1 Remapping {#tutorial_remap} 2 ========= 3 4 Goal 5 ---- 6 7 In this tutorial you will learn how to: 8 9 a. Use the OpenCV function @ref cv::remap to implement simple remapping routines. 10 11 Theory 12 ------ 13 14 ### What is remapping? 15 16 - It is the process of taking pixels from one place in the image and locating them in another 17 position in a new image. 18 - To accomplish the mapping process, it might be necessary to do some interpolation for 19 non-integer pixel locations, since there will not always be a one-to-one-pixel correspondence 20 between source and destination images. 21 - We can express the remap for every pixel location \f$(x,y)\f$ as: 22 23 \f[g(x,y) = f ( h(x,y) )\f] 24 25 where \f$g()\f$ is the remapped image, \f$f()\f$ the source image and \f$h(x,y)\f$ is the mapping function 26 that operates on \f$(x,y)\f$. 27 28 - Let's think in a quick example. Imagine that we have an image \f$I\f$ and, say, we want to do a 29 remap such that: 30 31 \f[h(x,y) = (I.cols - x, y )\f] 32 33 What would happen? It is easily seen that the image would flip in the \f$x\f$ direction. For 34 instance, consider the input image: 35 36  37 38 observe how the red circle changes positions with respect to x (considering \f$x\f$ the horizontal 39 direction): 40 41  42 43 - In OpenCV, the function @ref cv::remap offers a simple remapping implementation. 44 45 Code 46 ---- 47 48 -# **What does this program do?** 49 - Loads an image 50 - Each second, apply 1 of 4 different remapping processes to the image and display them 51 indefinitely in a window. 52 - Wait for the user to exit the program 53 54 -# The tutorial code's is shown lines below. You can also download it from 55 [here](https://github.com/Itseez/opencv/tree/master/samples/cpp/tutorial_code/ImgTrans/Remap_Demo.cpp) 56 @include samples/cpp/tutorial_code/ImgTrans/Remap_Demo.cpp 57 58 Explanation 59 ----------- 60 61 -# Create some variables we will use: 62 @code{.cpp} 63 Mat src, dst; 64 Mat map_x, map_y; 65 char* remap_window = "Remap demo"; 66 int ind = 0; 67 @endcode 68 -# Load an image: 69 @code{.cpp} 70 src = imread( argv[1], 1 ); 71 @endcode 72 -# Create the destination image and the two mapping matrices (for x and y ) 73 @code{.cpp} 74 dst.create( src.size(), src.type() ); 75 map_x.create( src.size(), CV_32FC1 ); 76 map_y.create( src.size(), CV_32FC1 ); 77 @endcode 78 -# Create a window to display results 79 @code{.cpp} 80 namedWindow( remap_window, WINDOW_AUTOSIZE ); 81 @endcode 82 -# Establish a loop. Each 1000 ms we update our mapping matrices (*mat_x* and *mat_y*) and apply 83 them to our source image: 84 @code{.cpp} 85 while( true ) 86 { 87 /// Each 1 sec. Press ESC to exit the program 88 int c = waitKey( 1000 ); 89 90 if( (char)c == 27 ) 91 { break; } 92 93 /// Update map_x & map_y. Then apply remap 94 update_map(); 95 remap( src, dst, map_x, map_y, INTER_LINEAR, BORDER_CONSTANT, Scalar(0,0, 0) ); 96 97 /// Display results 98 imshow( remap_window, dst ); 99 } 100 @endcode 101 The function that applies the remapping is @ref cv::remap . We give the following arguments: 102 103 - **src**: Source image 104 - **dst**: Destination image of same size as *src* 105 - **map_x**: The mapping function in the x direction. It is equivalent to the first component 106 of \f$h(i,j)\f$ 107 - **map_y**: Same as above, but in y direction. Note that *map_y* and *map_x* are both of 108 the same size as *src* 109 - **INTER_LINEAR**: The type of interpolation to use for non-integer pixels. This is by 110 default. 111 - **BORDER_CONSTANT**: Default 112 113 How do we update our mapping matrices *mat_x* and *mat_y*? Go on reading: 114 115 -# **Updating the mapping matrices:** We are going to perform 4 different mappings: 116 -# Reduce the picture to half its size and will display it in the middle: 117 \f[h(i,j) = ( 2*i - src.cols/2 + 0.5, 2*j - src.rows/2 + 0.5)\f] 118 for all pairs \f$(i,j)\f$ such that: \f$\dfrac{src.cols}{4}<i<\dfrac{3 \cdot src.cols}{4}\f$ and 119 \f$\dfrac{src.rows}{4}<j<\dfrac{3 \cdot src.rows}{4}\f$ 120 -# Turn the image upside down: \f$h( i, j ) = (i, src.rows - j)\f$ 121 -# Reflect the image from left to right: \f$h(i,j) = ( src.cols - i, j )\f$ 122 -# Combination of b and c: \f$h(i,j) = ( src.cols - i, src.rows - j )\f$ 123 124 This is expressed in the following snippet. Here, *map_x* represents the first coordinate of 125 *h(i,j)* and *map_y* the second coordinate. 126 @code{.cpp} 127 for( int j = 0; j < src.rows; j++ ) 128 { for( int i = 0; i < src.cols; i++ ) 129 { 130 switch( ind ) 131 { 132 case 0: 133 if( i > src.cols*0.25 && i < src.cols*0.75 && j > src.rows*0.25 && j < src.rows*0.75 ) 134 { 135 map_x.at<float>(j,i) = 2*( i - src.cols*0.25 ) + 0.5 ; 136 map_y.at<float>(j,i) = 2*( j - src.rows*0.25 ) + 0.5 ; 137 } 138 else 139 { map_x.at<float>(j,i) = 0 ; 140 map_y.at<float>(j,i) = 0 ; 141 } 142 break; 143 case 1: 144 map_x.at<float>(j,i) = i ; 145 map_y.at<float>(j,i) = src.rows - j ; 146 break; 147 case 2: 148 map_x.at<float>(j,i) = src.cols - i ; 149 map_y.at<float>(j,i) = j ; 150 break; 151 case 3: 152 map_x.at<float>(j,i) = src.cols - i ; 153 map_y.at<float>(j,i) = src.rows - j ; 154 break; 155 } // end of switch 156 } 157 } 158 ind++; 159 } 160 @endcode 161 162 Result 163 ------ 164 165 -# After compiling the code above, you can execute it giving as argument an image path. For 166 instance, by using the following image: 167 168  169 170 -# This is the result of reducing it to half the size and centering it: 171 172  173 174 -# Turning it upside down: 175 176  177 178 -# Reflecting it in the x direction: 179 180  181 182 -# Reflecting it in both directions: 183 184  185
