使用 imread OpenCV 读取图像时出错

Question

我正在使用 findHomography() 比较两个图像。我在 OpenCV 3.1.0 中添加了来自 opencv_contrib 的额外模块以使用 Surf 和 Sift 算法并针对最新的 Android 架构进行编译。我可以使用 ndk-build 成功编译库。

问题： 当我运行 LG Nexus 5 上的应用程序时，我可以使用 imread 读取图像，但是当我运行 [=] 上的相同应用程序时22=]LG Nexus 5X, imread 不读取图像。我已经在 Samsung S6 和 OnePlus X 上进行了测试，但遇到了同样的问题。下面是我的原生方法：

#include <jni.h>
#include <string.h>
#include <stdio.h>
#include <android/log.h>

#include "opencv2/core/core.hpp"
#include "opencv2/features2d/features2d.hpp"
#include "opencv2/highgui/highgui.hpp"
#include "opencv2/calib3d/calib3d.hpp"
#include "opencv2/xfeatures2d/nonfree.hpp"
#include "opencv2/opencv.hpp"

using namespace std;
using namespace cv;

#define  LOG_TAG    "nonfree_jni"
#define  LOGI(...)  __android_log_print(ANDROID_LOG_INFO,LOG_TAG,__VA_ARGS__)

jboolean detect_features(JNIEnv * env, jstring scenePath, jstring objectPath) {

    const char *nativeScenePath = (env)->GetStringUTFChars(scenePath, NULL);
    const char *nativeObjectPath = (env)->GetStringUTFChars(objectPath, NULL);

    nativeScenePath = env->GetStringUTFChars(scenePath, 0);
    nativeObjectPath = env->GetStringUTFChars(objectPath, 0);

    (env)->ReleaseStringUTFChars(scenePath, nativeScenePath);
    (env)->ReleaseStringUTFChars(objectPath, nativeObjectPath);

    __android_log_print(ANDROID_LOG_VERBOSE, LOG_TAG, "Object path: ----- %s \n", nativeObjectPath);
    __android_log_print(ANDROID_LOG_VERBOSE, LOG_TAG, "Scene path: ----- %s \n", nativeScenePath);

    Mat img_object = imread( nativeObjectPath, CV_LOAD_IMAGE_GRAYSCALE );
    Mat img_scene = imread( nativeScenePath, CV_LOAD_IMAGE_GRAYSCALE );


    if( !img_object.data || !img_scene.data){
        LOGI(" --(!) Error reading images ");
        return false;
    }

        //-- Step 1: Detect the keypoints using SURF Detector
        int minHessian = 400;

    __android_log_print(ANDROID_LOG_VERBOSE, LOG_TAG, "Image comparison rows: ----- %d \n", img_object.rows);
    __android_log_print(ANDROID_LOG_VERBOSE, LOG_TAG, "Image comparison colums: ----- %d \n", img_object.cols);

//        cv::xfeatures2d::SurfFeatureDetector detector( minHessian );
        Ptr<cv::xfeatures2d::SurfFeatureDetector> detector = cv::xfeatures2d::SurfFeatureDetector::create(minHessian);

        std::vector<KeyPoint> keypoints_object, keypoints_scene;
        detector->detect( img_object, keypoints_object );
        detector->detect( img_scene, keypoints_scene );

        //-- Step 2: Calculate descriptors (feature vectors)
//        cv::xfeatures2d::SurfDescriptorExtractor extractor;
        Ptr<cv::xfeatures2d::SurfDescriptorExtractor> extractor = cv::xfeatures2d::SurfDescriptorExtractor::create();

        Mat descriptors_object, descriptors_scene;

        extractor->compute( img_object, keypoints_object, descriptors_object );
        extractor->compute( img_scene, keypoints_scene, descriptors_scene );

        //-- Step 3: Matching descriptor vectors using FLANN matcher
        FlannBasedMatcher matcher;
        std::vector< DMatch > matches;
        matcher.match( descriptors_object, descriptors_scene, matches );

        double max_dist = 0; double min_dist = 100;

        //-- Quick calculation of max and min distances between keypoints
        for( int i = 0; i < descriptors_object.rows; i++ )
        {
            double dist = matches[i].distance;
            if (dist == 0) continue;
            if( dist < min_dist ) min_dist = dist;
            if( dist > max_dist ) max_dist = dist;
        }

        __android_log_print(ANDROID_LOG_VERBOSE, LOG_TAG, "-- Max dist : %f \n", max_dist);
        __android_log_print(ANDROID_LOG_VERBOSE, LOG_TAG, "-- Min dist : %f \n", min_dist);

        //-- Draw only "good" matches (i.e. whose distance is less than 3*min_dist )
        std::vector< DMatch > good_matches;

        for( int i = 0; i < descriptors_object.rows; i++ )
        {
            if( matches[i].distance <= 0.1 ) //3*min_dist
            {
                good_matches.push_back( matches[i]);
            }
        }

        __android_log_print(ANDROID_LOG_VERBOSE, LOG_TAG, "FLANN total matches -----: %zu \n", matches.size());
        __android_log_print(ANDROID_LOG_VERBOSE, LOG_TAG, "FLANN good matches -----: %zu \n", good_matches.size());

        Mat img_matches;
        drawMatches( img_object, keypoints_object, img_scene, keypoints_scene,
                    good_matches, img_matches, Scalar::all(-1), Scalar::all(-1),
                    vector<char>(), DrawMatchesFlags::NOT_DRAW_SINGLE_POINTS );

        //-- Localize the object
        std::vector<Point2f> obj;
        std::vector<Point2f> scene;

        for( int i = 0; i < good_matches.size(); i++ )
        {
            //-- Get the keypoints from the good matches
            obj.push_back( keypoints_object[ good_matches[i].queryIdx ].pt );
            scene.push_back( keypoints_scene[ good_matches[i].trainIdx ].pt );
        }

        if (good_matches.size() >= 5)
        {
            Mat H = findHomography( obj, scene, CV_RANSAC );

            //-- Get the corners from the image_1 ( the object to be "detected" )
            std::vector<Point2f> obj_corners(4);
            obj_corners[0] = cvPoint(0,0); obj_corners[1] = cvPoint( img_object.cols, 0 );
            obj_corners[2] = cvPoint( img_object.cols, img_object.rows ); obj_corners[3] = cvPoint( 0, img_object.rows );
            std::vector<Point2f> scene_corners(4);

            Mat output, matrix;

            warpPerspective(img_object, output, H, { img_scene.cols, img_scene.rows });

            ////////////////////////////////////////////////////////////////////////////////

            detector->detect( output, keypoints_object );

            //-- Step 2: Calculate descriptors (feature vectors)
            //cv::xfeatures2d::SurfDescriptorExtractor extractor;
            Ptr<cv::xfeatures2d::SurfDescriptorExtractor> extractor = cv::xfeatures2d::SurfDescriptorExtractor::create();

            extractor->compute( output, keypoints_object, descriptors_object );
            extractor->compute( img_scene, keypoints_scene, descriptors_scene );

            std::vector<std::vector<cv::DMatch>> matches2;
            BFMatcher matcher;
            matcher.knnMatch(descriptors_object, descriptors_scene, matches2, 2);
            vector<cv::DMatch> good_matches2;

            for (int i = 0; i < matches2.size(); ++i)
            {
                const float ratio = 0.8; // As in Lowe's paper; can be tuned
                if (matches2[i][0].distance < ratio * matches2[i][1].distance)
                {
                    good_matches2.push_back(matches2[i][0]);
                }
            }

            if (matches2.size() == 0 || good_matches2.size() == 0) {
            LOGI( "End run!\n");
                return false;
            }

            double ratioOfSimilarity =  static_cast<double>(good_matches2.size()) / static_cast<double>(matches2.size());

            __android_log_print(ANDROID_LOG_VERBOSE, LOG_TAG, "Bruteforce total matches -----: %zu \n", matches2.size());
            __android_log_print(ANDROID_LOG_VERBOSE, LOG_TAG, "Bruteforce good matches -----: %zu \n", good_matches2.size());
            __android_log_print(ANDROID_LOG_VERBOSE, LOG_TAG, "Bruteforce similarity ratio -----: %f \n", ratioOfSimilarity);

            if(ratioOfSimilarity >= 0.3) {
            LOGI( "End run!\n");
                return true;
            }

            LOGI( "End run!\n");
            return false;

        }
        LOGI( "End run!\n");
        return false;
}

方法在这一行中断：

if( !img_object.data || !img_scene.data){
        LOGI(" --(!) Error reading images ");
        return false;
}

Answer 1

应该是!img_object.data？现在您会在有数据而不是没有数据时记录错误和 return false。

Answer 2

我在 Nexus 5x android 7.0 设备上测试了您的未读问题，所以我只在我的 android 项目中使用了 imread 命令。

我的 opencv 库是 OpenCV 3.1.0 预建库。

经过一些测试，我只能读取nexus 5x中的图像：

/sdcard 正常
/storage/emulated/0/失败

我认为实际上是相同的路径，但它不会使用第二个选项加载图像。

Mat flag=imread("/sdcard/Pictures/mytest.jpg", CV_LOAD_IMAGE_GRAYSCALE);

根据我的开发经验，我在外部存储路径方面遇到了问题，因为有些设备模拟了外部存储而有些则没有。

所以通常，为了避免这个问题，我在执行时将我的资源复制到内部.APK。

我将资源存储在 res.raw 文件夹中，并使用

获取内部路径

config_path = m_context.getApplicationContext().getFilesDir().toString();

希望我的测试能帮助解决您的问题。

干杯。

乌奈.

使用 imread OpenCV 读取图像时出错

Error reading images using imread OpenCV

android

surf

sift

android-ndk

opencv3.1