Issue 1:
System information (version)

OpenCV => 4.0.0

Operating System / Platform => Ubuntu 18.04

Compiler => gcc 7.3.0

Detailed description

Hi all,

I am trying to compile OpenCV 4.0.0 release version on Ubuntu 18.04 but got this

[ 72%] Building CXX object modules/world/CMakeFiles/opencv_perf_gapi.dir/__/gapi/perf/internal/gapi_compiler_perf_tests.cpp.o
/home/owen/Downloads/opencv-4.0.0/modules/gapi/perf/internal/gapi_compiler_perf_tests.cpp: In member function ‘virtual void opencv_test::CompilerPerfTest_TestPerformance_Test::PerfTestBody()’:
/home/owen/Downloads/opencv-4.0.0/modules/gapi/perf/internal/gapi_compiler_perf_tests.cpp:26:29: error: ‘split3’ is not a member of ‘cv::gapi’
auto splitted = cv::gapi::split3(in);
^~~~~~
/home/owen/Downloads/opencv-4.0.0/modules/gapi/perf/internal/gapi_compiler_perf_tests.cpp:27:25: error: ‘addC’ is not a member of ‘cv::gapi’
auto add1 = cv::gapi::addC({1}, std::get<0>(splitted));
^~~~
/home/owen/Downloads/opencv-4.0.0/modules/gapi/perf/internal/gapi_compiler_perf_tests.cpp:28:25: error: ‘addC’ is not a member of ‘cv::gapi’
auto add2 = cv::gapi::addC({2}, std::get<1>(splitted));
^~~~
/home/owen/Downloads/opencv-4.0.0/modules/gapi/perf/internal/gapi_compiler_perf_tests.cpp:29:25: error: ‘addC’ is not a member of ‘cv::gapi’
auto add3 = cv::gapi::addC({3}, std::get<2>(splitted));
^~~~
/home/owen/Downloads/opencv-4.0.0/modules/gapi/perf/internal/gapi_compiler_perf_tests.cpp:30:24: error: ‘merge3’ is not a member of ‘cv::gapi’
auto out = cv::gapi::merge3(add1, add2, add3);
^~~~~~
/home/owen/Downloads/opencv-4.0.0/modules/gapi/perf/internal/gapi_compiler_perf_tests.cpp:35:65: error: ‘cv::gapi::core’ has not been declared
c.apply(in_mat1, out_mat_gapi, cv::compile_args(cv::gapi::core::fluid::kernels()));
^~~~
modules/world/CMakeFiles/opencv_perf_gapi.dir/build.make:230: recipe for target 'modules/world/CMakeFiles/opencv_perf_gapi.dir/__/gapi/perf/internal/gapi_compiler_perf_tests.cpp.o' failed
make[2]: *** [modules/world/CMakeFiles/opencv_perf_gapi.dir/__/gapi/perf/internal/gapi_compiler_perf_tests.cpp.o] Error 1
CMakeFiles/Makefile2:8188: recipe for target 'modules/world/CMakeFiles/opencv_perf_gapi.dir/all' failed
make[1]: *** [modules/world/CMakeFiles/opencv_perf_gapi.dir/all] Error 2
Makefile:162: recipe for target 'all' failed
make: *** [all] Error 2

Any idea? Thanks!

Issue 2:
System information (version)

OpenCV => : 4.1.0.25

Operating System / Platform => :Unbuntu16.04 64 Bit

Compiler => :Pycharm Community Edition2017.2

Detailed description

I use Opencv 4.1.0.25 for DIS Optical Flow, code as below , but I got the result of UseMeanNormalization to calc distances of current imgs and reference image, I got results parameter "flows" all zeros. This could not be true result, for I picture the moving object. I don't know why this happened. And I try on OpenCV Online Document for help, but I could not find detail descriptions on how to use python code of DIS Optical flow

Steps to reproduce

```.python
ref = np.uint8(allimgs[0] * 255)
motions = []
flows = []
h, w = ref.shape
count = h * w
totalMotions = 0
#
dis = cv2.DISOpticalFlow_create(1) #for use PRESET_FAST = 1
dis.setUseMeanNormalization(True)
flows.append(np.zeros((h, w), np.float32))
for i in range(1, len(allimgs)):
curr_img = np.uint8(allimgs[i] * 255)
flow = np.zeros((h, w), np.float32)
dis.calc(ref, curr_img, flow)
flows.append(flow)
```