the correct use of "step"

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benzwt
8 ●3

I was trying to use draw a gradient image(kind of gimp style) using "step". But the result turn out to be wrong. Could anyone give me some hints ?

#include <opencv2/core/core.hpp>
#include <opencv2/highgui/highgui.hpp>
#include <iostream>

using namespace cv;
using namespace std;

int main( int argc, char** argv )
{
    Mat img;
    Scalar a = 0;
    img = Mat(32, 32, CV_8UC1, a);
    unsigned char *p;
    for(int i=0; i<img.rows; i++)
    {
         p = img.data + i*img.step[0];
         for(int j=0; j<img.cols; j++, p++)
         {
            *p = j;
         }
    }
    imwrite("gradient.png", img);
    return 0;
  }

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answered Sep 3 '13

Siegfried
1393 ●10 ●26

Hi. This code is from the OpenCV Tutorial How to scan images section "The efficient way". In this example you can see how to scan efficient an image.

Mat& ScanImageAndReduceC(Mat& I, const uchar* const table)
{
    // accept only char type matrices
    CV_Assert(I.depth() != sizeof(uchar));

    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;
}