converting from unsigned char to uchar4 - c++

Question

i'm learning CUDA and i came across a course that is helping even though the code is very old and i'm having problems running it i'm trying to understand it, so he reads images using openCV imread which gives a Mat obj i guess but the data is saved as a uchar*
cv::Mat image = cv::imread(filename.c_str(), CV_LOAD_IMAGE_COLOR);
but after i was stuck in converting uchar to uchar4 and i was reading the code from the teacher and he wrote .
cv::Mat image = cv::imread(filename.c_str(), CV_LOAD_IMAGE_COLOR);
if (image.empty()) {
std::cerr << "Couldn't open file: " << filename << std::endl;
exit(1);
}
cv::cvtColor(image, imageInputRGBA, CV_BGR2RGBA);
//allocate memory for the output
imageOutputRGBA.create(image.rows, image.cols, CV_8UC4);
//This shouldn't ever happen given the way the images are created
//at least based upon my limited understanding of OpenCV, but better to check
if (!imageInputRGBA.isContinuous() || !imageOutputRGBA.isContinuous()) {
std::cerr << "Images aren't continuous!! Exiting." << std::endl;
exit(1);
}
*h_inputImageRGBA = (uchar4 *)imageInputRGBA.ptr<unsigned char>(0);
*h_outputImageRGBA = (uchar4 *)imageOutputRGBA.ptr<unsigned char>(0);
are the two last lines the ones where he subtly converts from uchar to uchar4 ...
h_inputImageRGBA
h_outputImageRGBA
are both uchar4**
can somebody help me understand the code
here is the link to the source
function name : Preprocess