Linux 安裝 OpenCV

Linux 查詢 OpenCV 版本:

opencv_version

3.4.4

1. get opencv_contrib

git clone https://github.com/opencv/opencv_contrib.git
cd opencv_contrib
git checkout 4.1.0
cd ..

2. lets create a build directory inside opencv folder and install opencv where we will refer opencv_contrib

wget https://github.com/opencv/opencv/archive/4.5.2.zip
unzip 4.5.2.zip
cd opencv-4.5.2/
mkdir build
cd build
cmake -D CMAKE_BUILD_TYPE=RELEASE -D OPENCV_ENABLE_NONFREE=ON -D OPENCV_EXTRA_MODULES_PATH=../../opencv_contrib/modules -D WITH_GTK=ON -D CMAKE_INSTALL_PREFIX=/usr/local ..
make
sudo make install
ldconfig

Reference

• Ubuntu 18.04 安裝 OpenCV 與建置 C++ 編譯環境

Feature Detection 特徵提取

在 OpenCV 中提供下面三種演算法可以對一張影像進行特徵點的萃取。在進行特徵點萃取前必須將影像轉為灰階影像，因為色彩易受光照影響，因此難以提供關鍵訊息故將影像進行灰階化，同時也可以加快特徵提取的速度。另外要辨識物體，最關鍵的因素是梯度也就是邊緣物體位置。自然就用到灰度影像，因此可以把灰階理解為影像的強度。

SURF	SIFT	ORB
901 個關鍵點	547 個關鍵點	492 個關鍵點

#include <opencv2/opencv.hpp>
#include <opencv2/xfeatures2d.hpp>
#include <iostream>
using namespace cv;
using namespace cv::xfeatures2d;
using namespace std;

int main()
{
    Mat srcImage = imread("./images/target.jpg", 0);

//     int numFeatures = 20; // 可以指定擷取特徵數量 create(numFeatures)
//     Ptr<SIFT> dectector = SIFT::create();
    Ptr<SURF> dectector = SURF::create();
//     Ptr<ORB> dectector = ORB::create();

    vector<KeyPoint> keypoints;
    dectector->detect(srcImage, keypoints, Mat());
    printf("Total keypoints: %ld\n", keypoints.size());

    Mat keypoint_img;
    drawKeypoints(srcImage, keypoints, keypoint_img, Scalar::all(-1), DrawMatchesFlags::DEFAULT);
    cv::imwrite("result.jpg", keypoint_img);
}

程式碼參考Feature_Detection.cpp

• tutorial_feature_detection
• sift、surf、orb 特徵提取及最優特徵點匹配

使用 Features2D + Homography 尋找物件

參考 Features2D_Homography.cpp，透過 SURF 演算法尋找照片中的特徵點，接著關鍵點配對透過 knnMatch 尋找距離最近的相似點。最後計算 Homography，可以找出目標影像中欲辨識物體的位置並且將它框起來。

• Features2D + Homography to find a known object
• 【OpenCV】OpenCV 4 下 SIFT、SURF的使用
• SIFT特徵和SURF特徵比較

如何編譯程式

方法一使用 Cmake

參考以下指令透過 Cmake 建構專案。CMakeLists.txt 待編譯的檔案為 Features2D_Homography.cpp(第三行)。

mkdir build
cd build
sudo cmake -D CMAKE_INSTALL_PREFIX=/home/jovyan/project/cpp_opencv/ ..
sudo make install
sudo make
./main

方法二透過 g++ 編譯

編譯 Feature_Detection.cpp：

g++ -o main Feature_Detection.cpp -I /usr/local/include/opencv4/ -L /usr/local/lib  -lopencv_features2d  -lopencv_imgcodecs -lopencv_core
./main

編譯 Features2D_Homography.cpp：

g++ -o main Features2D_Homography.cpp -I /usr/local/include/opencv4/ -L /usr/local/lib  -lopencv_features2d -lopencv_xfeatures2d -lopencv_calib3d -lopencv_imgcodecs -lopencv_imgproc -lopencv_core
./main

編譯 Features2D_Homography_SIFT.cpp：

g++ -o main Features2D_Homography_SIFT.cpp -I /usr/local/include/opencv4/ -L /usr/local/lib  -lopencv_features2d -lopencv_xfeatures2d -lopencv_calib3d -lopencv_imgcodecs -lopencv_imgproc -lopencv_core
./main

Linux 編譯 OpenCV

使用指令編譯程式

g++ -o main main.cpp -I /usr/local/include/opencv4/ -L /usr/local/lib  -lopencv_features2d  -lopencv_imgcodecs -lopencv_core

建立 pkg-config

我們可以透過建立 pkg-config 文件來幫我們管理每個套件的路徑（標頭文件路徑/include，庫文件路徑/lib）。透過 pkg-config 提供的參數(–cflags, –libs)，將所需信息提取出來供編譯和連接使用。因此可以省去在終端機用 g++ 編譯時後面添加的 -I、-L、-l 這些 flag。

cd /usr/local/lib
sudo mkdir pkgconfig && cd pkgconfig
sudo vim opencv.pc

prefix=/usr/local/
exec_prefix=${prefix}
includedir=${exec_prefix}/include/opencv4
libdir=${exec_prefix}/lib

Name: OpenCV
Description: Open Source Computer Vision Library
Version: 4.5.2
Libs: -L${libdir} -lopencv_stitching -lopencv_superres -lopencv_videostab -lopencv_aruco -lopencv_bgsegm -lopencv_bioinspired -lopencv_ccalib -lopencv_dnn_objdetect -lopencv_dpm -lopencv_face -lopencv_photo -lopencv_freetype -lopencv_fuzzy -lopencv_hdf -lopencv_hfs -lopencv_img_hash -lopencv_line_descriptor -lopencv_optflow -lopencv_reg -lopencv_rgbd -lopencv_saliency -lopencv_stereo -lopencv_structured_light -lopencv_phase_unwrapping -lopencv_surface_matching -lopencv_tracking -lopencv_datasets -lopencv_text -lopencv_dnn -lopencv_plot -lopencv_xfeatures2d -lopencv_shape -lopencv_video -lopencv_ml -lopencv_ximgproc -lopencv_calib3d -lopencv_features2d -lopencv_highgui -lopencv_videoio -lopencv_flann -lopencv_xobjdetect -lopencv_imgcodecs -lopencv_objdetect -lopencv_xphoto -lopencv_imgproc -lopencv_core
Libs.private: -ldl -lm -lpthread -lrt
Cflags: -I${includedir}

不確定要不要：

export PKG_CONFIG_PATH=/usr/local/pkgconfig:$PKG_CONFIG_PATH
sudo ldconfig

透過 pkg-config 查看 OpenCV 版本

pkg-config --modversion opencv

使用 pkg-config 指令編譯程式

g++ -o main main.cpp `pkg-config --cflags --libs opencv`

g++ -o main Features2D_Homography.cpp `pkg-config --cflags --libs opencv`

Reference

• OpenCV 自定義環境變量pkg-config
• compiling opencv in c++