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Joachim 2ddeecd24c README refactor

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Detailed build procedure

The last way explains all the commands and modifications done to be able to compile and run OpenCV on the ESP32.

CMake command:

The following cmake command is launched in the build/ directory.

cmake 									\
-DCMAKE_BUILD_TYPE=Release				\
-DESP32=ON								\
-DBUILD_SHARED_LIBS=OFF 				\
-DCV_DISABLE_OPTIMIZATION=OFF 			\
-DWITH_IPP=OFF 							\
-DWITH_TBB=OFF 							\
-DWITH_OPENMP=OFF 						\
-DWITH_PTHREADS_PF=OFF 					\
-DWITH_QUIRC=OFF 						\
-DWITH_1394=OFF 						\
-DWITH_CUDA=OFF 						\
-DWITH_OPENCL=OFF 						\
-DWITH_OPENCLAMDFFT=OFF 				\
-DWITH_OPENCLAMDBLAS=OFF 				\
-DWITH_VA_INTEL=OFF 					\
-DWITH_EIGEN=OFF						\
										\
-DWITH_GSTREAMER=OFF 					\
-DWITH_GTK=OFF 							\
-DWITH_JASPER=OFF 						\
-DWITH_JPEG=OFF 						\
-DWITH_WEBP=OFF 						\
-DBUILD_ZLIB=ON							\
-DBUILD_PNG=ON 							\
-DWITH_TIFF=OFF 						\
-DWITH_V4L=OFF 							\
-DWITH_LAPACK=OFF						\
-DWITH_ITT=OFF 							\
-DWITH_PROTOBUF=OFF 					\
-DWITH_IMGCODEC_HDR=OFF 				\
-DWITH_IMGCODEC_SUNRASTER=OFF 			\
-DWITH_IMGCODEC_PXM=OFF 				\
-DWITH_IMGCODEC_PFM=OFF 				\
										\
-DBUILD_LIST=core,imgproc,imgcodecs		\
-DBUILD_JAVA=OFF						\
-DBUILD_opencv_python=OFF 				\
-DBUILD_opencv_java=OFF 				\
										\
-DBUILD_opencv_apps=OFF					\
-DBUILD_PACKAGE=OFF						\
-DBUILD_PERF_TESTS=OFF					\
-DBUILD_TESTS=OFF						\
-DCV_ENABLE_INTRINSICS=OFF				\
-DCV_TRACE=OFF							\
-DOPENCV_ENABLE_MEMALIGN=OFF			\
										\
-DCMAKE_TOOLCHAIN_FILE=~/esp/esp-idf/tools/cmake/toolchain-esp32.cmake \
..

-DCMAKE_BUILD_TYPE can be set to Debug to simplify debugging, but uses more RAM
-DBUILD_ZLIB and -DBUILD_PNG are enabled to statically compile the libs instead of using dynamic libs of the system.
-DOPENCV_ENABLE_MEMALIGN is disabled because the ESP32 doesn't support memory alignment for now (https://github.com/espressif/esp-idf/issues/4218)

The toolchain file toolchain-esp32.cmake is taken from esp-idf github:

set(CMAKE_SYSTEM_NAME Generic)

set(CMAKE_C_COMPILER xtensa-esp32-elf-gcc)
set(CMAKE_CXX_COMPILER xtensa-esp32-elf-g++)
set(CMAKE_ASM_COMPILER xtensa-esp32-elf-gcc)

set(CMAKE_C_FLAGS "-mlongcalls -Wno-frame-address" CACHE STRING "C Compiler Base Flags")
set(CMAKE_CXX_FLAGS "-mlongcalls -Wno-frame-address" CACHE STRING "C++ Compiler Base Flags")

# Can be removed after gcc 5.2.0 support is removed (ref GCC_NOT_5_2_0)
set(CMAKE_EXE_LINKER_FLAGS "-nostdlib" CACHE STRING "Linker Base Flags")

When the cmake command works, the following summary is given:

-- General configuration for OpenCV 4.2.0-dev =====================================
--   Version control:               4.2.0-375-g8808aaccff-dirty
-- 
--   Platform:
--     Timestamp:                   2020-03-23T09:54:18Z
--     Host:                        Linux 4.19.10-041910-generic x86_64
--     Target:                      Generic
--     CMake:                       3.16.4
--     CMake generator:             Unix Makefiles
--     CMake build tool:            /usr/bin/make
--     Configuration:               Release
-- 
--   CPU/HW features:
--     Baseline:
--       requested:                 DETECT
-- 
--   C/C++:
--     Built as dynamic libs?:      NO
--     C++ standard:                11
--     C++ Compiler:                /home/joachim/.espressif/tools/xtensa-esp32-elf/esp-2019r2-8.2.0/xtensa-esp32-elf/bin/xtensa-esp32-elf-g++  (ver 8.2.0)
--     C++ flags (Release):         -mlongcalls -Wno-frame-address   -fsigned-char -W -Wall -Werror=return-type -Werror=non-virtual-dtor -Werror=address -Werror=sequence-point -Wformat -Werror=format-security -Wmissing-declarations -Wundef -Winit-self -Wpointer-arith -Wshadow -Wsign-promo -Wuninitialized -Winit-self -Wsuggest-override -Wno-delete-non-virtual-dtor -Wno-comment -Wimplicit-fallthrough=3 -Wno-strict-overflow -fdiagnostics-show-option -fomit-frame-pointer -ffunction-sections -fdata-sections  -fvisibility=hidden -fvisibility-inlines-hidden -O3 -DNDEBUG  -DNDEBUG
--     C++ flags (Debug):           -mlongcalls -Wno-frame-address   -fsigned-char -W -Wall -Werror=return-type -Werror=non-virtual-dtor -Werror=address -Werror=sequence-point -Wformat -Werror=format-security -Wmissing-declarations -Wundef -Winit-self -Wpointer-arith -Wshadow -Wsign-promo -Wuninitialized -Winit-self -Wsuggest-override -Wno-delete-non-virtual-dtor -Wno-comment -Wimplicit-fallthrough=3 -Wno-strict-overflow -fdiagnostics-show-option -fomit-frame-pointer -ffunction-sections -fdata-sections  -fvisibility=hidden -fvisibility-inlines-hidden -g  -O0 -DDEBUG -D_DEBUG
--     C Compiler:                  /home/joachim/.espressif/tools/xtensa-esp32-elf/esp-2019r2-8.2.0/xtensa-esp32-elf/bin/xtensa-esp32-elf-gcc
--     C flags (Release):           -mlongcalls -Wno-frame-address   -fsigned-char -W -Wall -Werror=return-type -Werror=non-virtual-dtor -Werror=address -Werror=sequence-point -Wformat -Werror=format-security -Wmissing-declarations -Wmissing-prototypes -Wstrict-prototypes -Wundef -Winit-self -Wpointer-arith -Wshadow -Wuninitialized -Winit-self -Wno-comment -Wimplicit-fallthrough=3 -Wno-strict-overflow -fdiagnostics-show-option -fomit-frame-pointer -ffunction-sections -fdata-sections  -fvisibility=hidden -O3 -DNDEBUG  -DNDEBUG
--     C flags (Debug):             -mlongcalls -Wno-frame-address   -fsigned-char -W -Wall -Werror=return-type -Werror=non-virtual-dtor -Werror=address -Werror=sequence-point -Wformat -Werror=format-security -Wmissing-declarations -Wmissing-prototypes -Wstrict-prototypes -Wundef -Winit-self -Wpointer-arith -Wshadow -Wuninitialized -Winit-self -Wno-comment -Wimplicit-fallthrough=3 -Wno-strict-overflow -fdiagnostics-show-option -fomit-frame-pointer -ffunction-sections -fdata-sections  -fvisibility=hidden -g  -O0 -DDEBUG -D_DEBUG
--     Linker flags (Release):      -Wl,--gc-sections  
--     Linker flags (Debug):        -Wl,--gc-sections  
--     ccache:                      NO
--     Precompiled headers:         NO
--     Extra dependencies:
--     3rdparty dependencies:       zlib libpng
-- 
--   OpenCV modules:
--     To be built:                 core imgcodecs imgproc
--     Disabled:                    world
--     Disabled by dependency:      calib3d features2d flann gapi highgui java_bindings_generator ml objdetect photo python_bindings_generator python_tests stitching video videoio
--     Unavailable:                 dnn java js python2 python3 ts
--     Applications:                -
--     Documentation:               NO
--     Non-free algorithms:         NO
-- 
--   GUI: 
-- 
--   Media I/O: 
--     ZLib:                        build (ver 1.2.11)
--     PNG:                         build (ver 1.6.37)
--     HDR:                         NO
--     SUNRASTER:                   NO
--     PXM:                         NO
--     PFM:                         NO
-- 
--   Video I/O:
--     FFMPEG:                      NO
--       avcodec:                   NO
--       avformat:                  NO
--       avutil:                    NO
--       swscale:                   NO
--       avresample:                NO
-- 
--   Parallel framework:            none
-- 
--   Other third-party libraries:
--     Custom HAL:                  NO
-- 
--   Python (for build):            /home/joachim/.espressif/python_env/idf4.2_py2.7_env/bin/python2.7
-- 
--   Install to:                    /home/joachim/Documents/HES/02_MSE/22_master_thesis/esp32-opencv/build/install
-- -----------------------------------------------------------------
-- 
-- Configuring done
-- Generating done
-- Build files have been written to: /home/joachim/Documents/HES/02_MSE/22_master_thesis/esp32-opencv/build

Make command:

When the cmake is done, the compilation is started with:

make -j5

When the compilation has ended, the libs are in the build/lib folder.

Compiling esp-idf project using OpenCV:

When the OpenCV library is cross-compiled, we have in result *.a files located in build/lib folder. We now want to try to compile an example project using OpenCV on the esp32. A basic example of esp-idf project can be found in esp32/examples/esp_opencv_basic. This project simply creates an OpenCV matrix, fill it with values and prints it on the console.

Esp-idf environment uses cmake and is separated in components. Because OpenCV libs were compiled outside this example project, we use the pre-built library functionality of esp-idf (https://docs.espressif.com/projects/esp-idf/en/latest/api-guides/build-system.html#using-prebuilt-libraries-with-components).

Here are the things done to add the OpenCV library to the project:

Create a folder named opencv/ into the main/ component's folder
Copy the generated libraries (libade.a, libopencv_core.a, libopencv_imgproc.a and libopencv_imgcodecs.a) into this directory
Create the folder opencv2/ into this directory, and copy into it the needed headers files :
- cvconfig.h
- opencv_modules.hpp
- opencv.hpp
- core.hpp
- imgproc.hpp
- imgcodecs.hpp
- core/ folder
- imgproc/ folder
- imgcodecs/ folder

Link the libraries to the project by modifying the CMakeList.txt of the main project's component is as below :

idf_component_register(
	SRC main.cpp
	INCLUDE_DIRS ./opencv
)

# Be aware that the order of the librairies is important
add_prebuilt_library(opencv_imgcodecs "opencv/libopencv_imgcodecs.a")
add_prebuilt_library(libpng "opencv/3rdparty/liblibpng.a")
add_prebuilt_library(libzlib "opencv/3rdparty/libzlib.a")
add_prebuilt_library(opencv_imgproc "opencv/libopencv_imgproc.a")
add_prebuilt_library(opencv_core "opencv/libopencv_core.a")
add_prebuilt_library(ade "opencv/libade.a")

target_link_libraries(${COMPONENT_LIB} PRIVATE opencv_imgcodecs)
target_link_libraries(${COMPONENT_LIB} PRIVATE libpng)
target_link_libraries(${COMPONENT_LIB} PRIVATE libzlib)
target_link_libraries(${COMPONENT_LIB} PRIVATE opencv_imgproc)
target_link_libraries(${COMPONENT_LIB} PRIVATE opencv_core)
target_link_libraries(${COMPONENT_LIB} PRIVATE ade)

Finally, include the OpenCV headers needed into your source files:

#include "opencv2/core.hpp"
#include "opencv2/imgproc.hpp"
#include "opencv2/imgcodecs.hpp"

Modified files for OpenCV to compile and run:

To get the cmake configuration and make compilation to work, some modifications on OpenCV files had to be done. They are listed below.

A cmake option ESP32 is added to the CmakeLists.txt :

OCV_OPTION(ESP32 "Compilation for esp32 target" OFF)

To enable it, add the -DESP32=ON in the cmake command. This will then be used to enable/disable some parts of the codes of the OpenCV project.

A preprocessor definition is also added in the CMakeLists.txt file, line 494:

if(ESP32)
  message(STATUS "Enabled ESP32 target specific build")
  add_definitions(-DESP32)
endif()

The cmake command is run, leading to the following errors:

Having error C++11 compiler must support std::atomic

The esp32 only has hardware support for 32-bit atomic operations but not wider (https://github.com/espressif/esp-idf/issues/3163)

FIX:
- Changed the file cmake/checks/atomic_check.cpp in OpenCV repo. The std::atomic<long long> was changed in std::atomic<long>, and also in file modules/core/include/opencv2/core/utils/allocator_stats.impl.hpp line 24.
CMake Error at /usr/share/cmake-3.16/Modules/TestBigEndian.cmake:50. The ESP32 is in little endian.

FIX:
- In the main CMakeLists.txt file, the line 654 is changed :
```
if(IOS OR ESP32)
```

After these fixes, the command make is run, with some new errors:

alloc.cpp:31:16: error: 'posix_memalign' was not declared in this scope

FIX: Modify alloc.cpp
- When there is an #if defined(ANDROID), add || defined(ESP32) after, so that malloc.h is included and memalign is used

#error "<dirent.h> not supported"

The ESP32 doesn't support directories (which are emulated with the filenames, like mydir/mysubdir/myfile.c).

FIX: Modify modules/core/src/glob.cpp

Add the following code after line 136:

#if defined(ESP32)
#include <sys/stat.h>
const char dir_separators[] = "/";

namespace {
    struct dirent {
        const char *d_name;
    };

    struct DIR {
        dirent ent;

        DIR() { }
        ~DIR()
        {
            if (ent.d_name)
                delete[] ent.d_name;
        }
    };

    DIR* opendir(const char* path)
    {
        DIR* dir = new DIR;
        dir->ent.d_name = 0;
        // TODO implement (point the first file starting with 'path' in its name)
        return dir;
    }

    dirent* readdir(DIR* dir)
    {
        // TODO: implement (point to the next file with 'path' in its name)
        return &(dir->ent);
    }

    void closedir(DIR* dir)
    {
        delete dir;
    }
}

The function are not implemented yet. Must be implemented if files reading/writing in SPIFFS needed.

system.cpp:1002:20: error: 'mkstemp' was not declared in this scope

FIX:
- Tried to include <stdlib.h> where mkstemp is, but didn't worked, so just commented lines 1003 to 1007 for now

color.simd_helpers.hpp:148:5: error: insn does not satisfy its constraints

FIX:

Disable optimization only for this function:

#pragma GCC push_options
#pragma GCC optimize ("-O0")
    virtual void operator()(const Range& range) const CV_OVERRIDE
    {
        // functon content ...
    }
#pragma GCC pop_options

histogram.cpp:1813:1: error: insn does not satisfy its constraints