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Cannot rotate warpAffine character in license plate?

asked 2016-06-08 01:52:32 -0500

Dinh Thap gravatar image

updated 2016-06-08 02:30:44 -0500

I warpAffine license plate before segmentation but cannot correct incline character.

            opencv_core.RotatedRect rr = opencv_imgproc.minAreaRect(currentPlate.countor);
            opencv_core.Point[] pt = ImageOpenCvUtils.getVerticesPoint(rr);
            for (int k = 0; k < 4; k++) {
                opencv_imgproc.line(input, pt[k], pt[(k + 1) % 4], opencv_core.Scalar.RED);
            double angle = rr.angle();
            if (angle < -45) {
                angle += 90;

            opencv_core.Mat rot_mat = opencv_imgproc.getRotationMatrix2D(, angle, 1);
            opencv_core.Mat rotated = new opencv_core.Mat();
            opencv_imgproc.warpAffine(clone, rotated, rot_mat, input.size());
            opencv_core.Size size = null;
            if (rr.angle() < -45) {
                size = new opencv_core.Size(Math.round(rr.size().height()), Math.round(rr.size().width()));
            } else {
                size = new opencv_core.Size((int) rr.size().width(), (int) rr.size().height());

            opencv_imgproc.rectangle(clone, rr.boundingRect(), opencv_core.Scalar.BLUE);

            opencv_core.Mat subImage = new opencv_core.Mat();
            opencv_imgproc.getRectSubPix(rotated, size,, subImage);


Can you help me? Thanks

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answered 2016-06-08 09:40:41 -0500

kbarni gravatar image

updated 2016-06-08 11:03:57 -0500

The license plate in the image is not only rotated but also skewed.

The skewing transformation matrix has the following form

[ 1  sy  0 ]
[ sx  1  0 ]

In your case, sy=-0.3, sx=0 (approximative value). You have to transform the rotated image with this matrix, or multiply the rotation and the skewing matrix and use warpAffine with the combined matrix.

To get a robust solution, I suggest to detect the four corners of the license plate and determine directly the affine transformation matrix using the findHomography function.

[Update] The unskewing operation should be something like (untested code):

Mat skew_mat = Mat::eye(2,3,CV_64F);<double>(0,1)=-0.3;<double>(1,0)=0;
Mat skewed;
warpAffine(rotated, skewed, skew_mat, rotated.size());

For the second idea, there are a lots of tutorials on the web: e.g. this one:

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How do you find out sx, sy value?Can you show me some code. That make me easy to understand. Thanks.

Dinh Thap gravatar imageDinh Thap ( 2016-06-08 10:44:55 -0500 )edit

That's great! I apply your matrix then get good result. Can you show me the code calculated sx,sy value? Thanks.

Dinh Thap gravatar imageDinh Thap ( 2016-06-08 11:06:56 -0500 )edit

Updated my answer with some code and a tutorial.

kbarni gravatar imagekbarni ( 2016-06-08 11:08:37 -0500 )edit

Thank you so much!

Dinh Thap gravatar imageDinh Thap ( 2016-06-08 11:14:20 -0500 )edit

I multiplied the rotation and the skewing matrix then I have bad result. If I do rotation first and skewed after that then I have good result. So How can I combined 2 matrix exactly?

Dinh Thap gravatar imageDinh Thap ( 2016-06-12 03:38:07 -0500 )edit

Matrix multiplication is not commutative! You have to follow the order of transformations when computing the final matrix. This is true when combining any kind of transformations.

Skew_mat*Rot_mat != Rot_mat*Skew_mat
kbarni gravatar imagekbarni ( 2016-06-13 02:48:17 -0500 )edit

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Asked: 2016-06-08 01:52:32 -0500

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Last updated: Jun 08 '16