It's not an error if some OpenCL platform has no devices. This makes
OpenCL device detection work correctly in the following scenario:

$ OPENCV_OPENCL_DEVICE=:GPU: ./opencv_test_dnn

OpenCV version: 4.1.2-dev
OpenCV VCS version: 4.1.2-80-g467748ee98-dirty
Build type: Debug
Compiler: /usr/bin/g++  (ver 7.4.0)
Parallel framework: pthreads
CPU features: SSE SSE2 SSE3 *SSE4.1 *SSE4.2 *FP16 *AVX *AVX2 *AVX512-SKX?
Intel(R) IPP version: ippIP AVX2 (l9) 2019.0.0 Gold (-) Jul 24 2018
OpenCL Platforms:
    AMD Accelerated Parallel Processing
    Portable Computing Language
        CPU: pthread-AMD Ryzen 7 2700X Eight-Core Processor (OpenCL 1.2 pocl HSTR: pthread-x86_64-pc-linux-gnu-znver1)
    NVIDIA CUDA
        dGPU: GeForce GTX 1080 (OpenCL 1.2 CUDA)
Current OpenCL device:
    Type = dGPU
    Name = GeForce GTX 1080
    Version = OpenCL 1.2 CUDA
    Driver version = 430.26

* Convert moments in tile algorithms to HAL (1.3x faster for VSX).

* Adding NEON code back in for non 64-bit platforms.

* Remove floats from post processing.

* Reduce code nesting
* Drop redundant .c_str() calls

Correct stereoRectify documentation

Fix wrong definition of viz::Color::navy()

Clarify stereoRectify() doc

The function stereoRectify() takes as input a coordinate transform between two cameras. It is ambiguous how it goes. I clarified that it goes from the second camera to the first.

* Use FlsAlloc/FlsFree/FlsGetValue/FlsSetValue instead of TlsAlloc/TlsFree/TlsGetValue/TlsSetValue to implment TLS value cleanup when thread has been terminated on Windows Vista and above

* Fix 32-bit build

* Fixed calling convention of cleanup callback

* WINAPI changed to NTAPI

* Use proper guard macro

* Vectorize flipHoriz and flipVert functions.

* Change v_load_mirror_1 to use vec_revb for VSX

* Only use vec_revb in ISA3.0

* Removing vec_revb code since some of the older compilers don't fully support it.

* Use new v_reverse intrinsic and cleanup code.

* Ensure there are no alignment issues with copies

Parallelize pyrDown & calcSharrDeriv

* ::pyrDown has been parallelized

* CalcSharrDeriv parallelized

* Fixed whitespace

* Set granularity based on amount of threads enabled

* Granularity changed to cv::getNumThreads, now each thread should receive 1/n sized stripes

* imgproc: move PyrDownInvoker<CastOp>::operator() implementation

* imgproc(pyramid): remove syloopboundary()

* video: SharrDerivInvoker replace 'Mat*' => 'Mat&' fields

* Changes disparity image to float representation
Signed-off-by: Connor James Smith <cjs.connor.smith@gmail.com>

* samples: update disparity multiplier handling in stereo_match.cpp

* Doc bugfix

The documentation page StereoBinaryBM and StereoBinarySGBM says that it returns a disparity that is scaled multiplied by 16. This scaling must be undone before calling reprojectImageTo3D, otherwise the results are wrong. The function reprojectImageTo3D() could do this scaling internally, maybe, but at least the documentation must explain that this has to be done.

* calib3d: update reprojectImageTo3D documentation

* calib3d: add StereoBM/StereoSGBM into notes list

* Add flags for build js

* Add poi.json

* Rebase whitelist into JSON file

* Rework generator of white_list

* Fix small typos

* Transfer opencv_js.josn in opencv_js.config.py

* Edit OPENCV_JS_WHITELIST

* Write comment

* Add description

* Fix typos in desc

* flag's append deleeted

* Fix whitespace

* variable deleted

* fix comment on lines 229 and 235

If tests are run through GTest option `--gtest_also_run_disabled_tests`

* Use circular lut hustory buffer in computeGradient of HOG

* Initialize prefetch data outside main loop.  Avoid code duplication.

- OPENCV_TRACE_ITT_ENABLE

- move TLS & instrumentation code out of core/utility.hpp
- (*) TLSData lost .gather() method (to dispose thread data on thread termination)
- use TLSDataAccumulator for reliable collecting of thread data
- prefer using of .detachData() + .cleanupDetachedData() instead of .gather() method

(*) API is broken: replace TLSData => TLSDataAccumulator if gather required
(objects disposal on threads termination is not available in accumulator mode)

I think it would help to change all 3 of the the input file arguments to be "positional" for consistency with the other tutorials. This also simplifies the command line input to run this tutorial by reducing typing, and helpfully prints the "usage" info if any of the 3 required inputs are missing.

I'm new to OpenCV and working through the tutorials. I kept getting runtime errors with this one until I realized that the arguments weren't positional, and I was missing the "--input1", "--input2, "--input3" flags preceding the filenames. All of the previous tutorials had required filenames as positional arguments and didn't require this.

The original code would require each input to be specified like this:
./compareHist_Demo --input1 filename1 --input2 filename2 --input3 filename3 

But with this change, the above command is simplified to:
./compareHist_Demo  filename1 filename2 filename3

This avoids a confusing runtime error to make things simpler for newcomers like me :)

Fixing bug with comparison of v_int64x2 or v_uint64x2

* Casting v_uint64x2 to v_float64x2 and comparing does NOT work in all cases.  Rewrite using epi64 instructions - faster too.

* Fix bad merge.

* Fix equal comparsion for non-SSE4.1. Add test cases for v_int64x2 comparisons.

* Try to fix merge conflict.

* Only test v_int64x2 comparisons if CV_SIMD_64F

* Fix compiler warning.