Deskew scanned images

Question

I work in an OMR project and we use C #. When we come to scan answer sheets, the images are distorted. How can we copy them?

Answer 1

The VB.Net code for this is available here , however, since you asked for C #, this is a C # translation of their Deskew class (note: Binarize (strictly not necessary, but works much better) and Rotate are exercises left to the user).

public class Deskew { // Representation of a line in the image. private class HougLine { // Count of points in the line. public int Count; // Index in Matrix. public int Index; // The line is represented as all x,y that solve y*cos(alpha)-x*sin(alpha)=d public double Alpha; } // The Bitmap Bitmap _internalBmp; // The range of angles to search for lines const double ALPHA_START = -20; const double ALPHA_STEP = 0.2; const int STEPS = 40 * 5; const double STEP = 1; // Precalculation of sin and cos. double[] _sinA; double[] _cosA; // Range of d double _min; int _count; // Count of points that fit in a line. int[] _hMatrix; public Bitmap DeskewImage(Bitmap image, int type, int binarizeThreshold) { Size oldSize = image.Size; _internalBmp = BitmapFunctions.Resize(image, new Size(1000, 1000), true, image.PixelFormat); Binarize(_internalBmp, binarizeThreshold); return Rotate(image, GetSkewAngle()); } // Calculate the skew angle of the image cBmp. private double GetSkewAngle() { // Hough Transformation Calc(); // Top 20 of the detected lines in the image. HougLine[] hl = GetTop(20); // Average angle of the lines double sum = 0; int count = 0; for (int i = 0; i <= 19; i++) { sum += hl[i].Alpha; count += 1; } return sum / count; } // Calculate the Count lines in the image with most points. private HougLine[] GetTop(int count) { HougLine[] hl = new HougLine[count]; for (int i = 0; i <= count - 1; i++) { hl[i] = new HougLine(); } for (int i = 0; i <= _hMatrix.Length - 1; i++) { if (_hMatrix[i] > hl[count - 1].Count) { hl[count - 1].Count = _hMatrix[i]; hl[count - 1].Index = i; int j = count - 1; while (j > 0 && hl[j].Count > hl[j - 1].Count) { HougLine tmp = hl[j]; hl[j] = hl[j - 1]; hl[j - 1] = tmp; j -= 1; } } } for (int i = 0; i <= count - 1; i++) { int dIndex = hl[i].Index / STEPS; int alphaIndex = hl[i].Index - dIndex * STEPS; hl[i].Alpha = GetAlpha(alphaIndex); //hl[i].D = dIndex + _min; } return hl; } // Hough Transforamtion: private void Calc() { int hMin = _internalBmp.Height / 4; int hMax = _internalBmp.Height * 3 / 4; Init(); for (int y = hMin; y <= hMax; y++) { for (int x = 1; x <= _internalBmp.Width - 2; x++) { // Only lower edges are considered. if (IsBlack(x, y)) { if (!IsBlack(x, y + 1)) { Calc(x, y); } } } } } // Calculate all lines through the point (x,y). private void Calc(int x, int y) { int alpha; for (alpha = 0; alpha <= STEPS - 1; alpha++) { double d = y * _cosA[alpha] - x * _sinA[alpha]; int calculatedIndex = (int)CalcDIndex(d); int index = calculatedIndex * STEPS + alpha; try { _hMatrix[index] += 1; } catch (Exception ex) { System.Diagnostics.Debug.WriteLine(ex.ToString()); } } } private double CalcDIndex(double d) { return Convert.ToInt32(d - _min); } private bool IsBlack(int x, int y) { Color c = _internalBmp.GetPixel(x, y); double luminance = (cR * 0.299) + (cG * 0.587) + (cB * 0.114); return luminance < 140; } private void Init() { // Precalculation of sin and cos. _cosA = new double[STEPS]; _sinA = new double[STEPS]; for (int i = 0; i < STEPS; i++) { double angle = GetAlpha(i) * Math.PI / 180.0; _sinA[i] = Math.Sin(angle); _cosA[i] = Math.Cos(angle); } // Range of d: _min = -_internalBmp.Width; _count = (int)(2 * (_internalBmp.Width + _internalBmp.Height) / STEP); _hMatrix = new int[_count * STEPS]; } private static double GetAlpha(int index) { return ALPHA_START + index * ALPHA_STEP; } } 

Answer 2

The scanned document is always skewed for the average angle [-10; +10] degrees. It’s easy to expose them using the Hugh transform, as Lou Franco said. This conversion defines the lines in your image for several angles. You just need to select the appropriate one for the horizontal lines of the document, and then rotate it.

• try isolating the pixel that matches your horizontal lines in the document (for example, black pixels with a white pixel at the bottom).
  

• Perform a Hough conversion. Remember to use the "unsafe" mode in C # to lock the entire image process with a pointer.
  

• Rotate your document from a different angle.
  

Works like the charm of binary documents (easily extends to gray level)

Answer 3

Disclaimer: I work in Atalasoft , a DotImage Imaging Document can do this with a few lines of code.

Deskew is an art term that describes what you are trying to do. As Ben Voigt said, it is technically spinning, not skewed, however when searching you will find algorithms under the automatic table.

The usual way to do this is to do a hough conversion to find the prevailing lines in the image. With normal documents, many of them will be orthogonal to the sides of the paper.

Answer 4

Are you sure that this is a “skew” and not a “rotation” (rotation preserves angles, skew does not work).

• Use some kind of registration mark (at least in two places) that you can recognize even when turning.
• Find the coordinates of these marks and calculate the angle of rotation.
• Apply a rotation transformation matrix to the image.