OpenCV get pixel channel value from Mat image

Question

Maybe I'm not looking hard enough, but everything seems to want me to use an array. Thus, how do I get the channel value for a particular pixel for foo if foo is something like Mat foo = imread("bar.png")?

Answer 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...     } } 

Answer 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