OpenCV color image to gray trouble

Question

I'm new to OpenCV and I'm trying to proccess the image from the directory, make it black and white (grayscale) and then write it down to another file. But the output image is quite different from what I expected. Maybe you can help me and indicate the errors in code?

#include <iostream>
#include <opencv2/opencv.hpp>
#include <conio.h>
#include <string.h>
#include <string>
#include <opencv2/highgui/highgui.hpp>
#include <opencv2/core/core.hpp>
#include <stdio.h>
#include <stdlib.h>
#include "cuda_runtime.h"
#include "device_launch_parameters.h"

using namespace std;

void faktorial(int InSize, char *DataIn, char *DataOut)// заголовок функции
{
    for(int i = 0,  j = 0; i < InSize; i += 4, j++)
    {
        DataOut[j] = (DataIn[i] + DataIn[i + 1] + DataIn[i + 2]) / 3;
    }

}

int main() 
{        
        char* c = "E:\henrik-evensen-castle-valley-v03.jpg";

        printf("Input source of image\n Example of right directory file: E:\henrik-evensen-castle-valley-v03.jpg\n Your try:\n");
        char *tbLEN;
        tbLEN = new char [1024];

        cin.getline(tbLEN,1024);

        cout << tbLEN;


        IplImage* image;
        image = cvLoadImage(tbLEN, 1);
        int height1 = image->height;
        int width1 = image->width;
        int step = image->widthStep;
        int SizeIn = step*height1;
        char* DatIn = image->imageData;

        IplImage *image2 = cvCreateImage(cvSize(image->width, image->height), IPL_DEPTH_8U, 1);
        char* DatOut = image2->imageData;

        faktorial(SizeIn, DatIn, DatOut);

        cvNamedWindow("Imagecolor");
        cvShowImage("Imagecolor", image);

        cvNamedWindow("Gray");
        cvShowImage("Gray", image2);
        cvWaitKey(0);
        return 0;
}

EDIT: I don't need CvtColor function, I need to use that one factorial function.

Is there any particular reason you are using the C-API and not the C++ API? You *are* including the C++ headers anyway, and the C-API is as far as I know deprecated. Unless someone is forcing you to, I would recommend not using the C-API. Your error is also likely to disappear if you use the C++ API since it takes care of some things for you. — Hannes Ovrén, Apr 20 '16 at 14:57
Possible duplicate of [Convert RGB to Black & White in OpenCV](http://stackoverflow.com/questions/1585535/convert-rgb-to-black-white-in-opencv) — Samer, Apr 20 '16 at 15:03
**Samer** a lot of thanks for that link, it's quite useful for me, don't know how I could pass that theme. **Hannes Ovrén** So IplImage, cvSaveImage, cvCreateImage, etc are C-API, right? And in my case I need to use Mat, imwrite and others which are from C++ API, am I right? — Generwp, Apr 20 '16 at 15:07
Check [this](http://stackoverflow.com/a/34311841/5008845). Just sum up pixels and divide by 3, instead of ORing them. Remember to convert to int before summing pixel up, or you saturate. Also, forget about **obsolete C api**, and use C++ api — Miki, Apr 20 '16 at 16:04
as an addition to @Miki's comment take a look at [this page](http://www.johndcook.com/blog/2009/08/24/algorithms-convert-color-grayscale/) — sturkmen, Apr 20 '16 at 17:07

score 1 · Answer 1 · answered Apr 20 '16 at 15:09

1

Try cvtColor(src, bwsrc, CV_RGB2GRAY); http://docs.opencv.org/2.4/modules/imgproc/doc/miscellaneous_transformations.html (look for cvtColor).

answered Apr 20 '16 at 15:09

Leonardo Alves Machado

2,747
10
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The problem is: I really need to do that with that function (faktorial), without using cvtColor and stuff like that. So I need to take the image, give it to the function and get the black&white image out of it. Still thanks for try. – Generwp Apr 20 '16 at 15:15
So, check the number of channels of your final image. That's probably why you are having this effect, – Leonardo Alves Machado Apr 20 '16 at 15:46
And how can I do that? Can you explain please – Generwp Apr 20 '16 at 15:53
I left for lunch, and you got two new answers that will help you with that. – Leonardo Alves Machado Apr 20 '16 at 17:31

score 1 · Accepted Answer · answered Apr 20 '16 at 16:41

In faktorial you assume you have 3 channels. So you need to increase i by 3, and not by 4. Also, you need to convert char* data to uchar* data, so that accumulation works ok:

You end up with:

void faktorial(int InSize, uchar *DataIn, uchar *DataOut)
{
    for (int i = 0, j = 0; i < InSize; i += 3, j++)
    {
        DataOut[j] = (DataIn[i] + DataIn[i + 1] + DataIn[i + 2]) / 3;
    }
}

You can easily extend this to multiple channels, like:

void faktorial2(int InSize, int nChannels, uchar *DataIn, uchar *DataOut)
{
    for (int i = 0, j = 0; i < InSize; i += nChannels, j++)
    {
        int accum = 0;
        for (int c = 0; c < nChannels; ++c)
        {
            accum += DataIn[i + c];
        }
        DataOut[j] = uchar(accum / nChannels);
    }
}

You in general need also to take image stride into account:

void faktorial3(int rows, int cols, int in_step, int in_channels, int out_step, uchar *in, uchar *out)
{
    for (int r = 0; r < rows; ++r)
    {
        for (int c = 0; c < cols; ++c)
        {
            int accum = 0;
            for (int i = 0; i < in_channels; ++i)
            {
                accum += in[r*in_step + c * in_channels + i];
            }
            out[r*out_step + c] = uchar(accum / in_channels);
        }
    }
}

Here the full code with the calls:

#include <opencv2/opencv.hpp>
using namespace std;

void faktorial3(int rows, int cols, int in_step, int in_channels, int out_step, uchar *in, uchar *out)
{
    for (int r = 0; r < rows; ++r)
    {
        for (int c = 0; c < cols; ++c)
        {
            int accum = 0;
            for (int i = 0; i < in_channels; ++i)
            {
                accum += in[r*in_step + c * in_channels + i];
            }
            out[r*out_step + c] = uchar(accum / in_channels);
        }
    }
}

void faktorial(int InSize, uchar *DataIn, uchar *DataOut)
{
    for (int i = 0, j = 0; i < InSize; i += 3, j++)
    {
        DataOut[j] = (DataIn[i] + DataIn[i + 1] + DataIn[i + 2]) / 3;
    }

}

void faktorial2(int InSize, int nChannels, uchar *DataIn, uchar *DataOut)
{
    for (int i = 0, j = 0; i < InSize; i += nChannels, j++)
    {
        int accum = 0;
        for (int c = 0; c < nChannels; ++c)
        {
            accum += DataIn[i + c];
        }
        DataOut[j] = uchar(accum / nChannels);
    }
}

int main()
{
    char tbLEN[] = "D:\\SO\\img\\barns.jpg";

    IplImage* image;
    image = cvLoadImage(tbLEN, 1);

    IplImage *image2 = cvCreateImage(cvSize(image->width, image->height), IPL_DEPTH_8U, 1);

    int height1 = image->height;
    int width1 = image->width;
    int step = image->widthStep;
    int SizeIn = step*height1;
    int nChannels = image->nChannels;
    uchar* DatIn = (uchar*)image->imageData;
    uchar* DatOut = (uchar*)image2->imageData;

    faktorial(SizeIn, DatIn, DatOut);
    //faktorial2(SizeIn, nChannels, DatIn, DatOut);
    //faktorial3(image->height, image->width, image->widthStep, image->nChannels, image2->widthStep, (uchar*)image->imageData, (uchar*)image2->imageData);

    cvNamedWindow("Imagecolor");
    cvShowImage("Imagecolor", image);

    cvNamedWindow("Gray");
    cvShowImage("Gray", image2);
    cvWaitKey(0);
    return 0;
}

Remember that C api is obsolete. You should switch to C++ api.

score 0 · Answer 3 · answered Apr 20 '16 at 16:09

Your faktorial is intended for 4 byte per pixel images (and it doesn't take into account possible line padding).

Loaded from JPG image has 3 byte per pixel, that is why you see 4 shifted ghosts. You can modify faktorial or just convert loaded image to 4-byte format

 image = cvLoadImage(tbLEN, 1);
 cvtColor(image, image, CV_RGB2RGBA);

OpenCV color image to gray trouble

3 Answers3