converting RGB to YCbCr in OpenCV - opencv

Question

I am currently converting image from RGB to YCrCb format using OpenCV function -cvtColor. I would like perform the conversion on my own with equations similar to
//equations for RGB to YUV conversion
Y' = 0.299 R + 0.587 G + 0.114 B
U = -0.147 R - 0.289 G + 0.436 B
V = 0.615 R - 0.515 G - 0.100 B.
I am not able to understand OpenCV image matrix operation. I would like to access RGB pixel values from image Mat so that I can perform the conversion myself. How can I get R,G,B values from image and then how to apply the transformation ? My current code below.
int main (int argc, char *argv[])
{
// Load in image
cv::Mat src = cv ::imread("C:\\openv2410\\frames\\frame_0.png",1);
// Create a vector for the channels and split the original image into B G R colour channels.
// Keep in mind that OpenCV uses BGR and not RGB images
vector<cv::Mat> spl;
split(src,spl);
// Create an zero pixel image for filling purposes - will become clear later
// Also create container images for B G R channels as colour images
cv::Mat empty_image = cv::Mat::zeros(src.rows, src.cols, CV_8UC1);
cv::Mat empty_channel = cv::Mat::zeros(src.rows, src.cols, CV_8UC1);
cv::Mat result_blue(src.rows, src.cols, CV_8UC3); // notice the 3 channels here!
cv::Mat result_green(src.rows, src.cols, CV_8UC3); // notice the 3 channels here!
cv::Mat result_red(src.rows, src.cols, CV_8UC3); // notice the 3 channels here!
// Create blue channel
cv::Mat in1[] = { spl[0], empty_image, empty_image };
int from_to1[] = { 0,0, 1,1, 2,2 };
mixChannels( in1, 3, &result_blue, 1, from_to1, 3 );
// Create green channel
cv::Mat in2[] = { empty_channel, spl[1], empty_image };
int from_to2[] = { 0,0, 1,1, 2,2 };
mixChannels( in2, 3, &result_green, 1, from_to2, 3 );
// Create red channel
cv::Mat in3[] = { empty_channel, empty_channel, spl[2]};
int from_to3[] = { 0,0, 1,1, 2,2 };
mixChannels( in3, 3, &result_red, 1, from_to3, 3 );
imshow("blue channel",result_blue);
imshow("green channel",result_green);
imshow("red channel",result_red);
cv::waitKey(0);
return 0;
}

Answer 1

Sample code for conversion from BGR to YCrCb. Source(1).
//sample input and output
float data[3][1] = { 98,76,88 };
Mat input( 1, 1, CV_32FC3, data) ;
Mat output( 1, 1, CV_32FC3 );
//iterate over all pixels
for(int i = 0; i < input.rows; i++) {
for(int j = 0; j < input.cols; j++) {
//get bgr pixel
Vec3f bgrPixel = input.at<Vec3f>(i, j);
float B = bgrPixel[0];
float G = bgrPixel[1];
float R = bgrPixel[2];
//actual conversion from BGR to YCrCb
float delta = 0.5f;
float Y = 0.299f * R + 0.587f * G + 0.114f * B;
float Cb = (B - Y) * 0.564f + delta;
float Cr = (R - Y) * 0.713f + delta;
//store into result image
Vec3f yCrCbPixel( Y, Cr, Cb );
output.at<Vec3f>(i, j) = yCrCbPixel;
}
}