cv::Mat.data always returns the same value

Question

I have a red image - just for testing. The RGB Color is (217/18/36).

I perform the following code:

void QuaterImage(Mat& SrcImage, Mat& DestImage, bool Downsample) {
    int newWidth = 0, newHeight = 0;
    int newOrigWidth = 0, newOrigHeight = 0;

    if (SrcImage.cols % 2 > 0) { newOrigWidth = SrcImage.cols - 1; } else { newOrigWidth = SrcImage.cols; }
    if (SrcImage.rows % 2 > 0) { newOrigHeight = SrcImage.rows - 1; } else { newOrigHeight = SrcImage.rows; }

    if (SrcImage.depth() != CV_8U) { return; }

    newHeight = newOrigHeight / 2;
    newWidth = newOrigWidth / 2;

    DestImage = Mat(newWidth, newHeight, SrcImage.type());
    int r = 0, c = 0;
    uchar* DataPtr = SrcImage.ptr<uchar>(0);

    std::cout << std::to_string(*DataPtr) << std::endl;
    return;
}

It always returns "205".

If I change the image to be complettely yellow, it returns the exact same value. How can that be?

Regards, Jan

you only consider the first byte of the bmp, is really what you want ? — bruno, Apr 08 '19 at 16:23
Hi, postet the complette function. I know, I'm currently only accessing the first byte. And thats not, what I want - but currently, I'm searching for the error... If thats fixed, I'll continue. In the end, I like to design a function which can downsample an image bei half of the width and half of the hight as fast as possible. therefor I only want to take the average of four pixel for one pixel in the cores image. — Jan021981, Apr 08 '19 at 17:29
no, I only cut the part, where I calculate the new width and new hight. You can delete that part, because it's not used until now. The assigning of the pointer and the output is exactly the same. — Jan021981, Apr 08 '19 at 17:32
Maybe you just want this: https://stackoverflow.com/questions/10431919/downsampling-without-smoothing — drescherjm, Apr 08 '19 at 17:33
something like that, but first I would like to know, where my mistake is... — Jan021981, Apr 08 '19 at 18:27

score 0 · Answer 1 · answered Apr 08 '19 at 18:35

0

Ok, sorry, I got it. You could not see it. I was giving the same image for SrcImage and DestImage, so the line DestImage = Mat(newWidth, newHeight, SrcImage.type());overwrote the image. Before that line, the values are correct.

Sorry for that...

answered Apr 08 '19 at 18:35

Jan021981

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for future errors, it is Mat(height, width, ...) order – Micka Apr 08 '19 at 19:45

cv::Mat.data always returns the same value

1 Answers1