How to Efficiently Access Pixel Channel Values in OpenCV Mat Images?

Retrieving Pixel Channel Values from OpenCV Mat Representations

To obtain the channel value for a specific pixel within an OpenCV Mat image, you can utilize the following methods:

Method 1: Using OpenCV's Vec3b

For Mat images of type CV_8UC3, containing three channels (blue, green, and red), the Vec3b data structure can be employed:

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

        // Access individual channel values:
        uint8_t blue = bgrPixel[0];
        uint8_t green = bgrPixel[1];
        uint8_t red = bgrPixel[2];
    }
}

Method 2: Direct Buffer Access

For improved performance, direct access to the image data buffer can be utilized:

uint8_t* pixelPtr = (uint8_t*)foo.data;
int cn = foo.channels(); // Number of channels per pixel

for(int i = 0; i < foo.rows; i++)
{
    for(int j = 0; j < foo.cols; j++)
    {
        // Retrieve channel values:
        uint8_t blue = pixelPtr[i*foo.cols*cn + j*cn + 0];
        uint8_t green = pixelPtr[i*foo.cols*cn + j*cn + 1];
        uint8_t red = pixelPtr[i*foo.cols*cn + j*cn + 2];
    }
}

Note: OpenCV internally stores pixel data in BGR (blue, green, red) format, rather than RGB.

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