More details on supported platform

[fcoulombe]

I was trying to force opencl to use my nvidia gpu since the default was always using the intel version.

I found page http://docs.opencv.org/modules/ocl/doc/introduction.html which explained that I should set the environment variable OPENCV_OPENCL_DEVICE but after reading this, I wasn't exactly sure of the string that I should write. I guessed "NVIDIA" but I wasn't sure if it was supposed to be in lower case, upper case, whether it mattered.

Maybe showing the common options might help the next reader

[alalek]

For example, for GTX 650 these variants work fine:

OPENCV_OPENCL_DEVICE=:GPU:650
or
OPENCV_OPENCL_DEVICE=NVIDIA:GPU:650
or
OPENCV_OPENCL_DEVICE=NVIDIA:GPU:

If it doesn't help please provide more information:

• Output dump from clinfo tool? (I'm not sure that it was installed with NVIDIA OpenCL SDK, but It exists in AMD OpenCL SDK and can be used anywhere)
• Or build and run test for ocl module (OpenCV 2.4.x) or core module (OpenCV 3.0+). Please dump here "OpenCL platform" section, like this:

OpenCL Platforms: 
    AMD Accelerated Parallel Processing
        CPU: Intel(R) Core(TM) i5-2500 CPU @ 3.30GHz (OpenCL 1.2 AMD-APP (1214.3))
    Intel(R) OpenCL
        CPU:         Intel(R) Core(TM) i5-2500 CPU @ 3.30GHz (OpenCL 1.2 (Build 76921))
    NVIDIA CUDA
        dGPU: GeForce GTX 650 (OpenCL 1.1 CUDA)

[fcoulombe]

I figured out what to put in the variable. my point is that the documentation is lacking. I'm a bit new to opencv and really wasnt sure what I was supposed to set as a value.

[StevenPuttemans]

@fcoulombe you might be up to providing a PR with some more information added in the documentation?