Error reading images using imread OpenCV

Issue

I am comparing two images using findHomography(). I have added extra modules from opencv_contrib in OpenCV 3.1.0 to use Surf and Sift algorithms and to compile for latest Android architectures. I can successfully compile the libraries using ndk-build.

Problem: When I run the application on LG Nexus 5, I am able to read images using imread but when I run the same application on LG Nexus 5X, imread does not read image. I have tested on Samsung S6 and OnePlus X and have the same issue. Below is my native method:

#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;
}

and the method breaks at this line:

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

Solution

I test your imread problem on Nexus 5x android 7.0 device, so I have only taken the imread command in my android project.

My opencv libraries are OpenCV 3.1.0 prebuilt libraries.

After some test, I only can read the image in the nexus 5x:

• /sdcard OK
• /storage/emulated/0/ Fails

I think actually are the same path but it does not load the image with the second option.

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

In my developing experience, I had problems with external storage paths, because some devices have emulated external storage and others not.

So usually, to avoid this problem, I copy my resources to internal .APK in execution time.

I store my resources on res.raw folder and I get the internal path with

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

I hope my test helps to solve your problem.

Cheers.

Unai.

Answered By - uelordi