I am using OpenCV on android. I try to detect largest rectangle and extract this rectangle from image. I am using this code below. When the photo is taken, some image processing is done. My aim is, I want to detect a rectangle which is like border of paper and if its area is bigger than threshold_area, I want to crop it from original bitmap and draw on an imageview.
Mat cropped_mat;
PictureCallback _jpgCallBack = new PictureCallback(){
@Override
public void onPictureTaken(byte[] data, Camera camera) {
mCamera.startPreview();
bitmap = BitmapFactory.decodeByteArray(data , 0, data .length);
Mat ImageMat = new Mat();
Utils.bitmapToMat(cropped_mat, ImageMat);
double area = findLargestRectangle(ImageMat);
if(area > THRESHOLD_AREA){
mCamera.stopPreview();
try{
bitmap=Bitmap.createBitmap(cropped_mat.cols(), cropped_mat.rows(),Bitmap.Config.ARGB_8888);
Utils.matToBitmap(cropped_mat, bitmap);
}
catch(Exception e){}
img.setImageBitmap(bitmap);
}
else{
takePicture();
}
}
};
This is my findLargestRectangle method.
private double findLargestRectangle(Mat imgSource) {
Imgproc.Canny(imgSource, imgSource, 50, 50);
Imgproc.GaussianBlur(imgSource, imgSource, new Size(5, 5), 5);
List<MatOfPoint> contours = new ArrayList<MatOfPoint>();
Imgproc.findContours(imgSource, contours, new Mat(), Imgproc.RETR_LIST, Imgproc.CHAIN_APPROX_SIMPLE);
double maxArea = -1;
int maxAreaIdx = -1;
MatOfPoint temp_contour = contours.get(0); //the largest is at the index 0 for starting point
MatOfPoint2f approxCurve = new MatOfPoint2f();
Mat largest_contour = contours.get(0);
List<MatOfPoint> largest_contours = new ArrayList<MatOfPoint>();
for (int idx = 0; idx < contours.size(); idx++) {
temp_contour = contours.get(idx);
double contourarea = Imgproc.contourArea(temp_contour);
//compare this contour to the previous largest contour found
if (contourarea > maxArea) {
//check if this contour is a square
MatOfPoint2f new_mat = new MatOfPoint2f( temp_contour.toArray() );
int contourSize = (int)temp_contour.total();
Imgproc.approxPolyDP(new_mat, approxCurve, contourSize*0.05, true);
if (approxCurve.total() == 4) {
maxArea = contourarea;
maxAreaIdx = idx;
largest_contours.add(temp_contour);
largest_contour = temp_contour;
}
}
}
MatOfPoint temp_largest = largest_contours.get(largest_contours.size()-1);
Rect rect = Imgproc.boundingRect(temp_largest);
largest_contours = new ArrayList<MatOfPoint>();
largest_contours.add(temp_largest);
Core.rectangle(imgSource, new Point(rect.x,rect.y), new Point(rect.x+rect.width,rect.y+rect.height),new Scalar(0,0,255));
cropped_mat= imgSource.submat(rect.y , rect.y + rect.height, rect.x, rect.x + rect.width);
return Imgproc.contourArea(temp_largest);
}
But result is like below. Where am i wrong ?
original frame
result frame
intended bitmap