315 lines
8.7 KiB
ReStructuredText
315 lines
8.7 KiB
ReStructuredText
.. _remap:
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Remapping
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*********
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Goal
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====
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In this tutorial you will learn how to:
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a. Use the OpenCV function :remap:`remap <>` to implement simple remapping routines.
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Theory
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======
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What is remapping?
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------------------
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* It is the process of taking pixels from one place in the image and locating them in another position in a new image.
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* 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.
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* We can express the remap for every pixel location :math:`(x,y)` as:
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.. math::
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g(x,y) = f ( h(x,y) )
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where :math:`g()` is the remapped image, :math:`f()` the source image and :math:`h(x,y)` is the mapping function that operates on :math:`(x,y)`.
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* Let's think in a quick example. Imagine that we have an image :math:`I` and, say, we want to do a remap such that:
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.. math::
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h(x,y) = (I.cols - x, y )
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What would happen? It is easily seen that the image would flip in the :math:`x` direction. For instance, consider the input image:
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.. image:: images/Remap_Tutorial_Theory_0.jpg
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:alt: Original test image
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:width: 120pt
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:align: center
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observe how the red circle changes positions with respect to x (considering :math:`x` the horizontal direction):
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.. image:: images/Remap_Tutorial_Theory_1.jpg
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:alt: Original test image
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:width: 120pt
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:align: center
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* In OpenCV, the function :remap:`remap <>` offers a simple remapping implementation.
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Code
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====
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#. **What does this program do?**
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* Loads an image
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* Each second, apply 1 of 4 different remapping processes to the image and display them indefinitely in a window.
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* Wait for the user to exit the program
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#. The tutorial code's is shown lines below. You can also download it from `here <http://code.opencv.org/projects/opencv/repository/revisions/master/raw/samples/cpp/tutorial_code/ImgTrans/Remap_Demo.cpp>`_
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.. code-block:: cpp
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#include "opencv2/highgui/highgui.hpp"
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#include "opencv2/imgproc/imgproc.hpp"
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#include <iostream>
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#include <stdio.h>
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using namespace cv;
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/// Global variables
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Mat src, dst;
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Mat map_x, map_y;
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char* remap_window = "Remap demo";
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int ind = 0;
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/// Function Headers
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void update_map( void );
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/**
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* @function main
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*/
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int main( int argc, char** argv )
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{
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/// Load the image
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src = imread( argv[1], 1 );
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/// Create dst, map_x and map_y with the same size as src:
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dst.create( src.size(), src.type() );
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map_x.create( src.size(), CV_32FC1 );
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map_y.create( src.size(), CV_32FC1 );
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/// Create window
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namedWindow( remap_window, CV_WINDOW_AUTOSIZE );
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/// Loop
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while( true )
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{
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/// Each 1 sec. Press ESC to exit the program
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int c = waitKey( 1000 );
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if( (char)c == 27 )
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{ break; }
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/// Update map_x & map_y. Then apply remap
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update_map();
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remap( src, dst, map_x, map_y, CV_INTER_LINEAR, BORDER_CONSTANT, Scalar(0,0, 0) );
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/// Display results
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imshow( remap_window, dst );
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}
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return 0;
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}
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/**
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* @function update_map
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* @brief Fill the map_x and map_y matrices with 4 types of mappings
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*/
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void update_map( void )
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{
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ind = ind%4;
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for( int j = 0; j < src.rows; j++ )
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{ for( int i = 0; i < src.cols; i++ )
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{
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switch( ind )
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{
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case 0:
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if( i > src.cols*0.25 && i < src.cols*0.75 && j > src.rows*0.25 && j < src.rows*0.75 )
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{
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map_x.at<float>(j,i) = 2*( i - src.cols*0.25 ) + 0.5 ;
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map_y.at<float>(j,i) = 2*( j - src.rows*0.25 ) + 0.5 ;
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}
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else
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{ map_x.at<float>(j,i) = 0 ;
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map_y.at<float>(j,i) = 0 ;
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}
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break;
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case 1:
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map_x.at<float>(j,i) = i ;
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map_y.at<float>(j,i) = src.rows - j ;
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break;
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case 2:
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map_x.at<float>(j,i) = src.cols - i ;
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map_y.at<float>(j,i) = j ;
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break;
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case 3:
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map_x.at<float>(j,i) = src.cols - i ;
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map_y.at<float>(j,i) = src.rows - j ;
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break;
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} // end of switch
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}
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}
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ind++;
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}
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Explanation
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===========
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#. Create some variables we will use:
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.. code-block:: cpp
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Mat src, dst;
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Mat map_x, map_y;
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char* remap_window = "Remap demo";
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int ind = 0;
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#. Load an image:
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.. code-block:: cpp
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src = imread( argv[1], 1 );
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#. Create the destination image and the two mapping matrices (for x and y )
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.. code-block:: cpp
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dst.create( src.size(), src.type() );
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map_x.create( src.size(), CV_32FC1 );
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map_y.create( src.size(), CV_32FC1 );
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#. Create a window to display results
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.. code-block:: cpp
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namedWindow( remap_window, CV_WINDOW_AUTOSIZE );
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#. Establish a loop. Each 1000 ms we update our mapping matrices (*mat_x* and *mat_y*) and apply them to our source image:
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.. code-block:: cpp
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while( true )
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{
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/// Each 1 sec. Press ESC to exit the program
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int c = waitKey( 1000 );
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if( (char)c == 27 )
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{ break; }
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/// Update map_x & map_y. Then apply remap
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update_map();
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remap( src, dst, map_x, map_y, CV_INTER_LINEAR, BORDER_CONSTANT, Scalar(0,0, 0) );
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/// Display results
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imshow( remap_window, dst );
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}
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The function that applies the remapping is :remap:`remap <>`. We give the following arguments:
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* **src**: Source image
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* **dst**: Destination image of same size as *src*
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* **map_x**: The mapping function in the x direction. It is equivalent to the first component of :math:`h(i,j)`
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* **map_y**: Same as above, but in y direction. Note that *map_y* and *map_x* are both of the same size as *src*
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* **CV_INTER_LINEAR**: The type of interpolation to use for non-integer pixels. This is by default.
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* **BORDER_CONSTANT**: Default
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How do we update our mapping matrices *mat_x* and *mat_y*? Go on reading:
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#. **Updating the mapping matrices:** We are going to perform 4 different mappings:
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a. Reduce the picture to half its size and will display it in the middle:
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.. math::
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h(i,j) = ( 2*i - src.cols/2 + 0.5, 2*j - src.rows/2 + 0.5)
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for all pairs :math:`(i,j)` such that: :math:`\dfrac{src.cols}{4}<i<\dfrac{3 \cdot src.cols}{4}` and :math:`\dfrac{src.rows}{4}<j<\dfrac{3 \cdot src.rows}{4}`
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b. Turn the image upside down: :math:`h( i, j ) = (i, src.rows - j)`
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c. Reflect the image from left to right: :math:`h(i,j) = ( src.cols - i, j )`
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d. Combination of b and c: :math:`h(i,j) = ( src.cols - i, src.rows - j )`
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This is expressed in the following snippet. Here, *map_x* represents the first coordinate of *h(i,j)* and *map_y* the second coordinate.
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.. code-block:: cpp
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for( int j = 0; j < src.rows; j++ )
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{ for( int i = 0; i < src.cols; i++ )
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{
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switch( ind )
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{
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case 0:
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if( i > src.cols*0.25 && i < src.cols*0.75 && j > src.rows*0.25 && j < src.rows*0.75 )
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{
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map_x.at<float>(j,i) = 2*( i - src.cols*0.25 ) + 0.5 ;
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map_y.at<float>(j,i) = 2*( j - src.rows*0.25 ) + 0.5 ;
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}
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else
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{ map_x.at<float>(j,i) = 0 ;
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map_y.at<float>(j,i) = 0 ;
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}
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break;
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case 1:
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map_x.at<float>(j,i) = i ;
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map_y.at<float>(j,i) = src.rows - j ;
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break;
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case 2:
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map_x.at<float>(j,i) = src.cols - i ;
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map_y.at<float>(j,i) = j ;
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break;
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case 3:
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map_x.at<float>(j,i) = src.cols - i ;
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map_y.at<float>(j,i) = src.rows - j ;
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break;
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} // end of switch
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}
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}
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ind++;
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}
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Result
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======
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#. After compiling the code above, you can execute it giving as argument an image path. For instance, by using the following image:
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.. image:: images/Remap_Tutorial_Original_Image.jpg
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:alt: Original test image
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:width: 250pt
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:align: center
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#. This is the result of reducing it to half the size and centering it:
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.. image:: images/Remap_Tutorial_Result_0.jpg
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:alt: Result 0 for remapping
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:width: 250pt
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:align: center
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#. Turning it upside down:
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.. image:: images/Remap_Tutorial_Result_1.jpg
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:alt: Result 0 for remapping
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:width: 250pt
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:align: center
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#. Reflecting it in the x direction:
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.. image:: images/Remap_Tutorial_Result_2.jpg
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:alt: Result 0 for remapping
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:width: 250pt
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:align: center
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#. Reflecting it in both directions:
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.. image:: images/Remap_Tutorial_Result_3.jpg
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:alt: Result 0 for remapping
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:width: 250pt
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:align: center
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