I have been through the documentation and didn't get a clear detailed description about UMat; however I think it has something to relate with GPU and CPU. Please help me out. Thank you.
Perhaps section 3 of this document will help: [link now broken] https://software.intel.com/sites/default/files/managed/2f/19/inde_opencv_3.0_arch_guide.pdf
Specifically, section 3.1:
A unified abstraction cv::UMat that enables the same APIs to be implemented using CPU or OpenCL code, without a requirement to call OpenCL accelerated version explicitly. These functions use an OpenCL-enabled GPU if exists in the system, and automatically switch to CPU operation otherwise.
and section 3.3:
Generally, the cv::UMat is the C++ class, which is very similar to cv::Mat. But the actual UMat data can be located in a regular system memory, dedicated video memory, or shared memory.
Link to usage suggested in the comments by @BourbonCreams:
https://docs.opencv.org/3.0-rc1/db/dfa/tutorial_transition_guide.html#tutorial_transition_hints_opencl
来源：https://stackoverflow.com/questions/33602675/what-is-the-difference-between-umat-and-mat-in-opencv