samples/cpp/tutorial_code/core/how_to_scan_images/how_to_scan_images.cpp

Check the corresponding tutorial for more details

#include <opencv2/core.hpp>
#include <opencv2/core/utility.hpp>
#include "opencv2/imgcodecs.hpp"
#include <opencv2/highgui.hpp>
#include <iostream>
#include <sstream>
 
using namespace std;
using namespace cv;
 
static void help()
{
    cout
        << "\n--------------------------------------------------------------------------" << endl
        << "This program shows how to scan image objects in OpenCV (cv::Mat). As use case"
        << " we take an input image and divide the native color palette (255) with the "  << endl
        << "input. Shows C operator[] method, iterators and at function for on-the-fly item address calculation."<< endl
        << "Usage:"                                                                       << endl
        << "./how_to_scan_images <imageNameToUse> <divideWith> [G]"                       << endl
        << "if you add a G parameter the image is processed in gray scale"                << endl
        << "--------------------------------------------------------------------------"   << endl
        << endl;
}
 
Mat& ScanImageAndReduceC(Mat& I, const uchar* table);
Mat& ScanImageAndReduceIterator(Mat& I, const uchar* table);
Mat& ScanImageAndReduceRandomAccess(Mat& I, const uchar * table);
 
int main( int argc, char* argv[])
{
    help();
    if (argc < 3)
    {
        cout << "Not enough parameters" << endl;
        return -1;
    }
 
    Mat I, J;
    if( argc == 4 && !strcmp(argv[3],"G") )
        I = imread(argv[1], IMREAD_GRAYSCALE);
    else
        I = imread(argv[1], IMREAD_COLOR);
 
    if (I.empty())
    {
        cout << "The image" << argv[1] << " could not be loaded." << endl;
        return -1;
    }
 
    int divideWith = 0; // convert our input string to number - C++ style
    stringstream s;
    s << argv[2];
    s >> divideWith;
    if (!s || !divideWith)
    {
        cout << "Invalid number entered for dividing. " << endl;
        return -1;
    }
 
    uchar table[256];
    for (int i = 0; i < 256; ++i)
       table[i] = (uchar)(divideWith * (i/divideWith));
 
    const int times = 100;
    double t;
 
    t = (double)getTickCount();
 
    for (int i = 0; i < times; ++i)
    {
        cv::Mat clone_i = I.clone();
        J = ScanImageAndReduceC(clone_i, table);
    }
 
    t = 1000*((double)getTickCount() - t)/getTickFrequency();
    t /= times;
 
    cout << "Time of reducing with the C operator [] (averaged for "
         << times << " runs): " << t << " milliseconds."<< endl;
 
    t = (double)getTickCount();
 
    for (int i = 0; i < times; ++i)
    {
        cv::Mat clone_i = I.clone();
        J = ScanImageAndReduceIterator(clone_i, table);
    }
 
    t = 1000*((double)getTickCount() - t)/getTickFrequency();
    t /= times;
 
    cout << "Time of reducing with the iterator (averaged for "
        << times << " runs): " << t << " milliseconds."<< endl;
 
    t = (double)getTickCount();
 
    for (int i = 0; i < times; ++i)
    {
        cv::Mat clone_i = I.clone();
        ScanImageAndReduceRandomAccess(clone_i, table);
    }
 
    t = 1000*((double)getTickCount() - t)/getTickFrequency();
    t /= times;
 
    cout << "Time of reducing with the on-the-fly address generation - at function (averaged for "
        << times << " runs): " << t << " milliseconds."<< endl;
 
    Mat lookUpTable(1, 256, CV_8U);
    uchar* p = lookUpTable.ptr();
    for( int i = 0; i < 256; ++i)
        p[i] = table[i];
 
    t = (double)getTickCount();
 
    for (int i = 0; i < times; ++i)
        LUT(I, lookUpTable, J);
 
    t = 1000*((double)getTickCount() - t)/getTickFrequency();
    t /= times;
 
    cout << "Time of reducing with the LUT function (averaged for "
        << times << " runs): " << t << " milliseconds."<< endl;
    return 0;
}
 
Mat& ScanImageAndReduceC(Mat& I, const uchar* const table)
{
    // accept only char type matrices
    CV_Assert(I.depth() == CV_8U);
 
    int channels = I.channels();
 
    int nRows = I.rows;
    int nCols = I.cols * channels;
 
    if (I.isContinuous())
    {
        nCols *= nRows;
        nRows = 1;
    }
 
    int i,j;
    uchar* p;
    for( i = 0; i < nRows; ++i)
    {
        p = I.ptr<uchar>(i);
        for ( j = 0; j < nCols; ++j)
        {
            p[j] = table[p[j]];
        }
    }
    return I;
}
 
Mat& ScanImageAndReduceIterator(Mat& I, const uchar* const table)
{
    // accept only char type matrices
    CV_Assert(I.depth() == CV_8U);
 
    const int channels = I.channels();
    switch(channels)
    {
    case 1:
        {
            MatIterator_<uchar> it, end;
            for( it = I.begin<uchar>(), end = I.end<uchar>(); it != end; ++it)
                *it = table[*it];
            break;
        }
    case 3:
        {
            MatIterator_<Vec3b> it, end;
            for( it = I.begin<Vec3b>(), end = I.end<Vec3b>(); it != end; ++it)
            {
                (*it)[0] = table[(*it)[0]];
                (*it)[1] = table[(*it)[1]];
                (*it)[2] = table[(*it)[2]];
            }
        }
    }
 
    return I;
}
 
Mat& ScanImageAndReduceRandomAccess(Mat& I, const uchar* const table)
{
    // accept only char type matrices
    CV_Assert(I.depth() == CV_8U);
 
    const int channels = I.channels();
    switch(channels)
    {
    case 1:
        {
            for( int i = 0; i < I.rows; ++i)
                for( int j = 0; j < I.cols; ++j )
                    I.at<uchar>(i,j) = table[I.at<uchar>(i,j)];
            break;
        }
    case 3:
        {
         Mat_<Vec3b> _I = I;
 
         for( int i = 0; i < I.rows; ++i)
            for( int j = 0; j < I.cols; ++j )
               {
                   _I(i,j)[0] = table[_I(i,j)[0]];
                   _I(i,j)[1] = table[_I(i,j)[1]];
                   _I(i,j)[2] = table[_I(i,j)[2]];
            }
         I = _I;
         break;
        }
    }
 
    return I;
}