Segmentation of perspectively distorted barcodes

Question

There are images with perspectively distorted barcodes in them.

They are located and decoded using ZBar.

Now I do not only need the rough location, but the four real corner points of the barcode, that define the enclosing 4-point polygon.

I tried different approaches, but did not yet get the desired result. One of them was:

• convert image to grayscale
• threshold image
• erode image
• floodFill beginning with a pixel known to be part of barcode
• obtain the contour around the floodFill result

But around this contour I now would need to find the minimum best fitting 4-point polygon, which seems to be not that easy.

Do you have ideas for better approaches?

Answer 1

You could use the following code and try to reduce your contour to 4-point polygon via approxPoly

vector approx;

for (size_t i = 0; i < contours.size(); i++)
{
    approxPolyDP(Mat(contours[i]), approx, 
             arcLength(Mat(contours[i]), true)*0.02, true);

    if (approx.size() == 4 &&
        fabs(contourArea(Mat(approx))) > 1000 &&
        isContourConvex(Mat(approx)))
    {
        double maxCosine = 0;

        for( int j = 2; j < 5; j++ )
        {
            double cosine = fabs(angle(approx[j%4], approx[j-2], approx[j-1]));
            maxCosine = MAX(maxCosine, cosine);
        }

        if( maxCosine < 0.3 )
            squares.push_back(approx);
    }
}

http://opencv-code.com/tutorials/detecting-simple-shapes-in-an-image/

You can also try the following methods, maybe they will produce good enough results for you: http://docs.opencv.org/modules/imgproc/doc/structural_analysis_and_shape_descriptors.html?highlight=minarearect#minarearect

or

http://docs.opencv.org/modules/imgproc/doc/structural_analysis_and_shape_descriptors.html?highlight=convexhull#convexhull