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There is a common technique used in OCR software that does the following (pseudocode):

threshold the image with an automatic global method (i.e. Otsu)
foreach (angle in angleRange)
{
    rotate original image by angle degrees
    count the number of white pixels (background) in each row
    store this number in a vector together with the associated angle
}
find the angle corresponding to the maximum value in the vector

This works because when the image is straight, in every row there is the maximum value of background pixels.