restore CUDA module introduction

put it into core documentation

modules/core/doc/cuda.markdown

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+CUDA Module Introduction {#cuda_intro}
+========================
+
+General Information
+-------------------
+
+The OpenCV CUDA module is a set of classes and functions to utilize CUDA computational capabilities.
+It is implemented using NVIDIA\* CUDA\* Runtime API and supports only NVIDIA GPUs. The OpenCV CUDA
+module includes utility functions, low-level vision primitives, and high-level algorithms. The
+utility functions and low-level primitives provide a powerful infrastructure for developing fast
+vision algorithms taking advantage of CUDA whereas the high-level functionality includes some
+state-of-the-art algorithms (such as stereo correspondence, face and people detectors, and others)
+ready to be used by the application developers.
+
+The CUDA module is designed as a host-level API. This means that if you have pre-compiled OpenCV
+CUDA binaries, you are not required to have the CUDA Toolkit installed or write any extra code to
+make use of the CUDA.
+
+The OpenCV CUDA module is designed for ease of use and does not require any knowledge of CUDA.
+Though, such a knowledge will certainly be useful to handle non-trivial cases or achieve the highest
+performance. It is helpful to understand the cost of various operations, what the GPU does, what the
+preferred data formats are, and so on. The CUDA module is an effective instrument for quick
+implementation of CUDA-accelerated computer vision algorithms. However, if your algorithm involves
+many simple operations, then, for the best possible performance, you may still need to write your
+own kernels to avoid extra write and read operations on the intermediate results.
+
+To enable CUDA support, configure OpenCV using CMake with WITH\_CUDA=ON . When the flag is set and
+if CUDA is installed, the full-featured OpenCV CUDA module is built. Otherwise, the module is still
+built but at runtime all functions from the module throw Exception with CV\_GpuNotSupported error
+code, except for cuda::getCudaEnabledDeviceCount(). The latter function returns zero GPU count in
+this case. Building OpenCV without CUDA support does not perform device code compilation, so it does
+not require the CUDA Toolkit installed. Therefore, using the cuda::getCudaEnabledDeviceCount()
+function, you can implement a high-level algorithm that will detect GPU presence at runtime and
+choose an appropriate implementation (CPU or GPU) accordingly.
+
+Compilation for Different NVIDIA\* Platforms
+--------------------------------------------
+
+NVIDIA\* compiler enables generating binary code (cubin and fatbin) and intermediate code (PTX).
+Binary code often implies a specific GPU architecture and generation, so the compatibility with
+other GPUs is not guaranteed. PTX is targeted for a virtual platform that is defined entirely by the
+set of capabilities or features. Depending on the selected virtual platform, some of the
+instructions are emulated or disabled, even if the real hardware supports all the features.
+
+At the first call, the PTX code is compiled to binary code for the particular GPU using a JIT
+compiler. When the target GPU has a compute capability (CC) lower than the PTX code, JIT fails. By
+default, the OpenCV CUDA module includes:
+
+\*
+   Binaries for compute capabilities 1.3 and 2.0 (controlled by CUDA\_ARCH\_BIN in CMake)
+
+\*
+   PTX code for compute capabilities 1.1 and 1.3 (controlled by CUDA\_ARCH\_PTX in CMake)
+
+This means that for devices with CC 1.3 and 2.0 binary images are ready to run. For all newer
+platforms, the PTX code for 1.3 is JIT'ed to a binary image. For devices with CC 1.1 and 1.2, the
+PTX for 1.1 is JIT'ed. For devices with CC 1.0, no code is available and the functions throw
+Exception. For platforms where JIT compilation is performed first, the run is slow.
+
+On a GPU with CC 1.0, you can still compile the CUDA module and most of the functions will run
+flawlessly. To achieve this, add "1.0" to the list of binaries, for example,
+CUDA\_ARCH\_BIN="1.0 1.3 2.0" . The functions that cannot be run on CC 1.0 GPUs throw an exception.
+
+You can always determine at runtime whether the OpenCV GPU-built binaries (or PTX code) are
+compatible with your GPU. The function cuda::DeviceInfo::isCompatible returns the compatibility
+status (true/false).
+
+Utilizing Multiple GPUs
+-----------------------
+
+In the current version, each of the OpenCV CUDA algorithms can use only a single GPU. So, to utilize
+multiple GPUs, you have to manually distribute the work between GPUs. Switching active devie can be
+done using cuda::setDevice() function. For more details please read Cuda C Programming Guide.
+
+While developing algorithms for multiple GPUs, note a data passing overhead. For primitive functions
+and small images, it can be significant, which may eliminate all the advantages of having multiple
+GPUs. But for high-level algorithms, consider using multi-GPU acceleration. For example, the Stereo
+Block Matching algorithm has been successfully parallelized using the following algorithm:
+
+1.  Split each image of the stereo pair into two horizontal overlapping stripes.
+2.  Process each pair of stripes (from the left and right images) on a separate Fermi\* GPU.
+3.  Merge the results into a single disparity map.
+
+With this algorithm, a dual GPU gave a 180% performance increase comparing to the single Fermi GPU.
+For a source code example, see <https://github.com/Itseez/opencv/tree/master/samples/gpu/>.