Understanding OpenCV's undistort function - c++

Question

I'm looking to undistort an image using the distortion coefficients that I've computed for my camera, without changing the camera matrix. This is exactly what undistort() does, but I wanted to draw the output to a larger canvas image.
When I tried this:
Mat drawtransform = getOptimalNewCameraMatrix(cameraMatrix, distCoeffs, size, 1.0, size * 2);
undistort(inputimage, undistorted, cameraMatrix, distCoeffs, drawtransform);
It still wrote out the same sized image, but only the top left quarter of the scaled-up-by-two undistorted result. Like the documentation says, undistort writes into a target image of the same size.
It's pretty obvious that I can just go copy out and reimplement a slightly tweaked version of undistort() but I am having some trouble understanding what it is doing. Here's the source:
void cv::undistort( InputArray _src, OutputArray _dst, InputArray _cameraMatrix,
InputArray _distCoeffs, InputArray _newCameraMatrix )
{
Mat src = _src.getMat(), cameraMatrix = _cameraMatrix.getMat();
Mat distCoeffs = _distCoeffs.getMat(), newCameraMatrix = _newCameraMatrix.getMat();
_dst.create( src.size(), src.type() );
Mat dst = _dst.getMat();
CV_Assert( dst.data != src.data );
int stripe_size0 = std::min(std::max(1, (1 << 12) / std::max(src.cols, 1)), src.rows);
Mat map1(stripe_size0, src.cols, CV_16SC2), map2(stripe_size0, src.cols, CV_16UC1);
Mat_<double> A, Ar, I = Mat_<double>::eye(3,3);
cameraMatrix.convertTo(A, CV_64F);
if( distCoeffs.data )
distCoeffs = Mat_<double>(distCoeffs);
else
{
distCoeffs.create(5, 1, CV_64F);
distCoeffs = 0.;
}
if( newCameraMatrix.data )
newCameraMatrix.convertTo(Ar, CV_64F);
else
A.copyTo(Ar);
double v0 = Ar(1, 2);
for( int y = 0; y < src.rows; y += stripe_size0 )
{
int stripe_size = std::min( stripe_size0, src.rows - y );
Ar(1, 2) = v0 - y;
Mat map1_part = map1.rowRange(0, stripe_size),
map2_part = map2.rowRange(0, stripe_size),
dst_part = dst.rowRange(y, y + stripe_size);
initUndistortRectifyMap( A, distCoeffs, I, Ar, Size(src.cols, stripe_size),
map1_part.type(), map1_part, map2_part );
remap( src, dst_part, map1_part, map2_part, INTER_LINEAR, BORDER_CONSTANT );
}
}
About half of the lines here are for sanity checking and initializing input parameters. What I'm confused about is what's going on with map1 and map2. These names are sadly less descriptive than most. I must be missing some explanation, maybe it's tucked away in some introduction page, or under the doc for another function.
map1 is a two channel signed short integer matrix and map2 is an unsigned short integer matrix, both are of dimension (height, max(4096/width, 1)). The question is, why? What will these maps contain? What is the significance and purpose of this striping? What is the significance and purpose of the strange dimension of the stripes?

Answer 1

Use initUndistortRectifyMap to obtain the transformation to the scale you desire , then apply its output (the two matrices you mention) to remap .
The first map is used to compute the transform the x coordinate at each pixel position, the second is used to transform the y coordinate.

Answer 2

You might want to read the description for the function remap. The map represents the pixel X,Y location in the source image for every pixel in the destination image. Map1_part is every X location in the source, and Map2_part is every Y location in the source.
Without reading into it much, the striping could be a method of speeding up the transformation process.
EDIT:
Also, if you are looking to just scale your image to a larger dimension you could just re-size the output image.
double scaleX = 2.0;
double scaleY = 2.0;
cv::Mat undistortedScaled;
cv::resize(undistorted, undistortedScaled, cv::Size(0,0), scaleX, scaleY);