OpenCV Mat class: Accessing elements of a multi-channel matrix - opencv

Question

I currently want to read in some values into a 3-channel, 480 row by 640 column matrix of 8 bit unsigned integer values. I am initializing the matrix like this:
Declaration:
rgbMatrix = Mat::zeros(480,640,CV_8UC3);
When I try to iterate through the entire matrix I am unable to assign/grab values using the following method. The values simply stay 0. My code looks like this:
for (int i = 0; i < rgbMatrix.rows; i++)
{
for (int j = 0; j < rgbMatrix.cols; j++)
{
(rgbMatrix.data + rgbMatrix.step * i)[j * rgbMatrix.channels() + 0] = *value0*;
(rgbMatrix.data + rgbMatrix.step * i)[j * rgbMatrix.channels() + 1] = *value1*;
(rgbMatrix.data + rgbMatrix.step * i)[j * rgbMatrix.channels() + 2] = *value2*;
}
}
However, when I declare three separate 1-channel matrices (also 480 row by 640 column of 8 bit unsigned integer values) and attempt to access elements of those matrices the following code works:
Declaration:
rgbMatrix0 = Mat::zeros(480,640,CV_8UC1);
rgbMatrix1 = Mat::zeros(480,640,CV_8UC1);
rgbMatrix2 = Mat::zeros(480,640,CV_8UC1);
for (int i = 0; i < rgbMatrix0.rows; i++)
{
for (int j = 0; j < rgbMatrix0.cols; j++)
{
(rgbMatrix0.data + rgbMatrix0.step * i)[j] = *value0*;
(rgbMatrix1.data + rgbMatrix1.step * i)[j] = *value1*;
(rgbMatrix2.data + rgbMatrix2.step * i)[j] = *value2*;
}
}
Now, I want to use just one matrix for these operations, as having to keep track of three separate variables will get tiresome after a while. I have a feeling that I am not accessing the right point in memory for the three-channel matrix. Does anyone know how I can accomplish what I did in the second portion of code but using one three-channel matrix instead of three separate one-channel matrices?
Thanks.

Answer 1

There are plenty of ways to do it, for example:
cv::Mat rgbMatrix(480,640,CV_8UC3);
for (int i = 0; i < rgbMatrix.rows; i++)
for (int j = 0; j < rgbMatrix.cols; j++)
for (int k = 0; k < 3; k++)
rgbMatrix.at<cv::Vec3b>(i,j)[k] = value;
[k] here is the channel value.
To set the all the matrix elements to a specific value like 5 for example you can do this:
cv::Mat rgbMatrix2(cv::Size(480,640), CV_8UC3, cv::Scalar(5,5,5));
std::cout << rgbMatrix2 << std::endl;

Answer 2

Sorry I can't see your code since I am writing from iPhone. When you use 3 channel matrix you can get the pixel using:
Vec3b pix = rgbMatrix.at(row,col);
Now you can access channel using:
pix[0] = 255; pix[1] += pix[2];
P.s. Generally rgbMatrix pixel is of type vec3b or vec3d. Always cast image.at<> with relevant type

Answer 3

Very Simple using Vec3b - for uchar, Vec3i - for int, Vec3f - for float, Vec3d - for double
Mat rgbMatrix = Mat::zeros(480,640,CV_8UC1);
for (int i = 0; i < rgbMatrix.rows; i++)
{
for (int j = 0; j < rgbMatrix.cols; j++)
{
rgbMatrix.at<Vec3b>(i,j)[0] = *value0;
rgbMatrix.at<Vec3b>(i,j)[1] = *value1;
rgbMatrix.at<Vec3b>(i,j)[2] = *value2;
}
}

Answer 4

vector<cv::Point3f> xyzBuffer;
cv::Mat xyzBuffMat = cv::Mat(307200, 1, CV_32FC3);
for (int i = 0; i < xyzBuffer.size(); i++) {
xyzBuffMat.at<cv::Vec3f>(i, 1, 0) = xyzBuffer[i].x;
xyzBuffMat.at<cv::Vec3f>(i, 1, 1) = xyzBuffer[i].y;
xyzBuffMat.at<cv::Vec3f>(i, 1, 2) = xyzBuffer[i].z;
}
Here, 0, 1, and 2 are respectively the channels that store x, y and z values.