glDrawPixels to upload to stencil buffer problem

OpenGL OpenGL: Basic Coding
1

I’m creating a image using OpenCV which I would like to use as a stencil. However, when I upload the image using glDrawPixels and then read it back using glReadPixels, I only get a fraction of the image. It is not obvious to me what is the problem since the alignment appears correct. Help would be appreciated. The code is below.

//---------------------------------------------------------------------
// createStencilBuffer
//---------------------------------------------------------------------
void RenderRadiance::createStencilBuffer(void)
{
cv::Mat stencilImg(_radDim.y,_radDim.x,CV_8UC1);

stencilImg.setTo(0);

for (size_t i = 0;i < _hPntLst.size();i++)
{
HPnt hPnt = _hPntLst[i];

cv::circle(stencilImg,cv::Point(hPnt._hCen.x,hPnt._hCen.y),(int)_hogelDiameter/2,cv::Scalar(255),-1);

}

glPixelStorei(GL_UNPACK_ALIGNMENT,1);
glPixelStorei(GL_UNPACK_ROW_LENGTH,_radDim.x);
glPixelStorei(GL_UNPACK_SKIP_ROWS,0);
glPixelStorei(GL_UNPACK_SKIP_PIXELS,0);

glPixelStorei(GL_PACK_ALIGNMENT,1);
glPixelStorei(GL_PACK_ROW_LENGTH,_radDim.x);
glPixelStorei(GL_PACK_SKIP_ROWS,0);
glPixelStorei(GL_PACK_SKIP_PIXELS,0);

glGenRenderbuffers(1,&_renderStencilBuffer);
glBindRenderbuffer(GL_RENDERBUFFER,_renderStencilBuffer);
glRenderbufferStorage(GL_RENDERBUFFER,GL_STENCIL_INDEX,_radDim.x,_radDim.y);

glDrawPixels(_radDim.x,_radDim.y,GL_STENCIL_INDEX,GL_UNSIGNED_BYTE,stencilImg.data);

if (1)
{
cv::Mat stencilImgRead(_radDim.y,_radDim.x,CV_8UC1);

glReadPixels(0,0,_radDim.x,_radDim.y,GL_STENCIL_INDEX,GL_UNSIGNED_BYTE,stencilImgRead.data);

cv::imwrite("Stencil.png",stencilImg);
cv::imwrite("StencilGPU.png",stencilImgRead);

}
}