals[1] = predicted_y - T(observed_y);
return true;
}
// Factory to hide the construction of the CostFunction object from
// the client code.
static ceres::CostFunction* Create(const double observed_x,
const double observed_y) {
// 使用AutoDiffCostFunction，2个残差，参数1有9维，参数2有3维
return (new ceres::AutoDiffCostFunction2, 9, 3>(
new SnavelyReprojectionError(observed_x, observed_y)));
}
double observed_x;
double observed_y;
};
// Templated pinhole camera model for used with Ceres. The camera is
// parameterized using 10 parameters. 4 for rotation, 3 for
// translation, 1 for focal length and 2 for radial distortion. The
// principal point is not modeled (i.e. it is assumed be located at
// the image center).
struct SnavelyReprojectionErrorWithQuaternions {
// (u, v): the position of the observation with respect to the image
// center point.
SnavelyReprojectionErrorWithQuaternions(double observed_x, double observed_y)
: observed_x(observed_x), observed_y(observed_y) {}
template <typename T>
bool operator()(const T* const camera,
const T* const point,
T* residuals) const {
// camera[0,1,2,3] is are the rotation of the camera as a quaternion.
//
// We use QuaternionRotatePoint as it does not assume that the
// quaternion is normalized, since one of the ways to run the
// bundle adjuster is to let Ceres optimize all 4 quaternion
// parameters without a local parameterization.
T p[3];
QuaternionRotatePoint(camera, point, p);
p[0] += camera[4];
p[1] += camera[5];
p[2] += camera[6];
// Compute the center of distortion. The sign change comes from
// the camera model that Noah Snavely's Bundler assumes, whereby
// the camera coordinate system has a negative z axis.
const T xp = - p[0] / p[2];
const T yp = - p[1] / p[2];
// Apply second and fourth order radial distortion.
const T& l1 = camera[8];
const T& l2 = camera[9];
const T r2 = xp*xp + yp*yp;
const T distortion = T(1.0) + r2 * (l1 + l2 * r2);
// Compute final projected point position.
const T& focal = camera[7];
const T predicted_x = focal * distortion * xp;
const T predicted_y = focal * distortion * yp;
// The error is the difference between the predicted and observed position.
residuals[0] = predicted_x - T(observed_x);
residuals[1] = predicted_y - T(observed_y);
return true;
}
// Factory to hide the construction of the CostFunction object from
// the client code.
static ceres::CostFunction* Create(const double observed_x,
const double observed_y) {
return (new ceres::AutoDiffCostFunction<
SnavelyReprojectionErrorWithQuaternions, 2, 10, 3>(
new SnavelyReprojectionErrorWithQuaternions(observed_x,
observed_y)));
}
double observed_x;
double observed_y;
}; 这个例程比较长，看起来很复杂，但是实际上只是将options的一些选项，列举了出来，具体的使用也没有很好的介绍，本质上并没有什么太难的。options的可设置参数非常之多，具体可以看solver.h文件。