Segmentation of perspectively distorted barcodes - opencv

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

Answer 2

OK, I found a solution that works good enough for my use case.
First a scanline is generated from the ZBar result.
Now the first and the last black pixels are found in verion of the image resulting from cv::adaptivethreshold with a large enough blockSize.
From there on the first and the last bar are segmented using cv::findContours.
Now for both end bars the two contour points with the most distance to each others are searched.
They finally define the enclosing 4-point-polygon.
Which is not exactly what I posted in my question, but the additional size due to the elongated guard patterns does not matter in my case.