UMat u_mat;
mat.copyTo(u_mat);
cv::cvtColor(u_mat, cv::BGR2GARY);
这样的代码反复执行，并未发现GPU占用提升。执行时间与不使用UMat相当。
由此证明，官网提供的opencv android sdk，并未把opencl的支持编译进去。
see: https://github.com/opencv/opencv/releases
下载地址：https://github.com/opencv/opencv/releases/download/3.4.6/opencv-3.4.6-android-sdk.zip
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网上找到一些帖子，貌似自己编译解决了这个问题，后面准备试一试：
https://www.slideshare.net/noritsuna/how-to-build-use-opencl-on-opencv-android-ndk
How to Build & Use OpenCL on OpenCV & Android NDK
https://github.com/soham24/Opencv-android-SDK-3.3.1-with-opencl-and-contrib
Opencv-android-SDK-3.3.1-with-opencl-and-contrib
https://gist.github.com/iago-suarez/13c82b416ce6b07a93b5b6eee6bd29f3
Setting Up OpenCL for OpenCV on Android, the full story