Remove black border lines around image after scan with opencv c++

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If you can determine the first border pixel accurately eg. occurs near (0,0) then you might be able to

1. floodFill all black pixels starting at (0,0) or where ever, generate a binary image such that 0 is for border pixels, and 1 is for everything else
2. use findContour + boundingRect to find the bounding box of the inside pixels. The findContour is very fast, it only visits pixels on the edge (not inside like a floodFill). findContour will return an array of possible contours, pick the largest one.
3. crop the borders out using the bounding rect of the non-border pixels

I would suggest running 4 nested loops on x,y, width, height and using the function countNonZero to count the number of white pixels. You can also take the negative image and use countNonZero to count the number of black pixels.

countNonZero:

http://docs.opencv.org/modules/core/doc/operations_on_arrays.html#countnonzero

negative image:

http://www.programming-techniques.com/2013/01/producing-nagative-of-grayscale-image-c.html