Ask Your Question

Android Opencv Perspective Transformation not working as expected

asked 2016-09-17 12:24:37 -0500

magicleon gravatar image

I'm working with perspective transformation in opencv for android for a personal project.
The thing I'd like to do is to acquire a document like CamScanner does. For this I import a picture and let the user set the four transformation points needed.
Everything works, except for the fact that the transformation seems to be applied to different points or something like that. I need some fresh eyes to point me to the right direction.
Note that myHandle is a custom class, its only interesting properties in this context are that they are points with x and y coordinates and getX() and getY() are its getters for the coordinates.
Here's my function for the transformation:

public Bitmap addrizzone(Bitmap image, ArrayList<myHandle> sourcePoints){
        // sourcePoints are  expected  to be clockwise ordered
        // [top_left,top_right,bottom_right,bottom_left]
        // getting the size of the output image
        double dst_width = Math.max(sourcePoints.get(0).distanceFrom(sourcePoints.get(1)),sourcePoints.get(3).distanceFrom(sourcePoints.get(2)));
        double dst_height = Math.max(sourcePoints.get(0).distanceFrom(sourcePoints.get(3)),sourcePoints.get(1).distanceFrom(sourcePoints.get(2)));

        //determining point sets to get the transformation matrix
        List<org.opencv.core.Point> srcPts = new ArrayList<org.opencv.core.Point>();
        for (myHandle ball : sourcePoints) {
            srcPts.add(new org.opencv.core.Point((ball.getX()),ball.getY()));

        List<org.opencv.core.Point> dstPoints= new ArrayList<org.opencv.core.Point>();
        dstPoints.add(new org.opencv.core.Point(0,0));
        dstPoints.add(new org.opencv.core.Point(dst_width-1,0));
        dstPoints.add(new org.opencv.core.Point(dst_width-1,dst_height-1));
        dstPoints.add(new org.opencv.core.Point(0,dst_height));

        Mat srcMat = Converters.vector_Point2f_to_Mat(srcPts);
        Mat dstMat = Converters.vector_Point2f_to_Mat(dstPoints);

        //getting the transformation matrix
        Mat perspectiveTransformation = Imgproc.getPerspectiveTransform(srcMat,dstMat);

        //getting the input matrix from the given bitmap
        Mat inputMat = new Mat(image.getHeight(),image.getWidth(),CvType.CV_8UC1);



        //getting the output matrix with the previously determined sizes
        Mat outputMat = new Mat((int) dst_height,(int) dst_width,CvType.CV_8UC1);

        //applying the transformation
        Imgproc.warpPerspective(inputMat,outputMat,perspectiveTransformation,new Size(dst_width,dst_height));

        //creating the output bitmap
        Bitmap outputBitmap = Bitmap.createBitmap((int)dst_width,(int)dst_height, Bitmap.Config.RGB_565);

        //Mat to Bitmap

        return outputBitmap;

Some example screenshots: User determined the points
User determined the points
Transformation is not as expected
Transformation is not as expected

edit retag flag offensive close merge delete

1 answer

Sort by ยป oldest newest most voted

answered 2016-09-19 00:24:42 -0500

    //applying the transformation
    Imgproc.warpPerspective(inputMat,outputMat,perspectiveTransformation,new Size(dst_width,dst_height));

try this?

    //applying the transformation
    Imgproc.warpPerspective(inputMat,outputMat,perspectiveTransformation,new Size(dst_height,dst_width));

IIRC, android camera does something strange with the orientation.

edit flag offensive delete link more

Question Tools

1 follower


Asked: 2016-09-17 12:24:37 -0500

Seen: 3,197 times

Last updated: Sep 17 '16