Convert Mat to char array (and vice-versa)

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Hi,

I tried converting a grayscale, single channel Mat (type: 8UC1) to a char[] array but somewhere it crashes. I am using android ndk and I'm coding in c++ in one of the native classes. I only get fatal signal 11 as error (probably refers to wrong memory access?).

Here's the code I am using:

unsigned char buffer[height * width];

for (int j = 0; j < height; j++) {
    for (int i = 0; i < width; i++) {
        uchar& uxy = myMat.at<uchar>(j, i);
        int color = (int) uxy;
        buffer[j * width + i] = color;
        }
}

I want to use a byte[] buffer to store the image (mat) and I need to access it's elements like this: y * width + x (where y=row index, x=column index). Because I don't have a byte[] structure in Android native, I decided to use a char[] array.

Is there any faster option? And I also want to convert the byte[] array (in my case, char[] array) back to a Mat. Thanks!

Edit:

Converting back code (untested!):

Mat convertToMat(unsigned char *buffer) {
    Mat tmp(width, height, CV_8UC1);
    for (int x = 0; x < height; x++) {
        for (int y = 0; y < width; y++) {
            int value = (int) buffer[x * width + y];
            tmp.at<int>(y, x) = value; 
        }
    }
    return tmp;
}

myMat = convertToMat(buffer);