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One of the best for thresholding the text paper for OCR is sauvola method.I have put the source code here.

Edit:

pixel = ( pixel > mean * ( 1 + k * ( standard_deviation / r - 1 ) ) ) ? object : background

Parameter 1: is the k value. The default value is 0.5. Any other number than 0 will change the default value.

Parameter 2: is the r value. The default value is 128. Any other number than 0 will change the default value

Sauvola, J & Pietaksinen, M (2000), "Adaptive Document Image Binarization", Pattern Recognition 33(2): 225-236,