在《学习OpenCV3，O'Reilly系列丛书》中的第11章常见的图像变换中讲到的“任意映射”，Remap函数。函数cv::remap通常用来纠正校准的立体图像，包括图像变形和图像扭曲。其实这个函数在无人机拍摄延时摄影（使用电子防抖算法）合成视频时也是常用到的函数。主要是处理无人机悬停或者行进中拍摄的照片出现比较大的形变。

remap( )函数解析

remap( )函数会根据我们指定的映射形式，将源图像进行重映射几何变换，基于的式子如下：

 

需要注意，此函数不支持就地（in-place）操作。看看其原型和参数。

void cv::remap	(	InputArray	src,
		OutputArray	dst,
		InputArray	map1,
		InputArray	map2,
		int	interpolation,
		int	borderMode = BORDER_CONSTANT,
		const Scalar &	borderValue = Scalar()
	)

 Parameters

src	Source image.
dst	Destination image. It has the same size as map1 and the same type as src .
map1	The first map of either (x,y) points or just x values having the type CV_16SC2 , CV_32FC1, or CV_32FC2. See convertMaps for details on converting a floating point representation to fixed-point for speed.
map2	The second map of y values having the type CV_16UC1, CV_32FC1, or none (empty map if map1 is (x,y) points), respectively.
interpolation	插值方式， Interpolation method (see InterpolationFlags). The method INTER_AREA is not supported by this function.
borderMode	图像边界处理方式 Pixel extrapolation method (see BorderTypes). When borderMode=BORDER_TRANSPARENT, it means that the pixels in the destination image that corresponds to the "outliers" in the source image are not modified by the function.
borderValue	Value used in case of a constant border. By default, it is 0.

使用OpenCV-3.4中的samples/cpp/tutorial_code/ImgTrans/Remap_Demo.cpp进行演示。

update_map函数中的参数ind意义：

ind=0表示提取源图像的25%～75%显示，1%～24%和76%～100%填充。

 

ind=1表示关于沿着x轴翻转（上下翻转）

ind=2表示关于沿着Y轴翻转（左右翻转）

 

ind=3表是上下和左右同时翻转。

/*** @function Remap_Demo.cpp* @brief Demo code for Remap* @author Ana Huaman*/#include "opencv2/imgcodecs.hpp"
#include "opencv2/highgui.hpp"
#include "opencv2/imgproc.hpp"
#include <iostream>using namespace cv;/// Function Headers
void update_map( int &ind, Mat &map_x, Mat &map_y );/*** @function main*/
int main(int argc, const char** argv)
{CommandLineParser parser(argc, argv, "{@image |chicky_512.png|input image name}");std::string filename = parser.get<std::string>(0);//! [Load]/// Load the imageMat src = imread( samples::findFile( filename ), IMREAD_COLOR );if (src.empty()){std::cout << "Cannot read image: " << filename << std::endl;return -1;}//! [Load]//! [Create]/// Create dst, map_x and map_y with the same size as src:Mat dst(src.size(), src.type());Mat map_x(src.size(), CV_32FC1);Mat map_y(src.size(), CV_32FC1);//! [Create]//! [Window]/// Create windowconst char* remap_window = "Remap demo";namedWindow( remap_window, WINDOW_AUTOSIZE );//! [Window]//! [Loop]/// Index to switch between the remap modesint ind = 0;for(;;){/// Update map_x & map_y. Then apply remapupdate_map(ind, map_x, map_y);remap( src, dst, map_x, map_y, INTER_LINEAR, BORDER_CONSTANT, Scalar(0, 0, 0) );/// Display resultsimshow( remap_window, dst );/// Each 1 sec. Press ESC to exit the programchar c = (char)waitKey( 1000 );if( c == 27 ){break;}}//! [Loop]return 0;
}/*** @function update_map* @brief Fill the map_x and map_y matrices with 4 types of mappings*/
//! [Update]void update_map( int &ind, Mat &map_x, Mat &map_y )
{for( int i = 0; i < map_x.rows; i++ ){for( int j = 0; j < map_x.cols; j++ ){switch( ind ){case 0:if( j > map_x.cols*0.25 && j < map_x.cols*0.75 && i > map_x.rows*0.25 && i < map_x.rows*0.75 ){map_x.at<float>(i, j) = 2*( j - map_x.cols*0.25f ) + 0.5f;map_y.at<float>(i, j) = 2*( i - map_x.rows*0.25f ) + 0.5f;}else{map_x.at<float>(i, j) = 0;map_y.at<float>(i, j) = 0;}break;case 1://行翻转map_x.at<float>(i, j) = (float)j;map_y.at<float>(i, j) = (float)(map_x.rows - i);break;case 2: //列翻转map_x.at<float>(i, j) = (float)(map_x.cols - j);map_y.at<float>(i, j) = (float)i;break;case 3: //上下左右均翻转map_x.at<float>(i, j) = (float)(map_x.cols - j);map_y.at<float>(i, j) = (float)(map_x.rows - i);break;default:break;} // end of switch}}ind = (ind+1) % 4;
}