Develop Reference

Overlay a image over live frame - OpenCV4Android

I'm trying to overlay on the live frame a little image previosuly selected by the user. I already have the image's path from a previous activity. My problem is that I am not able to show the image on the frame.
I'm trying to detect a rectangle on the frame, and over the rectangle display the image selected. I could detect the rectangle, but now i can't display the image on any part of the frame (I don't care about the rectangle right now).
I've been trying to do it with the explanations from Adding Image Overlay OpenCV for Android and add watermark small image to large image opencv4android but it didn't worked for me.
public Mat onCameraFrame(CameraBridgeViewBase.CvCameraViewFrame inputFrame) {
Mat gray = inputFrame.gray();
Mat dst = inputFrame.rgba();
Imgproc.pyrDown(gray, dsIMG, new Size(gray.cols() / 2, gray.rows() / 2));
Imgproc.pyrUp(dsIMG, usIMG, gray.size());
Imgproc.Canny(usIMG, bwIMG, 0, threshold);
Imgproc.dilate(bwIMG, bwIMG, new Mat(), new Point(-1, 1), 1);
List<MatOfPoint> contours = new ArrayList<MatOfPoint>();
cIMG = bwIMG.clone();
Imgproc.findContours(cIMG, contours, hovIMG, Imgproc.RETR_EXTERNAL, Imgproc.CHAIN_APPROX_SIMPLE);
for (MatOfPoint cnt : contours) {
MatOfPoint2f curve = new MatOfPoint2f(cnt.toArray());
Imgproc.approxPolyDP(curve, approxCurve, 0.02 * Imgproc.arcLength(curve, true), true);
int numberVertices = (int) approxCurve.total();
double contourArea = Imgproc.contourArea(cnt);
if (Math.abs(contourArea) < 100) {
continue;
}
//Rectangle detected
if (numberVertices >= 4 && numberVertices <= 6) {
List<Double> cos = new ArrayList<>();
for (int j = 2; j < numberVertices + 1; j++) {
cos.add(angle(approxCurve.toArray()[j % numberVertices], approxCurve.toArray()[j - 2], approxCurve.toArray()[j - 1]));
}
Collections.sort(cos);
double mincos = cos.get(0);
double maxcos = cos.get(cos.size() - 1);
if (numberVertices == 4 && mincos >= -0.3 && maxcos <= 0.5) {
//Small watermark image
Mat a = imread(img_path);
Mat bSubmat = dst.submat(0, dst.rows() -1 , 0, dst.cols()-1);
a.copyTo(bSubmat);
}
}
}
return dst;
}
NOTE: img_path is the path of the selected image I want to display over the frame. I got it from the previous activity.
By now, I just want to display the image over the frame. Later, I will try to display it on the same position where it found the rectangle.
Please, any suggestion or recommendation is welcome, as I am new with OpenCV. I'm sorry for my english, but feel free to ask me anything I didn't explain correctly. I'll do my best to explain it better.
Thanks a lot!

If you just want to display the image as an overlay, and not save it as part of the video, you may find it easier to simple display it in a separate view above the video view. This will likely use less processing, battery etc also.
If you want to draw onto the camera image bitmap then the following will allow you do that:
Bitmap cameraBitmap = BitmapFactory.decodeByteArray(bytes,0,bytes.length, opt);
Canvas camImgCanvas = new Canvas(cameraBitmap);
Drawable d = ContextCompat.getDrawable(getActivity(), R.drawable.myDrawable);
//Centre the drawing
int bitMapWidthCenter = cameraBitmap.getWidth()/2;
int bitMapheightCenter = cameraBitmap.getHeight()/2;
d.setBounds(bitMapWidthCenter, bitMapheightCenter, bitMapWidthCenter+d.getIntrinsicWidth(),
bitMapheightCenter+d.getIntrinsicHeight());
//And draw it...
d.draw(camImgCanvas);

Crop the largest rectangle using OpenCV

I am using openCV and tesseract to read cards. I am currently stuck at "how to crop the largest rectangle after I have detected all the rectangles and squares in from camera. Below is the code, please have a look.
public Mat onCameraFrame(CvCameraViewFrame inputFrame) {
if (Math.random()>0.80) {
findSquares(inputFrame.rgba().clone(),squares);
}
Mat image = inputFrame.rgba();
Imgproc.drawContours(image, squares, -1, new Scalar(0,0,255));
return image;
}
int thresh = 50, N = 11;
// helper function:
// finds a cosine of angle between vectors
// from pt0->pt1 and from pt0->pt2
double angle( Point pt1, Point pt2, Point pt0 ) {
double dx1 = pt1.x - pt0.x;
double dy1 = pt1.y - pt0.y;
double dx2 = pt2.x - pt0.x;
double dy2 = pt2.y - pt0.y;
return (dx1*dx2 + dy1*dy2)/Math.sqrt((dx1*dx1 + dy1*dy1)*(dx2*dx2 + dy2*dy2) + 1e-10);
}
// returns sequence of squares detected on the image.
// the sequence is stored in the specified memory storage
void findSquares( Mat image, List<MatOfPoint> squares )
{
squares.clear();
Mat smallerImg=new Mat(new Size(image.width()/2, image.height()/2),image.type());
Mat gray=new Mat(image.size(),image.type());
Mat gray0=new Mat(image.size(), CvType.CV_8U);
// down-scale and upscale the image to filter out the noise
Imgproc.pyrDown(image, smallerImg, smallerImg.size());
Imgproc.pyrUp(smallerImg, image, image.size());
// find squares in every color plane of the image
for( int c = 0; c < 3; c++ )
{
extractChannel(image, gray, c);
// try several threshold levels
for( int l = 1; l < N; l++ )
{
//Cany removed... Didn't work so well
Imgproc.threshold(gray, gray0, (l+1)*255/N, 255, Imgproc.THRESH_BINARY);
List<MatOfPoint> contours=new ArrayList<MatOfPoint>();
// find contours and store them all as a list
Imgproc.findContours(gray0, contours, new Mat(), Imgproc.RETR_LIST, Imgproc.CHAIN_APPROX_SIMPLE);
MatOfPoint approx=new MatOfPoint();
// test each contour
for( int i = 0; i < contours.size(); i++ )
{
// approximate contour with accuracy proportional
// to the contour perimeter
approx = approxPolyDP(contours.get(i), Imgproc.arcLength(new MatOfPoint2f(contours.get(i).toArray()), true)*0.02, true);
// square contours should have 4 vertices after approximation
// relatively large area (to filter out noisy contours)
// and be convex.
// Note: absolute value of an area is used because
// area may be positive or negative - in accordance with the
// contour orientation
if( approx.toArray().length == 4 &&
Math.abs(Imgproc.contourArea(approx)) > 1000 &&
Imgproc.isContourConvex(approx) )
{
double maxCosine = 0;
for( int j = 2; j < 5; j++ )
{
// find the maximum cosine of the angle between joint edges
double cosine = Math.abs(angle(approx.toArray()[j%4], approx.toArray()[j-2], approx.toArray()[j-1]));
maxCosine = Math.max(maxCosine, cosine);
}
// if cosines of all angles are small
// (all angles are ~90 degree) then write quandrange
// vertices to resultant sequence
if( maxCosine < 0.3 )
squares.add(approx);
}
}
}
}
}
void extractChannel(Mat source, Mat out, int channelNum) {
List<Mat> sourceChannels=new ArrayList<Mat>();
List<Mat> outChannel=new ArrayList<Mat>();
Core.split(source, sourceChannels);
outChannel.add(new Mat(sourceChannels.get(0).size(),sourceChannels.get(0).type()));
Core.mixChannels(sourceChannels, outChannel, new MatOfInt(channelNum,0));
Core.merge(outChannel, out);
}
MatOfPoint approxPolyDP(MatOfPoint curve, double epsilon, boolean closed) {
MatOfPoint2f tempMat=new MatOfPoint2f();
Imgproc.approxPolyDP(new MatOfPoint2f(curve.toArray()), tempMat, epsilon, closed);
return new MatOfPoint(tempMat.toArray());
}
}

So what you want to do at this point is sort the squares list for area.
If you are not worried about the efficiency or complexity of your algorithm (though you probably should be), you can simply add a line before you add the aprox to squares that calculates the contour area for your current approx. Then, have a loop that checks the contour area of an index in squares until it reaches either the end of squares or an element in squares where the area of your current contour is greater than the area in the contour at that index. This is called a selection sort, and while it is simple and slow, it does work.
If you are interested in earning further, sorting is a very interesting concept in computer science. For more efficient ways to sort your squares array, do some reading on heapsort, mergesort, and quicksort. You can learn the basics of sorting here
Good Luck!

Hough circle doesn't detect eyes iris

I want to detect eyes irises and their centers using Hough Circle algorithm.
I'm using this code:
private void houghCircle()
{
Bitmap obtainedBitmap = imagesList.getFirst();
/* convert bitmap to mat */
Mat mat = new Mat(obtainedBitmap.getWidth(),obtainedBitmap.getHeight(),
CvType.CV_8UC1);
Mat grayMat = new Mat(obtainedBitmap.getWidth(), obtainedBitmap.getHeight(),
CvType.CV_8UC1);
Utils.bitmapToMat(obtainedBitmap, mat);
/* convert to grayscale */
int colorChannels = (mat.channels() == 3) ? Imgproc.COLOR_BGR2GRAY : ((mat.channels() == 4) ? Imgproc.COLOR_BGRA2GRAY : 1);
Imgproc.cvtColor(mat, grayMat, colorChannels);
/* reduce the noise so we avoid false circle detection */
Imgproc.GaussianBlur(grayMat, grayMat, new Size(9, 9), 2, 2);
// accumulator value
double dp = 1.2d;
// minimum distance between the center coordinates of detected circles in pixels
double minDist = 100;
// min and max radii (set these values as you desire)
int minRadius = 0, maxRadius = 1000;
// param1 = gradient value used to handle edge detection
// param2 = Accumulator threshold value for the
// cv2.CV_HOUGH_GRADIENT method.
// The smaller the threshold is, the more circles will be
// detected (including false circles).
// The larger the threshold is, the more circles will
// potentially be returned.
double param1 = 70, param2 = 72;
/* create a Mat object to store the circles detected */
Mat circles = new Mat(obtainedBitmap.getWidth(), obtainedBitmap.getHeight(), CvType.CV_8UC1);
/* find the circle in the image */
Imgproc.HoughCircles(grayMat, circles, Imgproc.CV_HOUGH_GRADIENT, dp, minDist, param1, param2, minRadius, maxRadius);
/* get the number of circles detected */
int numberOfCircles = (circles.rows() == 0) ? 0 : circles.cols();
/* draw the circles found on the image */
for (int i=0; i<numberOfCircles; i++) {
/* get the circle details, circleCoordinates[0, 1, 2] = (x,y,r)
* (x,y) are the coordinates of the circle's center
*/
double[] circleCoordinates = circles.get(0, i);
int x = (int) circleCoordinates[0], y = (int) circleCoordinates[1];
Point center = new Point(x, y);
int radius = (int) circleCoordinates[2];
/* circle's outline */
Core.circle(mat, center, radius, new Scalar(0,
255, 0), 4);
/* circle's center outline */
Core.rectangle(mat, new Point(x - 5, y - 5),
new Point(x + 5, y + 5),
new Scalar(0, 128, 255), -1);
}
/* convert back to bitmap */
Utils.matToBitmap(mat, obtainedBitmap);
MediaStore.Images.Media.insertImage(getContentResolver(),obtainedBitmap, "testgray", "gray" );
}
But it doesn't detect iris in all images correctly. Specially, if the iris has a dark color like brown. How can I fix this code to detect the irises and their centers correctly?
EDIT: Here are some sample images (which I got from the web) that shows the performance of the algorithm (Please ignore the landmarks which are represented by the red squares):
In these images the algorithm doesn't detect all irises:
This image shows how the algorithm couldn't detect irises at all:
EDIT 2: Here is a code which uses Canny edge detection, but it causes the app to crash:
private void houghCircle()
{
Mat grayMat = new Mat();
Mat cannyEdges = new Mat();
Mat circles = new Mat();
Bitmap obtainedBitmap = imagesList.getFirst();
/* convert bitmap to mat */
Mat originalBitmap = new Mat(obtainedBitmap.getWidth(),obtainedBitmap.getHeight(),
CvType.CV_8UC1);
//Converting the image to grayscale
Imgproc.cvtColor(originalBitmap,grayMat,Imgproc.COLOR_BGR2GRAY);
Imgproc.Canny(grayMat, cannyEdges,10, 100);
Imgproc.HoughCircles(cannyEdges, circles,
Imgproc.CV_HOUGH_GRADIENT,1, cannyEdges.rows() / 15); //now circles is filled with detected circles.
//, grayMat.rows() / 8);
Mat houghCircles = new Mat();
houghCircles.create(cannyEdges.rows(),cannyEdges.cols()
,CvType.CV_8UC1);
//Drawing lines on the image
for(int i = 0 ; i < circles.cols() ; i++)
{
double[] parameters = circles.get(0,i);
double x, y;
int r;
x = parameters[0];
y = parameters[1];
r = (int)parameters[2];
Point center = new Point(x, y);
//Drawing circles on an image
Core.circle(houghCircles,center,r,
new Scalar(255,0,0),1);
}
//Converting Mat back to Bitmap
Utils.matToBitmap(houghCircles, obtainedBitmap);
MediaStore.Images.Media.insertImage(getContentResolver(),obtainedBitmap, "testgray", "gray" );
}
This is the error I get in the log
FATAL EXCEPTION: Thread-28685
CvException [org.opencv.core.CvException: cv::Exception: /hdd2/buildbot/slaves/slave_ardbeg1/50-SDK/opencv/modules/imgproc/src/color.cpp:3739: error: (-215) scn == 3 || scn == 4 in function void cv::cvtColor(cv::InputArray, cv::OutputArray, int, int)
]
at org.opencv.imgproc.Imgproc.cvtColor_1(Native Method)
at org.opencv.imgproc.Imgproc.cvtColor(Imgproc.java:4598)
Which is caused by this line: Imgproc.cvtColor(originalBitmap,grayMat,Imgproc.COLOR_BGR2GRAY);
Can anyone please tell me how this error can solved? Perhaps adding a canny edge detection will improve the results.

Hough circles work better on well defined circles. They are not good with things like iris.
After some thresholding, morphological operations or canny edge detection, feature detection methods like MSER work much better for iris detection.
Here is a similar question with a solution if you are looking for some code.

As you want to detect iris using hough transform (there are others), you had better studying the Canny edge detector and its parameters.
cv::HoughCircles takes the Canny-hysteresis threshold in param1. Investigating Canny alone, you get the impression of good threshold range.
Maybe instead of gaussian blur, you apply a better denoising (non local means with say h=32 and window sizes 5 and 15), and also try to harmonize the image contrast, e.g., using contrast limited adaptive histogram equalization (cv::CLAHE).
Harmonization is to make sure all (highlight and shadow) eyes map to similar intensity range.

I wanted to know if those images are the images you processed or if you like took a cell phone snapshot of your screen to upload them here. Because the irises are bigger than the maximum radius you set in your code. Therefor I don't understand how you could find any iris at all. The irises in the first image have a radius of over 20. So you shouldn't be able to detect them.
You should set the radii to the radius range you expect your irises to be.

Crop an image using multiple points and flexible size? [closed]

Closed. This question needs to be more focused. It is not currently accepting answers.
Want to improve this question? Update the question so it focuses on one problem only by editing this post.
Closed 3 years ago.
Improve this question
I want to make a an application like a cam scanner for cropping a document.
But I need same functionality like my two images..
First Images shown image captured by camera..
Second image recognize a captured image part like this..
I research more and more but not getting any out put so, I ask here if,any one done this tell me..
Thanks

I assume your problem is to detect the object to scan.
Object detection mechanisms like pattern matching or feature detection won't bring you the results you are looking for as you don't know what exactly is the object you are scanning.
Basically you search for a rectangular object in the picture.
A basic approach to this could be as following:
Run a canny edge detector on the image. It could help to blur the image a bit before doing this. The edges of the object should be clearly visible.
Now you want to do a Hough transform to find lines in the picture.
Search for lines with an angle around 90deg to each other. The problem would be to find the right ones. Maybe it is enough to use the lines closest to the frame of the picture that are reasonably parallel to them.
Find the intersecting points to define the edges of your object.
At least this should give you a hint where to research further.
As further steps in such an app you will have to calculate the projection of the points and do a affine transform of the object.
I hope this helps.
After writing all this i found this post. It should help you lot.
As my answer targets OpenCV you have to use the OpenCV library.
In Order to do this, you need to install the Android Native Development Kit (NDK).
There are some good tutorials on how to use OpenCV on Android on the OpenCV for Android page.
One thing to keep in mind is that almost each function of the Java wrapper calls a native method. That costs lots of time. So you want to do as much as possible in your native code before returning your results to the Java part.

I know I am too late to answer but it might be helpful to someone.
Try the following code.
#Override
protected void onDraw(Canvas canvas) {
super.onDraw(canvas);
path = new Path();
path.moveTo(x1, y1); // this should set the start point right
//path.lineTo(x1, y1); <-- this line should be drawn at the end of course,sorry
path.lineTo(x2, y2);
path.lineTo(x3, y3);
path.lineTo(x4, y4);
path.lineTo(x1, y1);
canvas.drawPath(path, currentPaint);
}

Pass your image mat in this method:
void findSquares(Mat image, List<MatOfPoint> squares) {
int N = 10;
squares.clear();
Mat smallerImg = new Mat(new Size(image.width() / 2, image.height() / 2), image.type());
Mat gray = new Mat(image.size(), image.type());
Mat gray0 = new Mat(image.size(), CvType.CV_8U);
// down-scale and upscale the image to filter out the noise
Imgproc.pyrDown(image, smallerImg, smallerImg.size());
Imgproc.pyrUp(smallerImg, image, image.size());
// find squares in every color plane of the image
Outer:
for (int c = 0; c < 3; c++) {
extractChannel(image, gray, c);
// try several threshold levels
Inner:
for (int l = 1; l < N; l++) {
Imgproc.threshold(gray, gray0, (l + 1) * 255 / N, 255, Imgproc.THRESH_BINARY);
List<MatOfPoint> contours = new ArrayList<MatOfPoint>();
// find contours and store them all as a list
Imgproc.findContours(gray0, contours, new Mat(), Imgproc.RETR_LIST, Imgproc.CHAIN_APPROX_SIMPLE);
MatOfPoint approx = new MatOfPoint();
// test each contour
for (int i = 0; i < contours.size(); i++) {
approx = approxPolyDP(contours.get(i), Imgproc.arcLength(new MatOfPoint2f(contours.get(i).toArray()), true) * 0.02, true);
// square contours should have 4 vertices after approximation
// relatively large area (to filter out noisy contours)
// and be convex.
// Note: absolute value of an area is used because
// area may be positive or negative - in accordance with the
// contour orientation
double area = Imgproc.contourArea(approx);
if (area > 5000) {
if (approx.toArray().length == 4 &&
Math.abs(Imgproc.contourArea(approx)) > 1000 &&
Imgproc.isContourConvex(approx)) {
double maxCosine = 0;
Rect bitmap_rect = null;
for (int j = 2; j < 5; j++) {
// find the maximum cosine of the angle between joint edges
double cosine = Math.abs(angle(approx.toArray()[j % 4], approx.toArray()[j - 2], approx.toArray()[j - 1]));
maxCosine = Math.max(maxCosine, cosine);
bitmap_rect = new Rect(approx.toArray()[j % 4], approx.toArray()[j - 2]);
}
// if cosines of all angles are small
// (all angles are ~90 degree) then write quandrange
// vertices to resultant sequence
if (maxCosine < 0.3)
squares.add(approx);
}
}
}
}
}
}
In this method you get four point of document then you can cut this image using below method:
public Bitmap warpDisplayImage(Mat inputMat) {
List<Point> newClockVisePoints = new ArrayList<>();
int resultWidth = inputMat.width();
int resultHeight = inputMat.height();
Mat startM = Converters.vector_Point2f_to_Mat(orderRectCorners(Previes method four poit list(like : List<Point> points)));
Point ocvPOut4 = new Point(0, 0);
Point ocvPOut1 = new Point(0, resultHeight);
Point ocvPOut2 = new Point(resultWidth, resultHeight);
Point ocvPOut3 = new Point(resultWidth, 0);
ocvPOut3 = new Point(0, 0);
ocvPOut4 = new Point(0, resultHeight);
ocvPOut1 = new Point(resultWidth, resultHeight);
ocvPOut2 = new Point(resultWidth, 0);
}
Mat outputMat = new Mat(resultWidth, resultHeight, CvType.CV_8UC4);
List<Point> dest = new ArrayList<Point>();
dest.add(ocvPOut3);
dest.add(ocvPOut2);
dest.add(ocvPOut1);
dest.add(ocvPOut4);
Mat endM = Converters.vector_Point2f_to_Mat(dest);
Mat perspectiveTransform = Imgproc.getPerspectiveTransform(startM, endM);
Imgproc.warpPerspective(inputMat, outputMat, perspectiveTransform, new Size(resultWidth, resultHeight), Imgproc.INTER_CUBIC);
Bitmap descBitmap = Bitmap.createBitmap(outputMat.cols(), outputMat.rows(), Bitmap.Config.ARGB_8888);
Utils.matToBitmap(outputMat, descBitmap);
return descBitmap;
}

Android OpenCV Find Largest Square or Rectangle

This might have been answered but I desperately need an answer for this. I want to find the largest square or rectangle in an image using OpenCV in Android. All of the solutions that I found are C++ and I tried converting it but it doesn't work and I do not know where I'm wrong.
private Mat findLargestRectangle(Mat original_image) {
Mat imgSource = original_image;
Imgproc.cvtColor(imgSource, imgSource, Imgproc.COLOR_BGR2GRAY);
Imgproc.Canny(imgSource, imgSource, 100, 100);
//I don't know what to do in here
return imgSource;
}
What I am trying to accomplish in here is to create a new image that is based on the largest square found in the original image (return value Mat image).
This is what I want to happen:
1 http://img14.imageshack.us/img14/7855/s7zr.jpg
It's also okay that I just get the four points of the largest square and I think I can take it from there. But it would be better if I can just return the cropped image.

After canny
1- you need to reduce noises with gaussian blur and find all the contours
2- find and list all the contours' areas.
3- the largest contour will be nothing but the painting.
4- now use perpective transformation to transform your shape to a rectangle.
check sudoku solver examples to see the similar processing problem. (largest contour + perspective)

Took me a while to convert the C++ code to Java, but here it is :-)
Warning ! Raw code, totally not optimized and all.
I decline any liability in cases of injury or lethal accident
List<MatOfPoint> squares = new ArrayList<MatOfPoint>();
public Mat onCameraFrame(CvCameraViewFrame inputFrame) {
if (Math.random()>0.80) {
findSquares(inputFrame.rgba().clone(),squares);
}
Mat image = inputFrame.rgba();
Imgproc.drawContours(image, squares, -1, new Scalar(0,0,255));
return image;
}
int thresh = 50, N = 11;
// helper function:
// finds a cosine of angle between vectors
// from pt0->pt1 and from pt0->pt2
double angle( Point pt1, Point pt2, Point pt0 ) {
double dx1 = pt1.x - pt0.x;
double dy1 = pt1.y - pt0.y;
double dx2 = pt2.x - pt0.x;
double dy2 = pt2.y - pt0.y;
return (dx1*dx2 + dy1*dy2)/Math.sqrt((dx1*dx1 + dy1*dy1)*(dx2*dx2 + dy2*dy2) + 1e-10);
}
// returns sequence of squares detected on the image.
// the sequence is stored in the specified memory storage
void findSquares( Mat image, List<MatOfPoint> squares )
{
squares.clear();
Mat smallerImg=new Mat(new Size(image.width()/2, image.height()/2),image.type());
Mat gray=new Mat(image.size(),image.type());
Mat gray0=new Mat(image.size(),CvType.CV_8U);
// down-scale and upscale the image to filter out the noise
Imgproc.pyrDown(image, smallerImg, smallerImg.size());
Imgproc.pyrUp(smallerImg, image, image.size());
// find squares in every color plane of the image
for( int c = 0; c < 3; c++ )
{
extractChannel(image, gray, c);
// try several threshold levels
for( int l = 1; l < N; l++ )
{
//Cany removed... Didn't work so well
Imgproc.threshold(gray, gray0, (l+1)*255/N, 255, Imgproc.THRESH_BINARY);
List<MatOfPoint> contours=new ArrayList<MatOfPoint>();
// find contours and store them all as a list
Imgproc.findContours(gray0, contours, new Mat(), Imgproc.RETR_LIST, Imgproc.CHAIN_APPROX_SIMPLE);
MatOfPoint approx=new MatOfPoint();
// test each contour
for( int i = 0; i < contours.size(); i++ )
{
// approximate contour with accuracy proportional
// to the contour perimeter
approx = approxPolyDP(contours.get(i), Imgproc.arcLength(new MatOfPoint2f(contours.get(i).toArray()), true)*0.02, true);
// square contours should have 4 vertices after approximation
// relatively large area (to filter out noisy contours)
// and be convex.
// Note: absolute value of an area is used because
// area may be positive or negative - in accordance with the
// contour orientation
if( approx.toArray().length == 4 &&
Math.abs(Imgproc.contourArea(approx)) > 1000 &&
Imgproc.isContourConvex(approx) )
{
double maxCosine = 0;
for( int j = 2; j < 5; j++ )
{
// find the maximum cosine of the angle between joint edges
double cosine = Math.abs(angle(approx.toArray()[j%4], approx.toArray()[j-2], approx.toArray()[j-1]));
maxCosine = Math.max(maxCosine, cosine);
}
// if cosines of all angles are small
// (all angles are ~90 degree) then write quandrange
// vertices to resultant sequence
if( maxCosine < 0.3 )
squares.add(approx);
}
}
}
}
}
void extractChannel(Mat source, Mat out, int channelNum) {
List<Mat> sourceChannels=new ArrayList<Mat>();
List<Mat> outChannel=new ArrayList<Mat>();
Core.split(source, sourceChannels);
outChannel.add(new Mat(sourceChannels.get(0).size(),sourceChannels.get(0).type()));
Core.mixChannels(sourceChannels, outChannel, new MatOfInt(channelNum,0));
Core.merge(outChannel, out);
}
MatOfPoint approxPolyDP(MatOfPoint curve, double epsilon, boolean closed) {
MatOfPoint2f tempMat=new MatOfPoint2f();
Imgproc.approxPolyDP(new MatOfPoint2f(curve.toArray()), tempMat, epsilon, closed);
return new MatOfPoint(tempMat.toArray());
}

There are some related questions here in SO. Check them out:
OpenCV C++/Obj-C: Detecting a sheet of paper / Square Detection
How do I recognize squares in this image?
There is also an example shipped with OpenCV:
https://code.ros.org/trac/opencv/browser/trunk/opencv/samples/cpp/squares.cpp?rev=4079
Once you have the rectangle, you can align the picture by computing the homography with the rectangle corners and applying a perspective transform.