perspectiveTransformation.py

def fourCornersSort(pts):
    """ Sort corners: top-left, bot-left, bot-right, top-right """
    # Difference and sum of x and y value
    # Inspired by http://www.pyimagesearch.com
    diff = np.diff(pts, axis=1)
    summ = pts.sum(axis=1)
    
    # Top-left point has smallest sum...
    # np.argmin() returns INDEX of min
    return np.array([pts[np.argmin(summ)],
                     pts[np.argmax(diff)],
                     pts[np.argmax(summ)],
                     pts[np.argmin(diff)]])


def contourOffset(cnt, offset):
    """ Offset contour, by 5px border """
    # Matrix addition
    cnt += offset
    
    # if value < 0 => replace it by 0
    cnt[cnt < 0] = 0
    return cnt


# Sort and offset corners
pageContour = fourCornersSort(pageContour[:, 0])
pageContour = contourOffset(pageContour, (-5, -5))

# Recalculate to original scale - start Points
sPoints = pageContour.dot(image.shape[0] / 800)
  
# Using Euclidean distance
# Calculate maximum height (maximal length of vertical edges) and width
height = max(np.linalg.norm(sPoints[0] - sPoints[1]),
             np.linalg.norm(sPoints[2] - sPoints[3]))
width = max(np.linalg.norm(sPoints[1] - sPoints[2]),
             np.linalg.norm(sPoints[3] - sPoints[0]))

# Create target points
tPoints = np.array([[0, 0],
                    [0, height],
                    [width, height],
                    [width, 0]], np.float32)

# getPerspectiveTransform() needs float32
if sPoints.dtype != np.float32:
    sPoints = sPoints.astype(np.float32)

# Wraping perspective
M = cv2.getPerspectiveTransform(sPoints, tPoints) 
newImage = cv2.warpPerspective(image, M, (int(width), int(height)))

# Saving the result. Yay! (don't forget to convert colors bact to BGR)
cv2.imwrite("folder/resultImage.jpg", cv2.cvtColor(newImage, cv2.COLOR_BGR2RGB))