源码如下，实现把一个JPG保存成灰度图格式的BMP

 1 //jpg2bmp.cpp
 2 #include <stdio.h>
 3 #include <inttypes.h>
 4 #include <opencv2/opencv.hpp>
 5 #include <opencv2/core.hpp>
 6 
 7 #ifndef P
 8 #define P(format, ...) do \
 9   { \
10     printf("%s %s %d " format "\n", __FILE__, __FUNCTION__, __LINE__, ##__VA_ARGS__); \
11     fflush(stdout); \
12   } \
13   while (0);
14 #endif
15 
16 int main(int argc, char* argv[]){
17     if (argc<3){
18         printf("usage:%s <from> <to>\n", argv[0]);
19         return 0;
20     }
21     cv::Mat image = cv::imread(argv[1], cv::IMREAD_UNCHANGED);
22     cv::UMat u_img = image.getUMat(cv::ACCESS_READ);
23     if (image.empty()){
24         P("load pic %s fail", argv[1]);
25         return -1;
26     }
27     cv::UMat bmp;
28     cv::cvtColor(u_img, bmp, cv::COLOR_BGR2GRAY);   
29     cv::imwrite(argv[2], bmp);
30     P("ok");
31     return 1;
32 }

编译命令行如下：

1 /Users/ahfu/code/android/android-ndk-r14b/toolchains/arm-linux-androideabi-4.9/prebuilt/darwin-x86_64/bin/arm-linux-androideabi-g++  \
2     -o jpg2bmp_1.o -c jpg2bmp_1.cpp -pie -fPIC \
3     -I"/Users/ahfu/code/android/OpenCV-android-sdk/sdk/native/jni/include" \
4     --sysroot=/Users/ahfu/code/android/android-ndk-r14b/platforms/android-22/arch-arm/ \
5     -I"/Users/ahfu/code/android/android-ndk-r14b/sources/cxx-stl/gnu-libstdc++/4.9/include" \
6     -I"/Users/ahfu/code/android/android-ndk-r14b/sources/cxx-stl/gnu-libstdc++/4.9/libs/armeabi-v7a/include/"

链接命令行如下：

1 /Users/ahfu/code/android/android-ndk-r14b/toolchains/arm-linux-androideabi-4.9/prebuilt/darwin-x86_64/bin/arm-linux-androideabi-g++ \
2     -o jpg2bmp_1 jpg2bmp_1.o -pie -fPIC \
3     --sysroot=/Users/ahfu/code/android/android-ndk-r14b/platforms/android-22/arch-arm/ \
4     -L"/Users/ahfu/code/android/OpenCV-android-sdk/sdk/native/staticlibs/armeabi-v7a" -lopencv_core -lopencv_imgcodecs -lopencv_imgproc -lm -lz -landroid -llog \
5     -L"/Users/ahfu/code/android/OpenCV-android-sdk/sdk/native/3rdparty/libs/armeabi-v7a/" -ltegra_hal -lcpufeatures -ltbb -llibpng -llibjpeg-turbo -llibtiff -llibjasper -llibwebp -lIlmImf -lopencv_core -lopencv_imgcodecs -lopencv_imgproc -ltegra_hal \
6     -L"/Users/ahfu/code/android/android-ndk-r14b/sources/cxx-stl/gnu-libstdc++/4.9/libs/armeabi-v7a" -lgnustl_static -lsupc++ \
7     -L"/Users/ahfu/code/android/android-ndk-r14b/platforms/android-22/arch-arm/usr/lib" -lstdc++

然后使用adb上传
adb root  #手机必要要root
adb push jpg2bmp 1.jpg /data/data/temp
adb shell
  cd /data/data/temp
  chmod +x jpg2bmp
  ./jpg2bmp 1.jpg 2.bmp

  #执行成功

ndk版本是：android-ndk-r14b
opencv android sdk版本是：opencv-3.4.6-android-sdk.zip