OpenCV get pixel channel value from Mat image

Solution 1:

Assuming the type is CV_8UC3 you would do this:

for(int i = 0; i < foo.rows; i++)
{
    for(int j = 0; j < foo.cols; j++)
    {
        Vec3b bgrPixel = foo.at<Vec3b>(i, j);

        // do something with BGR values...
    }
}

Here is the documentation for Vec3b. Hope that helps! Also, don't forget OpenCV stores things internally as BGR not RGB.

EDIT :
For performance reasons, you may want to use direct access to the data buffer in order to process the pixel values:

Here is how you might go about this:

uint8_t* pixelPtr = (uint8_t*)foo.data;
int cn = foo.channels();
Scalar_<uint8_t> bgrPixel;

for(int i = 0; i < foo.rows; i++)
{
    for(int j = 0; j < foo.cols; j++)
    {
        bgrPixel.val[0] = pixelPtr[i*foo.cols*cn + j*cn + 0]; // B
        bgrPixel.val[1] = pixelPtr[i*foo.cols*cn + j*cn + 1]; // G
        bgrPixel.val[2] = pixelPtr[i*foo.cols*cn + j*cn + 2]; // R

        // do something with BGR values...
    }
}

Or alternatively:

int cn = foo.channels();
Scalar_<uint8_t> bgrPixel;

for(int i = 0; i < foo.rows; i++)
{
    uint8_t* rowPtr = foo.row(i);
    for(int j = 0; j < foo.cols; j++)
    {
        bgrPixel.val[0] = rowPtr[j*cn + 0]; // B
        bgrPixel.val[1] = rowPtr[j*cn + 1]; // G
        bgrPixel.val[2] = rowPtr[j*cn + 2]; // R

        // do something with BGR values...
    }
}

Solution 2:

The below code works for me, for both accessing and changing a pixel value.

For accessing pixel's channel value :

for (int i = 0; i < image.cols; i++) {
    for (int j = 0; j < image.rows; j++) {
        Vec3b intensity = image.at<Vec3b>(j, i);
        for(int k = 0; k < image.channels(); k++) {
            uchar col = intensity.val[k]; 
        }   
    }
}

For changing a pixel value of a channel :

uchar pixValue;
for (int i = 0; i < image.cols; i++) {
    for (int j = 0; j < image.rows; j++) {
        Vec3b &intensity = image.at<Vec3b>(j, i);
        for(int k = 0; k < image.channels(); k++) {
            // calculate pixValue
            intensity.val[k] = pixValue;
        }
     }
}

`

Source : Accessing pixel value