OpenCV: Convert Mat into UChar4 - c++

Question

I've just completed the Udacity Parallel programming stage 2 course, and I'm now implementing what I've learnt into a basic app with OpenCV which applies a gaussian blur to a constant stream of images coming through a webcam.
I'm loading frames into a Mat object, and whilst in my loop I want to call a method gaussian_cpu, the only problem is it requires a uchar4 to be passed to both the input and output parameters. How would I convert a Mat object to uchar4?
// Keep processing frames - Do CPU First
while(cpu_frames > 0)
{
cout << cpu_frames << "\n";
camera >> frameIn;
gaussian_cpu(frameIn, frameOut, numRows(), numCols(), h_filter__, 9);
imshow("Source", frameIn);
imshow("Dest", frameOut);
// 2ms delay to prevent system from being interrupted whilst drawing the new frame
waitKey(2);
cpu_frames--;
}
My method signature then looks like this:
void gaussian_cpu(
const uchar4* const rgbaImage, // input image from the camera
uchar4* const outputImage, // The image we are writing back for display
size_t numRows, size_t numCols, // Width and Height of the input image (rows/cols)
const float* const filter, // The value of sigma
const int filterWidth // The size of the stencil (3x3) 9
)
I need to use uchar4 so I can split the channels, do my convolution and then recombine the channels to return the output image. Is there any way to do this?

Answer 1

opencv generally uses bgr, 3 channel Mats, but a basic:
Mat bgra;
cvtColor( frameIn, bgra, CV_BGR2BGRA );
will generate an (unused) 4th channel. now you probably have to allocate mem for you outputImage:
Mat frameOut( bgra.size(), bgra.type() );
then you can feed those into your gaussian_cpu():
int filterWidth=5;
float *filter = ... // your job, not mine ;)
gaussian_cpu( (uchar4*)(bgra.data), (uchar4*)(frameOut.data), bgra.rows, bgra.cols, filter, filterWidth );