Otsu thresholding.
The class implements Otsu thresholding, which is described in N. Otsu, "A threshold selection method from gray-level histograms", IEEE Trans. Systems, Man and Cybernetics 9(1), pp. 62–66, 1979.
This implementation instead of minimizing the weighted within-class variance does maximization of between-class variance, what gives the same result. The approach is described in this presentation.
The filter accepts 8 bpp grayscale images for processing.
Sample usage:
// create filter OtsuThreshold filter = new OtsuThreshold( ); // apply the filter filter.ApplyInPlace( image ); // check threshold value byte t = filter.ThresholdValue; // ... Initial image:
Result image (calculated threshold is 97):
private void SetFilter()
{
ImageType = ImageTypes.GrayscaleBT709;
Af.OtsuThreshold newFilter = new Af.OtsuThreshold();
imageFilter = newFilter;
}
public void GetBlobs3(string inputFile, string outputPath)
{
UnmanagedImage skewedImg = null;
{
var bmp = new Bitmap(inputFile);
var img = UnmanagedImage.FromManagedImage(bmp);
var grayImg = Accord.Imaging.Filters.Grayscale.CommonAlgorithms.BT709.Apply(img);
var bwImg = new Accord.Imaging.Filters.OtsuThreshold().Apply(grayImg);
var skewChecker = new DocumentSkewChecker();
double angle = skewChecker.GetSkewAngle(bwImg);
var rotateFilter = new Accord.Imaging.Filters.RotateBilinear(-angle);
skewedImg = rotateFilter.Apply(img);
bwImg.Dispose();
grayImg.Dispose();
img.Dispose();
bmp.Dispose();
}
// create filter
BlobsFiltering filter = new BlobsFiltering();
// configure filter
filter.CoupledSizeFiltering = true;
filter.MinHeight = 200; // 1" @ 200 DPI
filter.MinWidth = 200; // 1" @ 200 DPI
// apply the filter
filter.ApplyInPlace(skewedImg);
// save output file
Bitmap finalImg = skewedImg.ToManagedImage();
skewedImg.Dispose();
System.IO.File.Delete(outputPath);
finalImg.Save(outputPath);
finalImg.Dispose();
}
public void GetBlobs2(string inputFile, string outputPath)
{
UnmanagedImage skewedImg = null;
UnmanagedImage bwInvImg = null;
{
var bmp = new Bitmap(inputFile);
var img = UnmanagedImage.FromManagedImage(bmp);
var grayImg = Accord.Imaging.Filters.Grayscale.CommonAlgorithms.BT709.Apply(img);
var bwImg = new Accord.Imaging.Filters.OtsuThreshold().Apply(grayImg);
var skewChecker = new DocumentSkewChecker();
double angle = skewChecker.GetSkewAngle(bwImg);
var rotateFilter = new Accord.Imaging.Filters.RotateBilinear(-angle);
skewedImg = rotateFilter.Apply(img);
bwImg.Dispose();
grayImg.Dispose();
img.Dispose();
bmp.Dispose();
}
{
var grayImg = Accord.Imaging.Filters.Grayscale.CommonAlgorithms.BT709.Apply(skewedImg);
var bwImg = new Accord.Imaging.Filters.OtsuThreshold().Apply(grayImg);
var openingFilter = new Accord.Imaging.Filters.Opening();
openingFilter.ApplyInPlace(bwImg);
bwInvImg = new Accord.Imaging.Filters.Invert().Apply(bwImg);
bwImg.Dispose();
grayImg.Dispose();
}
var blobProc = new Accord.Imaging.BlobCounter();
blobProc.ProcessImage(bwInvImg);
var blobs = blobProc.GetObjectsInformation().ToList();
foreach (Accord.Imaging.Blob blob in blobs.OrderBy(b => b.Rectangle.Left).ThenBy(b => b.Rectangle.Top))
{
Console.WriteLine("{0} {1}", blob.Rectangle.ToString(), blob.Area.ToString());
}
//Layout parameters
var expectedLineMarkerSize = new System.Drawing.Size(25, 10); //new System.Drawing.Size(35, 15);
var expectedCellSize = new System.Drawing.Size(15, 10); //new System.Drawing.Size(20, 13);
int expectedNumlineMarkers = 19; // 23;
int tolerance = 3;
//Limits to determine in a cell is marked
double fullnessOk = .75;
double fullnessUnsure = .65;
var questions = new List <Tuple <int, Accord.Imaging.Blob, List <Accord.Imaging.Blob> > >();
{
var lineMarkers = blobs.Where(b =>
{
if (b.Rectangle.Width < expectedLineMarkerSize.Width - tolerance)
{
return(false);
}
if (b.Rectangle.Width > expectedLineMarkerSize.Width + tolerance)
{
return(false);
}
if (b.Rectangle.Height < expectedLineMarkerSize.Height - tolerance)
{
return(false);
}
if (b.Rectangle.Height > expectedLineMarkerSize.Height + tolerance)
{
return(false);
}
return(true);
})
.OrderBy(b => b.Rectangle.Left)
.ThenBy(b => b.Rectangle.Top)
.ToList();
if (lineMarkers.Count() != expectedNumlineMarkers)
{
throw new Exception(string.Format("Can't locate all line markers. Expected {0}, found {1}", expectedNumlineMarkers, lineMarkers.Count));
}
var cells = blobs.Where(b =>
{
if (b.Rectangle.Width < expectedCellSize.Width - tolerance)
{
return(false);
}
if (b.Rectangle.Width > expectedCellSize.Width + tolerance)
{
return(false);
}
if (b.Rectangle.Height < expectedCellSize.Height - tolerance)
{
return(false);
}
if (b.Rectangle.Height > expectedCellSize.Height + tolerance)
{
return(false);
}
return(true);
}).ToList();
int idxLine = 1;
foreach (var lineMarker in lineMarkers.OrderBy(b => b.CenterOfGravity.Y))
{
var cellsOfLine = cells.Where(b => Math.Abs(b.CenterOfGravity.Y - lineMarker.CenterOfGravity.Y) <= tolerance)
.Take(5)
.ToList()
.OrderBy(b => b.CenterOfGravity.X)
.ToList();
questions.Add(new Tuple <int, Accord.Imaging.Blob, List <Accord.Imaging.Blob> >(idxLine, lineMarker, cellsOfLine));
idxLine++;
}
}
{
var bmp = skewedImg.ToManagedImage();
using (Graphics g = Graphics.FromImage(bmp))
{
foreach (var question in questions)
{
g.FillRectangle(new SolidBrush(Color.Blue), question.Item2.Rectangle);
g.DrawString(question.Item1.ToString(), new System.Drawing.Font("Arial", 8), new SolidBrush(Color.White), question.Item2.Rectangle);
int column = 1;
foreach (Accord.Imaging.Blob blob in question.Item3.OrderBy(b => b.Rectangle.Left).ThenBy(b => b.Rectangle.Top))
{
if (System.Diagnostics.Debugger.IsAttached)
{
Console.WriteLine("Line {0}, Column {1}, Fullness {2}", question.Item1, column, Math.Round(blob.Fullness, 2));
}
if (blob.Fullness >= fullnessOk)
{
g.DrawRectangle(new Pen(Color.Green, 2), blob.Rectangle);
}
else if (blob.Fullness >= fullnessUnsure)
{
g.DrawRectangle(new Pen(Color.Yellow, 2), blob.Rectangle);
}
else
{
g.DrawRectangle(new Pen(Color.Red, 2), blob.Rectangle);
}
column++;
}
}
}
// bmp.Save(outp);
}
}
