diff --git a/examples/CMakeLists.txt b/examples/CMakeLists.txt
index 8cf10e8a9..eeacf2f11 100644
--- a/examples/CMakeLists.txt
+++ b/examples/CMakeLists.txt
@@ -1,56 +1,68 @@
 macro(ncnn_add_example name)
     add_executable(${name} ${name}.cpp)
-    target_include_directories(${name} PRIVATE ${OpenCV_INCLUDE_DIRS})
-    target_link_libraries(${name} PRIVATE ncnn ${OpenCV_LIBS})
+    if(OpenCV_FOUND)
+        target_include_directories(${name} PRIVATE ${OpenCV_INCLUDE_DIRS})
+        target_link_libraries(${name} PRIVATE ncnn ${OpenCV_LIBS})
+    elseif(NCNN_SIMPLEOCV)
+        target_compile_definitions(${name} PUBLIC USE_NCNN_SIMPLEOCV)
+        target_link_libraries(${name} PRIVATE ncnn)
+    endif()
 
     # add test to a virtual project group
     set_property(TARGET ${name} PROPERTY FOLDER "examples")
 endmacro()
 
-find_package(OpenCV QUIET COMPONENTS opencv_world)
-# for opencv 2.4 on ubuntu 16.04, there is no opencv_world but OpenCV_FOUND will be TRUE
-if("${OpenCV_LIBS}" STREQUAL "")
-    set(OpenCV_FOUND FALSE)
-endif()
-if(NOT OpenCV_FOUND)
-    find_package(OpenCV QUIET COMPONENTS core highgui imgproc imgcodecs videoio)
-endif()
-if(NOT OpenCV_FOUND)
-    find_package(OpenCV QUIET COMPONENTS core highgui imgproc)
-endif()
+if(NCNN_PIXEL)
+    find_package(OpenCV QUIET COMPONENTS opencv_world)
+    # for opencv 2.4 on ubuntu 16.04, there is no opencv_world but OpenCV_FOUND will be TRUE
+    if("${OpenCV_LIBS}" STREQUAL "")
+        set(OpenCV_FOUND FALSE)
+    endif()
+    if(NOT OpenCV_FOUND)
+        find_package(OpenCV QUIET COMPONENTS core highgui imgproc imgcodecs videoio)
+    endif()
+    if(NOT OpenCV_FOUND)
+        find_package(OpenCV QUIET COMPONENTS core highgui imgproc)
+    endif()
 
-if(NOT OpenCV_FOUND)
-    message(WARNING "OpenCV not found, examples won't be built")
-elseif(NOT NCNN_PIXEL)
-    message(WARNING "NCNN_PIXEL not enabled, examples won't be built")
-else()
-    message(STATUS "OpenCV library: ${OpenCV_INSTALL_PATH}")
-    message(STATUS "    version: ${OpenCV_VERSION}")
-    message(STATUS "    libraries: ${OpenCV_LIBS}")
-    message(STATUS "    include path: ${OpenCV_INCLUDE_DIRS}")
+    if(OpenCV_FOUND OR NCNN_SIMPLEOCV)
+        if(OpenCV_FOUND)
+            message(STATUS "OpenCV library: ${OpenCV_INSTALL_PATH}")
+            message(STATUS "    version: ${OpenCV_VERSION}")
+            message(STATUS "    libraries: ${OpenCV_LIBS}")
+            message(STATUS "    include path: ${OpenCV_INCLUDE_DIRS}")
 
-    if(${OpenCV_VERSION_MAJOR} GREATER 3)
-        set(CMAKE_CXX_STANDARD 11)
-    endif()
-    include_directories(${CMAKE_CURRENT_SOURCE_DIR}/../src)
-    include_directories(${CMAKE_CURRENT_BINARY_DIR}/../src)
+            if(${OpenCV_VERSION_MAJOR} GREATER 3)
+                set(CMAKE_CXX_STANDARD 11)
+            endif()
+        endif()
+
+        include_directories(${CMAKE_CURRENT_SOURCE_DIR}/../src)
+        include_directories(${CMAKE_CURRENT_BINARY_DIR}/../src)
 
-    ncnn_add_example(squeezenet)
-    ncnn_add_example(squeezenet_c_api)
-    ncnn_add_example(fasterrcnn)
-    ncnn_add_example(rfcn)
-    ncnn_add_example(yolov2)
-    ncnn_add_example(yolov3)
-    ncnn_add_example(yolov4)
-    ncnn_add_example(yolov5)
-    ncnn_add_example(mobilenetv2ssdlite)
-    ncnn_add_example(mobilenetssd)
-    ncnn_add_example(squeezenetssd)
-    ncnn_add_example(shufflenetv2)
-    ncnn_add_example(peleenetssd_seg)
-    ncnn_add_example(simplepose)
-    ncnn_add_example(retinaface)
-    ncnn_add_example(yolact)
-    ncnn_add_example(nanodet)
-    ncnn_add_example(scrfd)
+        ncnn_add_example(squeezenet)
+        ncnn_add_example(squeezenet_c_api)
+        ncnn_add_example(fasterrcnn)
+        ncnn_add_example(rfcn)
+        ncnn_add_example(yolov2)
+        ncnn_add_example(yolov3)
+        if(OpenCV_FOUND)
+            ncnn_add_example(yolov4)
+        endif()
+        ncnn_add_example(yolov5)
+        ncnn_add_example(mobilenetv2ssdlite)
+        ncnn_add_example(mobilenetssd)
+        ncnn_add_example(squeezenetssd)
+        ncnn_add_example(shufflenetv2)
+        ncnn_add_example(peleenetssd_seg)
+        ncnn_add_example(simplepose)
+        ncnn_add_example(retinaface)
+        ncnn_add_example(yolact)
+        ncnn_add_example(nanodet)
+        ncnn_add_example(scrfd)
+    else()
+        message(WARNING "OpenCV not found and NCNN_SIMPLEOCV disabled, examples won't be built")
+    endif()
+else()
+    message(WARNING "NCNN_PIXEL not enabled, examples won't be built")
 endif()
diff --git a/examples/fasterrcnn.cpp b/examples/fasterrcnn.cpp
index 64208490d..733acf18a 100644
--- a/examples/fasterrcnn.cpp
+++ b/examples/fasterrcnn.cpp
@@ -15,9 +15,13 @@
 #include "net.h"
 
 #include <math.h>
+#if defined(USE_NCNN_SIMPLEOCV)
+#include "simpleocv.h"
+#else
 #include <opencv2/core/core.hpp>
 #include <opencv2/highgui/highgui.hpp>
 #include <opencv2/imgproc/imgproc.hpp>
+#endif
 #include <stdio.h>
 
 struct Object
diff --git a/examples/mobilenetssd.cpp b/examples/mobilenetssd.cpp
index e4f32928a..7095ccc6e 100644
--- a/examples/mobilenetssd.cpp
+++ b/examples/mobilenetssd.cpp
@@ -14,9 +14,13 @@
 
 #include "net.h"
 
+#if defined(USE_NCNN_SIMPLEOCV)
+#include "simpleocv.h"
+#else
 #include <opencv2/core/core.hpp>
 #include <opencv2/highgui/highgui.hpp>
 #include <opencv2/imgproc/imgproc.hpp>
+#endif
 #include <stdio.h>
 #include <vector>
 
diff --git a/examples/mobilenetv2ssdlite.cpp b/examples/mobilenetv2ssdlite.cpp
index 6ed74e509..c1e998f9c 100644
--- a/examples/mobilenetv2ssdlite.cpp
+++ b/examples/mobilenetv2ssdlite.cpp
@@ -14,9 +14,13 @@
 
 #include "net.h"
 
+#if defined(USE_NCNN_SIMPLEOCV)
+#include "simpleocv.h"
+#else
 #include <opencv2/core/core.hpp>
 #include <opencv2/highgui/highgui.hpp>
 #include <opencv2/imgproc/imgproc.hpp>
+#endif
 #include <stdio.h>
 #include <vector>
 
diff --git a/examples/mobilenetv3ssdlite.cpp b/examples/mobilenetv3ssdlite.cpp
index b737f825d..9c97ffc18 100644
--- a/examples/mobilenetv3ssdlite.cpp
+++ b/examples/mobilenetv3ssdlite.cpp
@@ -15,9 +15,13 @@
 #include "net.h"
 #include "platform.h"
 
+#if defined(USE_NCNN_SIMPLEOCV)
+#include "simpleocv.h"
+#else
 #include <opencv2/core/core.hpp>
 #include <opencv2/highgui/highgui.hpp>
 #include <opencv2/imgproc/imgproc.hpp>
+#endif
 #include <stdio.h>
 #include <vector>
 #if NCNN_VULKAN
diff --git a/examples/nanodet.cpp b/examples/nanodet.cpp
index 2129271f9..8da7c4abd 100644
--- a/examples/nanodet.cpp
+++ b/examples/nanodet.cpp
@@ -14,9 +14,13 @@
 
 #include "net.h"
 
+#if defined(USE_NCNN_SIMPLEOCV)
+#include "simpleocv.h"
+#else
 #include <opencv2/core/core.hpp>
 #include <opencv2/highgui/highgui.hpp>
 #include <opencv2/imgproc/imgproc.hpp>
+#endif
 #include <stdlib.h>
 #include <float.h>
 #include <stdio.h>
diff --git a/examples/peleenetssd_seg.cpp b/examples/peleenetssd_seg.cpp
index 5b7e90f44..5ee393f6b 100644
--- a/examples/peleenetssd_seg.cpp
+++ b/examples/peleenetssd_seg.cpp
@@ -14,9 +14,13 @@
 
 #include "net.h"
 
+#if defined(USE_NCNN_SIMPLEOCV)
+#include "simpleocv.h"
+#else
 #include <opencv2/core/core.hpp>
 #include <opencv2/highgui/highgui.hpp>
 #include <opencv2/imgproc/imgproc.hpp>
+#endif
 #include <stdio.h>
 #include <vector>
 
diff --git a/examples/retinaface.cpp b/examples/retinaface.cpp
index 21defbff8..342d5661d 100644
--- a/examples/retinaface.cpp
+++ b/examples/retinaface.cpp
@@ -14,9 +14,13 @@
 
 #include "net.h"
 
+#if defined(USE_NCNN_SIMPLEOCV)
+#include "simpleocv.h"
+#else
 #include <opencv2/core/core.hpp>
 #include <opencv2/highgui/highgui.hpp>
 #include <opencv2/imgproc/imgproc.hpp>
+#endif
 #include <stdio.h>
 #include <vector>
 
diff --git a/examples/rfcn.cpp b/examples/rfcn.cpp
index 6e982761c..71fe74d67 100644
--- a/examples/rfcn.cpp
+++ b/examples/rfcn.cpp
@@ -15,9 +15,13 @@
 #include "net.h"
 
 #include <math.h>
+#if defined(USE_NCNN_SIMPLEOCV)
+#include "simpleocv.h"
+#else
 #include <opencv2/core/core.hpp>
 #include <opencv2/highgui/highgui.hpp>
 #include <opencv2/imgproc/imgproc.hpp>
+#endif
 #include <stdio.h>
 
 struct Object
diff --git a/examples/scrfd.cpp b/examples/scrfd.cpp
index 47e3e4e2a..5dc188cbe 100644
--- a/examples/scrfd.cpp
+++ b/examples/scrfd.cpp
@@ -14,9 +14,13 @@
 
 #include "net.h"
 
+#if defined(USE_NCNN_SIMPLEOCV)
+#include "simpleocv.h"
+#else
 #include <opencv2/core/core.hpp>
 #include <opencv2/highgui/highgui.hpp>
 #include <opencv2/imgproc/imgproc.hpp>
+#endif
 #include <stdio.h>
 #include <vector>
 
diff --git a/examples/shufflenetv2.cpp b/examples/shufflenetv2.cpp
index b7c7858df..5dd31abcf 100644
--- a/examples/shufflenetv2.cpp
+++ b/examples/shufflenetv2.cpp
@@ -15,8 +15,12 @@
 #include "net.h"
 
 #include <algorithm>
+#if defined(USE_NCNN_SIMPLEOCV)
+#include "simpleocv.h"
+#else
 #include <opencv2/core/core.hpp>
 #include <opencv2/highgui/highgui.hpp>
+#endif
 #include <stdio.h>
 #include <vector>
 
diff --git a/examples/simplepose.cpp b/examples/simplepose.cpp
index 8323292f7..0589f017d 100644
--- a/examples/simplepose.cpp
+++ b/examples/simplepose.cpp
@@ -15,9 +15,13 @@
 #include "net.h"
 
 #include <algorithm>
+#if defined(USE_NCNN_SIMPLEOCV)
+#include "simpleocv.h"
+#else
 #include <opencv2/core/core.hpp>
 #include <opencv2/highgui/highgui.hpp>
 #include <opencv2/imgproc/imgproc.hpp>
+#endif
 #include <stdio.h>
 #include <vector>
 
diff --git a/examples/squeezenet.cpp b/examples/squeezenet.cpp
index 30db592a4..ec86d71e6 100644
--- a/examples/squeezenet.cpp
+++ b/examples/squeezenet.cpp
@@ -15,8 +15,12 @@
 #include "net.h"
 
 #include <algorithm>
+#if defined(USE_NCNN_SIMPLEOCV)
+#include "simpleocv.h"
+#else
 #include <opencv2/core/core.hpp>
 #include <opencv2/highgui/highgui.hpp>
+#endif
 #include <stdio.h>
 #include <vector>
 
diff --git a/examples/squeezenet_c_api.cpp b/examples/squeezenet_c_api.cpp
index 132bb1008..01add23d1 100644
--- a/examples/squeezenet_c_api.cpp
+++ b/examples/squeezenet_c_api.cpp
@@ -15,8 +15,12 @@
 #include "c_api.h"
 
 #include <algorithm>
+#if defined(USE_NCNN_SIMPLEOCV)
+#include "simpleocv.h"
+#else
 #include <opencv2/core/core.hpp>
 #include <opencv2/highgui/highgui.hpp>
+#endif
 #include <stdio.h>
 #include <vector>
 
diff --git a/examples/squeezenetssd.cpp b/examples/squeezenetssd.cpp
index 113969bf9..64d461bf3 100644
--- a/examples/squeezenetssd.cpp
+++ b/examples/squeezenetssd.cpp
@@ -14,9 +14,13 @@
 
 #include "net.h"
 
+#if defined(USE_NCNN_SIMPLEOCV)
+#include "simpleocv.h"
+#else
 #include <opencv2/core/core.hpp>
 #include <opencv2/highgui/highgui.hpp>
 #include <opencv2/imgproc/imgproc.hpp>
+#endif
 #include <stdio.h>
 #include <vector>
 
diff --git a/examples/yolact.cpp b/examples/yolact.cpp
index 26d6a4f22..87cfeb8f7 100644
--- a/examples/yolact.cpp
+++ b/examples/yolact.cpp
@@ -14,9 +14,13 @@
 
 #include "net.h"
 
+#if defined(USE_NCNN_SIMPLEOCV)
+#include "simpleocv.h"
+#else
 #include <opencv2/core/core.hpp>
 #include <opencv2/highgui/highgui.hpp>
 #include <opencv2/imgproc/imgproc.hpp>
+#endif
 #include <stdio.h>
 #include <vector>
 
diff --git a/examples/yolov2.cpp b/examples/yolov2.cpp
index c89482104..d9488f756 100644
--- a/examples/yolov2.cpp
+++ b/examples/yolov2.cpp
@@ -14,9 +14,13 @@
 
 #include "net.h"
 
+#if defined(USE_NCNN_SIMPLEOCV)
+#include "simpleocv.h"
+#else
 #include <opencv2/core/core.hpp>
 #include <opencv2/highgui/highgui.hpp>
 #include <opencv2/imgproc/imgproc.hpp>
+#endif
 #include <stdio.h>
 #include <vector>
 
diff --git a/examples/yolov3.cpp b/examples/yolov3.cpp
index e2f57af5f..fd581376b 100644
--- a/examples/yolov3.cpp
+++ b/examples/yolov3.cpp
@@ -14,9 +14,13 @@
 
 #include "net.h"
 
+#if defined(USE_NCNN_SIMPLEOCV)
+#include "simpleocv.h"
+#else
 #include <opencv2/core/core.hpp>
 #include <opencv2/highgui/highgui.hpp>
 #include <opencv2/imgproc/imgproc.hpp>
+#endif
 #include <stdio.h>
 #include <vector>
 
diff --git a/examples/yolov5.cpp b/examples/yolov5.cpp
index c54b84008..3990fa950 100644
--- a/examples/yolov5.cpp
+++ b/examples/yolov5.cpp
@@ -15,9 +15,14 @@
 #include "layer.h"
 #include "net.h"
 
+#if defined(USE_NCNN_SIMPLEOCV)
+#include "simpleocv.h"
+#else
 #include <opencv2/core/core.hpp>
 #include <opencv2/highgui/highgui.hpp>
 #include <opencv2/imgproc/imgproc.hpp>
+#endif
+#include <float.h>
 #include <stdio.h>
 #include <vector>
 
diff --git a/src/simpleocv.cpp b/src/simpleocv.cpp
index 2ded7d397..f1270633d 100644
--- a/src/simpleocv.cpp
+++ b/src/simpleocv.cpp
@@ -211,6 +211,19 @@ bool imwrite(const std::string& path, const Mat& m, const std::vector<int>& para
     return success;
 }
 
+void imshow(const std::string& name, const Mat& m)
+{
+    NCNN_LOGE("imshow save image to %s.png", name.c_str());
+
+    imwrite(name + ".png", m);
+}
+
+int waitKey(int delay)
+{
+    NCNN_LOGE("waitKey stub");
+    return -1;
+}
+
 #if NCNN_PIXEL
 void resize(const Mat& src, Mat& dst, const Size& size, float sw, float sh, int flags)
 {
diff --git a/src/simpleocv.h b/src/simpleocv.h
index e01500120..19b8a7268 100644
--- a/src/simpleocv.h
+++ b/src/simpleocv.h
@@ -124,6 +124,11 @@ struct Point_
     {
     }
 
+    template<typename _Tp2> operator Point_<_Tp2>() const
+    {
+        return Point_<_Tp2>(saturate_cast<_Tp2>(x), saturate_cast<_Tp2>(y));
+    }
+
     _Tp x;
     _Tp y;
 };
@@ -143,6 +148,11 @@ struct Size_
     {
     }
 
+    template<typename _Tp2> operator Size_<_Tp2>() const
+    {
+        return Size_<_Tp2>(saturate_cast<_Tp2>(width), saturate_cast<_Tp2>(height));
+    }
+
     _Tp width;
     _Tp height;
 };
@@ -166,6 +176,11 @@ struct Rect_
     {
     }
 
+    template<typename _Tp2> operator Rect_<_Tp2>() const
+    {
+        return Rect_<_Tp2>(saturate_cast<_Tp2>(x), saturate_cast<_Tp2>(y), saturate_cast<_Tp2>(width), saturate_cast<_Tp2>(height));
+    }
+
     _Tp x;
     _Tp y;
     _Tp width;
@@ -437,6 +452,10 @@ enum ImwriteFlags
 
 NCNN_EXPORT bool imwrite(const std::string& path, const Mat& m, const std::vector<int>& params = std::vector<int>());
 
+NCNN_EXPORT void imshow(const std::string& name, const Mat& m);
+
+NCNN_EXPORT int waitKey(int delay = 0);
+
 #if NCNN_PIXEL
 NCNN_EXPORT void resize(const Mat& src, Mat& dst, const Size& size, float sw = 0.f, float sh = 0.f, int flags = 0);
 #endif // NCNN_PIXEL
diff --git a/toolchains/c906.toolchain.cmake b/toolchains/c906.toolchain.cmake
index fa726b6fb..6002fd783 100644
--- a/toolchains/c906.toolchain.cmake
+++ b/toolchains/c906.toolchain.cmake
@@ -15,9 +15,12 @@ set(CMAKE_CXX_COMPILER "${RISCV_ROOT_PATH}/bin/riscv64-unknown-linux-gnu-g++")
 
 set(CMAKE_FIND_ROOT_PATH "${RISCV_ROOT_PATH}/riscv64-unknown-linux-gnu")
 
+set(CMAKE_SYSROOT "${RISCV_ROOT_PATH}/sysroot")
+
 set(CMAKE_FIND_ROOT_PATH_MODE_PROGRAM NEVER)
 set(CMAKE_FIND_ROOT_PATH_MODE_LIBRARY ONLY)
 set(CMAKE_FIND_ROOT_PATH_MODE_INCLUDE ONLY)
+set(CMAKE_FIND_ROOT_PATH_MODE_PACKAGE ONLY)
 
 set(CMAKE_C_FLAGS "-march=rv64gcvxtheadc -mabi=lp64d -mtune=c906 -DRVV_SPEC_0_7 -D__riscv_zfh=1 -static")
 set(CMAKE_CXX_FLAGS "-march=rv64gcvxtheadc -mabi=lp64d -mtune=c906 -DRVV_SPEC_0_7 -D__riscv_zfh=1 -static")
@@ -26,5 +29,5 @@ set(CMAKE_CXX_FLAGS "-march=rv64gcvxtheadc -mabi=lp64d -mtune=c906 -DRVV_SPEC_0_
 set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS}" CACHE STRING "c flags")
 set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS}" CACHE STRING "c++ flags")
 
-# export RISCV_ROOT_PATH=/home/nihui/osd/riscv64-linux-x86_64-20210329
-# cmake -DCMAKE_TOOLCHAIN_FILE=../toolchains/c906.toolchain.cmake -DCMAKE_BUILD_TYPE=relwithdebinfo -DNCNN_BUILD_TESTS=ON -DNCNN_OPENMP=OFF -DNCNN_THREADS=OFF -DNCNN_RUNTIME_CPU=OFF -DNCNN_RVV=ON ..
+# export RISCV_ROOT_PATH=/home/nihui/osd/riscv64-linux-x86_64-20210512
+# cmake -DCMAKE_TOOLCHAIN_FILE=../toolchains/c906.toolchain.cmake -DCMAKE_BUILD_TYPE=relwithdebinfo -DNCNN_BUILD_TESTS=ON -DNCNN_OPENMP=OFF -DNCNN_THREADS=OFF -DNCNN_RUNTIME_CPU=OFF -DNCNN_RVV=ON -DNCNN_SIMPLEOCV=ON -DNCNN_BUILD_EXAMPLES=ON ..