How should I name my RGB channels, using cv::Mat_ - c++

Question

I want to access my matrix elements in the following manner:
frame[i][j].Red
, that is, the (i,j)-th pixe's red channel.
I have tried:
typedef struct{unsigned char Blue,Green,Red;}Pixel;
typedef cv::Mat_<Pixel> Image;
However when trying to imread(), imwrite() or whatever with the thus defined type, g++ greets me with:
OpenCV Error: Assertion failed (func != 0) in convertTo, file /home/users/mvitkov/projects/opencv-legacy/OpenCV-2.3.1/modules/core/src/convert.cpp, line 937
terminate called after throwing an instance of 'cv::Exception'
what(): /home/users/mvitkov/projects/opencv-legacy/OpenCV-2.3.1/modules/core/src/convert.cpp:937: error: (-215) func != 0 in function convertTo
Update: So no answer to my probably badly asked question. Too bad. The essence of the question is how to address the individual channels with sensible names (red, green, bkue), and not the c-era array indexing notation [2]. Duh!

Answer 1

Here's how you access each channel:
blue = frame.at<cv::Vec3b>(i,j)[0];
green = frame.at<cv::Vec3b>(i,j)[1];
red = frame.at<cv::Vec3b>(i,j)[2];
The above code assumes that you have a 3-channel image where each value is an 8-bit unsigned char (CV_8UC3). This type is used in many common image formats. However, if you have a different type of 3-channel image, here's what you do:
If the image type is 3-channel float (CV_32FC3), then replace cv::Vec3b with cv::Vec3f
If the image type is 3-channel double (CV_64FC3), then replace cv::Vec3b with cv::Vec3d
If the image type is 3-channel int (CV_32SC3), then replace cv::Vec3b with cv::Vec3i
If the image type is 3-channel short int (CV_16SC3) or 16-bit uchar (CV_16UC3), then replace cv::Vec3b with cv::Vec3s
Not sure what image format you're using? Try calling getImgType(frame) (see the code below).
string getImgType(cv::Mat frame)
{
int imgTypeInt = frame.type();
int numImgTypes = 28; // 7 base types, with 4 channel options each (C1, ..., C4)
int enum_ints[] = {CV_8UC1, CV_8UC2, CV_8UC3, CV_8UC4, CV_8SC1, CV_8SC2, CV_8SC3, CV_8SC4, CV_16UC1, CV_16UC2, CV_16UC3, CV_16UC4, CV_16SC1, CV_16SC2, CV_16SC3, CV_16SC4, CV_32SC1, CV_32SC2, CV_32SC3, CV_32SC4, CV_32FC1, CV_32FC2, CV_32FC3, CV_32FC4, CV_64FC1, CV_64FC2, CV_64FC3, CV_64FC4};
string enum_strings[] = {"CV_8U", "CV_8UC1", "CV_8UC2", "CV_8UC3", "CV_8UC4", "CV_8SC1", "CV_8SC2", "CV_8SC3", "CV_8SC4", "CV_16UC1", "CV_16UC2", "CV_16UC3", "CV_16UC4", "CV_16SC1", "CV_16SC2", "CV_16SC3", "CV_16SC4", "CV_32SC1", "CV_32SC2", "CV_32SC3", "CV_32SC4", "CV_32FC1", "CV_32FC2", "CV_32FC3", "CV_32FC4", "CV_64FC1", "CV_64FC2", "CV_64FC3", "CV_64FC4"};
for(int i=0; i<numImgTypes; i++)
{
if(imgTypeInt == enum_ints[i]) return enum_strings[i];
}
return "unknown image type";
}

Answer 2

If the image type is 3-channel short int (CV_16SC3) or 16-bit uchar (CV_16UC3), then replace cv::Vec3b with cv::Vec3s
this is simply not correct. I adressed a 3 channel 16 bit unsigned image once with < Vec3s>, and got a negative value G:29096 B:-21671 R:23413 returned.
If CV_16UC3, adress Mat with < Vec3w>. Using this I got G:29096 B:43865 R:23413
the "s" in CV_16SC3 stands for signed and not short.