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# Remapping

## Goal

In this tutorial you will learn how to:

1. Use the OpenCV function remap to implement simple remapping routines.

## Theory

### What is remapping?

- It is the process of taking pixels from one place in the image and locating them in another position in a new image.

- To accomplish the mapping process, it might be necessary to do some interpolation for non-integer pixel locations, since there will not always be a one-to-one-pixel correspondence between source and destination images.

- We can express the remap for every pixel location (x,y) as:

  g(x,y) = f ( h(x,y) )

  where g() is the remapped image, f() the source image and h(x,y) is the mapping function that operates on (x,y).

- Let’s think in a quick example. Imagine that we have an image I and, say, we want to do a remap such that:

  h(x,y) = (I.cols - x, y )

  What would happen? It is easily seen that the image would flip in the x direction. For instance, consider the input image:

  observe how the red circle changes positions with respect to x (considering x the horizontal direction):

- In OpenCV, the function remap offers a simple remapping implementation.

## Code

1. What does this program do?
   - Loads an image
   - Each second, apply 1 of 4 different remapping processes to the image and display them indefinitely in a window.
   - Wait for the user to exit the program
2. The tutorial code’s is shown lines below. You can also download it from here

     #include "opencv2/highgui/highgui.hpp"
     #include "opencv2/imgproc/imgproc.hpp"
     #include <iostream>
     #include <stdio.h>

     using namespace cv;

     /// Global variables
     Mat src, dst;
     Mat map_x, map_y;
     char* remap_window = "Remap demo";
     int ind = 0;

     /// Function Headers
     void update_map( void );

     /**
     * @function main
     */
     int main( int argc, char** argv )
     {
       /// Load the image
       src = imread( argv[1], 1 );

      /// Create dst, map_x and map_y with the same size as src:
      dst.create( src.size(), src.type() );
      map_x.create( src.size(), CV_32FC1 );
      map_y.create( src.size(), CV_32FC1 );

      /// Create window
      namedWindow( remap_window, CV_WINDOW_AUTOSIZE );

      /// Loop
      while( true )
      {
        /// Each 1 sec. Press ESC to exit the program
        int c = waitKey( 1000 );

        if( (char)c == 27 )
          { break; }

        /// Update map_x & map_y. Then apply remap
        update_map();
        remap( src, dst, map_x, map_y, CV_INTER_LINEAR, BORDER_CONSTANT, Scalar(0,0, 0) );

        /// Display results
        imshow( remap_window, dst );
      }
      return 0;
     }

     /**
     * @function update_map
     * @brief Fill the map_x and map_y matrices with 4 types of mappings
     */
     void update_map( void )
     {
       ind = ind%4;

       for( int j = 0; j < src.rows; j++ )
       { for( int i = 0; i < src.cols; i++ )
           {
             switch( ind )
             {
               case 0:
                 if( i > src.cols*0.25 && i < src.cols*0.75 && j > src.rows*0.25 && j < src.rows*0.75 )
                   {
                     map_x.at<float>(j,i) = 2*( i - src.cols*0.25 ) + 0.5 ;
                     map_y.at<float>(j,i) = 2*( j - src.rows*0.25 ) + 0.5 ;
                    }
                 else
                   { map_x.at<float>(j,i) = 0 ;
                     map_y.at<float>(j,i) = 0 ;
                   }
                     break;
               case 1:
                     map_x.at<float>(j,i) = i ;
                     map_y.at<float>(j,i) = src.rows - j ;
                     break;
               case 2:
                     map_x.at<float>(j,i) = src.cols - i ;
                     map_y.at<float>(j,i) = j ;
                     break;
               case 3:
                     map_x.at<float>(j,i) = src.cols - i ;
                     map_y.at<float>(j,i) = src.rows - j ;
                     break;
             } // end of switch
           }
        }
      ind++;
    }

## Explanation

1. Create some variables we will use:

       Mat src, dst;
       Mat map_x, map_y;
       char* remap_window = "Remap demo";
       int ind = 0;

2. Load an image:

       src = imread( argv[1], 1 );

3. Create the destination image and the two mapping matrices (for x and y )

       dst.create( src.size(), src.type() );
       map_x.create( src.size(), CV_32FC1 );
       map_y.create( src.size(), CV_32FC1 );

4. Create a window to display results

       namedWindow( remap_window, CV_WINDOW_AUTOSIZE );

5. Establish a loop. Each 1000 ms we update our mapping matrices (mat_x and mat_y) and apply them to our source image:

       while( true )
       {
         /// Each 1 sec. Press ESC to exit the program
         int c = waitKey( 1000 );

         if( (char)c == 27 )
           { break; }

         /// Update map_x & map_y. Then apply remap
         update_map();
         remap( src, dst, map_x, map_y, CV_INTER_LINEAR, BORDER_CONSTANT, Scalar(0,0, 0) );

         /// Display results
         imshow( remap_window, dst );
       }

   The function that applies the remapping is remap. We give the following arguments:

   - src: Source image
   - dst: Destination image of same size as src
   - map_x: The mapping function in the x direction. It is equivalent to the first component of h(i,j)
   - map_y: Same as above, but in y direction. Note that map_y and map_x are both of the same size as src
   - CV_INTER_LINEAR: The type of interpolation to use for non-integer pixels. This is by default.
   - BORDER_CONSTANT: Default

   How do we update our mapping matrices mat_x and mat_y? Go on reading:

6. Updating the mapping matrices: We are going to perform 4 different mappings:

   1. Reduce the picture to half its size and will display it in the middle:

      h(i,j) = ( 2*i - src.cols/2 + 0.5, 2*j - src.rows/2 + 0.5)

      for all pairs (i,j) such that: \dfrac{src.cols}{4}<i<\dfrac{3 \cdot src.cols}{4} and \dfrac{src.rows}{4}<j<\dfrac{3 \cdot src.rows}{4}

   2. Turn the image upside down: h( i, j ) = (i, src.rows - j)

   3. Reflect the image from left to right: h(i,j) = ( src.cols - i, j )

   4. Combination of b and c: h(i,j) = ( src.cols - i, src.rows - j )

> This is expressed in the following snippet. Here, map_x represents the first coordinate of h(i,j) and map_y the second coordinate.

>     for( int j = 0; j < src.rows; j++ )
>     { for( int i = 0; i < src.cols; i++ )
>         {
>           switch( ind )
>           {
>             case 0:
>               if( i > src.cols*0.25 && i < src.cols*0.75 && j > src.rows*0.25 && j < src.rows*0.75 )
>                 {
>                   map_x.at<float>(j,i) = 2*( i - src.cols*0.25 ) + 0.5 ;
>                   map_y.at<float>(j,i) = 2*( j - src.rows*0.25 ) + 0.5 ;
>                  }
>               else
>                 { map_x.at<float>(j,i) = 0 ;
>                   map_y.at<float>(j,i) = 0 ;
>                 }
>                   break;
>             case 1:
>                   map_x.at<float>(j,i) = i ;
>                   map_y.at<float>(j,i) = src.rows - j ;
>                   break;
>             case 2:
>                   map_x.at<float>(j,i) = src.cols - i ;
>                   map_y.at<float>(j,i) = j ;
>                   break;
>             case 3:
>                   map_x.at<float>(j,i) = src.cols - i ;
>                   map_y.at<float>(j,i) = src.rows - j ;
>                   break;
>           } // end of switch
>         }
>       }
>      ind++;
>     }

## Result

1. After compiling the code above, you can execute it giving as argument an image path. For instance, by using the following image:

2. This is the result of reducing it to half the size and centering it:

3. Turning it upside down:

4. Reflecting it in the x direction:

5. Reflecting it in both directions:

## Help and Feedback

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- Ask a question on the Q&A forum.
- If you think something is missing or wrong in the documentation, please file a bug report.

### Navigation

- index
- next |
- previous |
- OpenCV 2.4.12.3 documentation »
- OpenCV Tutorials »
- imgproc module. Image Processing »