How do you access pixel channel values in a Mat image in OpenCV?

Extracting Pixel Channel Values from a Mat Image in OpenCV

In OpenCV, there are several ways to access and manipulate pixel values within a Mat image. One common scenario is retrieving the channel value of a specific pixel.

Using Vec3b

If the image is stored in the CV_8UC3 format, which represents a three-channel image with 8-bit unsigned integer values (BGR), you can use the Vec3b data type to access the individual channels. For example, if foo is a Mat image of this type, you can iterate over the pixels using the following code:

for (int i = 0; i < foo.rows; i++)
{
    for (int j = 0; j < foo.cols; j++)
    {
        Vec3b bgrPixel = foo.at<Vec3b>(i, j);
        
        // Access the pixel's BGR values...
    }
}

Using Direct Data Buffer Access

For performance reasons, you can also access the pixel values directly by accessing the data buffer. This provides faster access but requires more manual handling of data conversion and channel offsets. Here is an example of how to do 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
        
        // Access the pixel's BGR values...
    }
}

Alternatively

Alternatively, you can also use the row() method to access individual rows of the image, as shown in the following code:

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

        // Access the pixel's BGR values...
    }
}

Remember that OpenCV stores pixel values in BGR format (Blue, Green, Red) instead of RGB, so you need to be mindful of that when accessing the channels.

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