Problem when transporting points to a bigger image

edit

Hi,

I'm developing a document scanner library from Android and I have almost everthing sorted but there's one thing that I can't get an appropiate solution. The library detects, crops and fixes the perspective for the document it finds, on the preview the contour is found perfectly adjusted to the document so it stores this contour and takes a full resolution picture, then multiplies (Core.multiply) the found set of points to crop the full-sized image and in that image I don't get the same adjustment that in the preview.

Here is an example of the preview (the document is lightly highlighted with a blue rectangle):

and the full-sized image (that has some annoying margin):

Has someone any idea what can be happening?

Thanks a lot.

EDIT:

The code I use to find the contour:

Imgproc.GaussianBlur(mat, mat, new Size(5.d, 5.d), 0);
Imgproc.Canny(mat, mat, 40, 180);
Imgproc.findContours(imageMat, points, new Mat(), Imgproc.RETR_LIST, Imgproc.CHAIN_APPROX_SIMPLE);

for (int i = 0; i < points.size(); ++i) {
    MatOfInt convexedPoints = new MatOfInt();
    Imgproc.convexHull(points.get(i), convexedPoints);
    points.set(i, convertIndexesToPoints(points.get(i), convexedPoints));
}

// sort points by size

MatOfPoint approxPoint = null;
for (MatOfPoint p : points) {
    MatOfPoint2f c = new MatOfPoint2f(p.toArray());
    double peri = Imgproc.arcLength(c, true);
    MatOfPoint2f approx = new MatOfPoint2f();
    Imgproc.approxPolyDP(c, approx, 0.1 * peri, true);

    if (approx.toArray().length == 4 && Imgproc.contourArea(p) >= (imageArea * 0.25)) {
        // this is the contour
    }
}

And the code to convert the detected contour to the full-sized image:

    double scaleWidth = imgMat.width() / previewSizeWidth;
double scaleHeight = imgMat.height() / previewSizeHeight;

MatOfPoint scaled = new MatOfPoint();
Core.multiply(detectedPoint, new Scalar(scaleWidth, scaleHeight), scaled);

// from scaled, sort points to get topLeft, topRight, ...

// calculate new width and height
double widthA = Math.sqrt(
        Math.pow(bottomRight.x - bottomLeft.x, 2) +
                Math.pow(bottomRight.y - bottomLeft.y, 2)
);
double widthB = Math.sqrt(
        Math.pow(topRight.x - topLeft.x, 2) +
                Math.pow(topRight.y - topLeft.y, 2)
);
double width = Math.max(widthA, widthB);

double heightA = Math.sqrt(
        Math.pow(topRight.x - bottomRight.x, 2) +
                Math.pow(topRight.y - bottomRight.y, 2)
);
double heightB = Math.sqrt(
        Math.pow(topLeft.x - bottomLeft.x, 2) +
                Math.pow(topLeft.y - bottomLeft.y, 2)
);
double height = Math.max(heightA, heightB);

// prepare mats for perspective and transform
MatOfPoint2f src = new MatOfPoint2f(
        topLeft,
        topRight,
        bottomRight,
        bottomLeft
);
MatOfPoint2f dest = new MatOfPoint2f(
        new Point(0, 0),
        new Point(width - 1, 0),
        new Point(width - 1, height - 1),
        new Point(0, height - 1)
);

// prepare mats for perspective and transform
MatOfPoint2f src = new MatOfPoint2f(
        topLeft,
        topRight,
        bottomRight,
        bottomLeft
);
MatOfPoint2f dest = new MatOfPoint2f(
        new Point(0, 0),
        new Point(width - 1, 0),
        new Point(width - 1, height - 1),
        new Point(0, height - 1)
);

// apply perspective and transform
Mat perspective = Imgproc.getPerspectiveTransform(src, dest);
Mat finalImage = new Mat((int) height, (int) width, CvType.CV_8UC1);
Imgproc.warpPerspective(image, finalImage, perspective, new Size(width, height));

Sorry for not attaching code earlier.