实时的Android使用OpenCV的纸张检测

Question

首先，我真的很感激http://opencv-code.com/tutorials/automatic-perspective-correction-for-quadrilateral-objects/

我下面这个博客上，但我不处理image (Used in the blog above)，而是我想检测纸张任何尺寸（A4，合法或任何普通长方形纸张尺寸）的实时相机预览。

问题我得到的是“扩展hough线段以适合图像”之后我得到大量的hough线，因此得到超过4个相交点（不是== 4）。

这样http://s17.postimg.org/i0a57fb8v/device_2015_04_07_171351.png

如何删除其余的无效分，我只需要4个角点？我正在使用OpenCV Library for Android

请主要关注detectPaperSheet()方法。这里是我的代码：

package org.opencv.samples.tutorial1; 

import java.util.ArrayList; 

import org.opencv.android.BaseLoaderCallback; 
import org.opencv.android.CameraBridgeViewBase; 
import org.opencv.android.CameraBridgeViewBase.CvCameraViewFrame; 
import org.opencv.android.CameraBridgeViewBase.CvCameraViewListener2; 
import org.opencv.android.LoaderCallbackInterface; 
import org.opencv.android.OpenCVLoader; 
import org.opencv.core.Core; 
import org.opencv.core.CvType; 
import org.opencv.core.Mat; 
import org.opencv.core.Point; 
import org.opencv.core.Scalar; 
import org.opencv.core.Size; 
import org.opencv.imgproc.Imgproc; 

import android.app.Activity; 
import android.os.Bundle; 
import android.util.Log; 
import android.view.Menu; 
import android.view.MenuItem; 
import android.view.SurfaceView; 
import android.view.WindowManager; 
import android.widget.Toast; 

public class Tutorial1Activity extends Activity implements 
     CvCameraViewListener2 { 
    private static final String TAG = "OCVSample::Activity"; 

private CameraBridgeViewBase mOpenCvCameraView; 
private boolean mIsJavaCamera = true; 
private MenuItem mItemSwitchCamera = null; 

private BaseLoaderCallback mLoaderCallback = new BaseLoaderCallback(this) { 
    @Override 
    public void onManagerConnected(int status) { 
     switch (status) { 
     case LoaderCallbackInterface.SUCCESS: { 
      Log.i(TAG, "OpenCV loaded successfully"); 
      mOpenCvCameraView.enableView(); 
     } 
      break; 
     default: { 
      super.onManagerConnected(status); 
     } 
      break; 
     } 
    } 
}; 

public Tutorial1Activity() { 
    Log.i(TAG, "Instantiated new " + this.getClass()); 
} 

/** Called when the activity is first created. */ 
@Override 
public void onCreate(Bundle savedInstanceState) { 
    Log.i(TAG, "called onCreate"); 
    super.onCreate(savedInstanceState); 
    getWindow().addFlags(WindowManager.LayoutParams.FLAG_KEEP_SCREEN_ON); 

    setContentView(R.layout.tutorial1_surface_view); 

    if (mIsJavaCamera) 
     mOpenCvCameraView = (CameraBridgeViewBase) findViewById(R.id.tutorial1_activity_java_surface_view); 
    else 
     mOpenCvCameraView = (CameraBridgeViewBase) findViewById(R.id.tutorial1_activity_native_surface_view); 

    mOpenCvCameraView.setVisibility(SurfaceView.VISIBLE); 

    mOpenCvCameraView.setCvCameraViewListener(this); 
} 

@Override 
public void onPause() { 
    super.onPause(); 
    if (mOpenCvCameraView != null) 
     mOpenCvCameraView.disableView(); 
} 

@Override 
public void onResume() { 
    super.onResume(); 
    OpenCVLoader.initAsync(OpenCVLoader.OPENCV_VERSION_2_4_9, this, 
      mLoaderCallback); 
} 

public void onDestroy() { 
    super.onDestroy(); 
    if (mOpenCvCameraView != null) 
     mOpenCvCameraView.disableView(); 
} 

@Override 
public boolean onCreateOptionsMenu(Menu menu) { 
    Log.i(TAG, "called onCreateOptionsMenu"); 
    mItemSwitchCamera = menu.add("Toggle Native/Java camera"); 
    return true; 
} 

@Override 
public boolean onOptionsItemSelected(MenuItem item) { 
    String toastMesage = new String(); 
    Log.i(TAG, "called onOptionsItemSelected; selected item: " + item); 

    if (item == mItemSwitchCamera) { 
     mOpenCvCameraView.setVisibility(SurfaceView.GONE); 
     mIsJavaCamera = !mIsJavaCamera; 

     if (mIsJavaCamera) { 
      mOpenCvCameraView = (CameraBridgeViewBase) findViewById(R.id.tutorial1_activity_java_surface_view); 
      toastMesage = "Java Camera"; 
     } else { 
      mOpenCvCameraView = (CameraBridgeViewBase) findViewById(R.id.tutorial1_activity_native_surface_view); 
      toastMesage = "Native Camera"; 
     } 

     mOpenCvCameraView.setVisibility(SurfaceView.VISIBLE); 
     mOpenCvCameraView.setCvCameraViewListener(this); 
     mOpenCvCameraView.enableView(); 
     Toast toast = Toast.makeText(this, toastMesage, Toast.LENGTH_LONG); 
     toast.show(); 
    } 

    return true; 
} 

public void onCameraViewStarted(int width, int height) { 
} 

public void onCameraViewStopped() { 
} 

public Mat onCameraFrame(CvCameraViewFrame inputFrame) { 

    return detectPaperSheet(inputFrame.rgba()); 

} 

private Mat detectPaperSheet(Mat original_image) { 
    Mat imgSource = original_image; 
    Mat untouched = original_image.clone(); 

    // Converting to grayscale 
    Mat mHsvMat = new Mat(imgSource.rows(), imgSource.cols(), 
      CvType.CV_8UC1, new Scalar(0)); 

    Imgproc.cvtColor(imgSource, mHsvMat, Imgproc.COLOR_BGRA2GRAY, 4); 

    // apply gaussian blur to smoothen lines of dots 
    Imgproc.GaussianBlur(imgSource, imgSource, new Size(11, 11), 0); 

    // Applying Canny 
    Imgproc.Canny(mHsvMat, mHsvMat, 80, 100); 

    Mat lines = new Mat(); 
    int threshold = 100; 
    int minLineSize = 150; 
    int lineGap = 40; 

    Imgproc.HoughLinesP(mHsvMat, lines, 1, Math.PI/180, threshold, 
      minLineSize, lineGap); 

    // Expanding the Lines To Image Width and Height 

    ArrayList<Point> corners = new ArrayList<Point>(); 

    for (int x = 0; x < lines.cols(); x++) { 

     double[] vec = lines.get(0, x); 
     double[] val = new double[4]; 

     val[0] = 0; 
     val[1] = ((float) vec[1] - vec[3])/(vec[0] - vec[2]) * -vec[0] 
       + vec[1]; 
     val[2] = imgSource.cols(); 
     val[3] = ((float) vec[1] - vec[3])/(vec[0] - vec[2]) 
       * (imgSource.cols() - vec[2]) + vec[3]; 

     lines.put(0, x, val); 
    } 

    for (int x = 0; x < lines.cols(); x++) { 
     double[] vec = lines.get(0, x); 
     double x1 = vec[0], y1 = vec[1], x2 = vec[2], y2 = vec[3]; 
     Point start = new Point(x1, y1); 
     Point end = new Point(x2, y2); 

     Core.line(imgSource, start, end, new Scalar(255, 0, 0), 1); 

    } 

    for (int i = 0; i < lines.cols(); i++) { 
     for (int j = i + 1; j < lines.cols(); j++) { 
      Point pt = computeIntersect(lines.get(0, i), lines.get(0, j)); 
      if (pt.x >= 0 && pt.y >= 0) 
       corners.add(pt); 
     } 
    } 

    if (corners.size() < 4) { 
     Log.e("Corner < 4", corners.size() + " |"); 
     return untouched; 
    } else { 
     Log.e("Corner > 4", corners.size() + " |"); 
    } 

    //Mat cornerPoints = new Mat(); 

    for (int j = 0; j < corners.size(); j++) { 

     Core.circle(imgSource, 
       new Point(corners.get(j).x, corners.get(j).y), 20, 
       new Scalar(0, 0, 255), 2); 
    } 

    return imgSource; 
} 

private static Point computeIntersect(double[] a, double[] b) { 
    double x1 = a[0], y1 = a[1], x2 = a[2], y2 = a[3], x3 = b[0], y3 = b[1], x4 = b[2], y4 = b[3]; 
    double denom = ((x1 - x2) * (y3 - y4)) - ((y1 - y2) * (x3 - x4)); 
    Point pt = new Point(); 
    if (denom != 0) { 

     pt.x = ((x1 * y2 - y1 * x2) * (x3 - x4) - (x1 - x2) 
       * (x3 * y4 - y3 * x4)) 
       /denom; 
     pt.y = ((x1 * y2 - y1 * x2) * (y3 - y4) - (y1 - y2) 
       * (x3 * y4 - y3 * x4)) 
       /denom; 
     return pt; 
    } else 
     return new Point(-1, -1); 
} 

} 

Answer 1

记得poly approximation是有一定的帮助，我在这种情况下（检测长方形）。不过，我用它与findContours方法（CV_CHAIN_APPROX_SIMPLE模式），因为它似乎产生比霍夫线更好的结果。

[编辑] 我做到了，在C++用JNI，但在Java中，我认为它应该是这个样子：

Mat srcImg; //you may want to apply Canny or some threshold before searching for contours 
    List<MatOfPoint> contours = new ArrayList<MatOfPoint>(); 
    Mat hierarchy; 
    Imgproc.findContours(srcImg, contours, hierarchy, Imgproc.RETR_LIST, Imgproc.CHAIN_APPROX_SIMPLE); 
    MatOfPoint2f mat2fsrc, mat2fdst; 
    Scalar color = new Scalar(250, 250, 255); 

    for (int i = 0; i < contours.size(); i++) { 
     contours.get(i).convertTo(mat2fsrc, CvType.CV_32FC2); 
     Imgproc.approxPolyDP(mat2fsrc, mat2fdst, 0.01 * Imgproc.arcLength(mat2fsrc, true), true); 
     mat2fdst.convertTo(contours.get(i), CvType.CV_32S); 
     Imgproc.drawContours(srcImg, contours, i, color, 2, 8, hierarchy, 0, new Point()); 
    }