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POUsing .Net to deskew an image
 

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  1. POUsing .Net to deskew an image
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    <p>I have been searching high and low for a reliable way to deskew an image in .Net, and am not having much luck.</p> <p>At the minute I am using Aforge. This is a pain as I am working with WPF, so the images I am working with are BitmapImage objects, as opposed to Bitmap objects, meaning I need to start with a BitmapImage object, save this to a memory stream, create a new Bitmap object from the memory stream, go through the deskewing process, save the deskewed image to a new memory stream and then create a new BitmapImage object from said memory stream. Not only that, but the deskewing isn't great.</p> <p>I am trying to read OMR data of a piece of paper scanned into a scanner, and therefore I need to rely on a particular OMR box being at the same co-ordinates every time, so the deskewing needs to be reliable.</p> <p>So I am using Aforge at the minute, I can't find any other free/open source libraries for image deskewing in .Net, everything I have found is either properly expensive or in C/C++.</p> <p>My question is do other free/open source libraries exist that assist in image deskewing in .Net? If so what are they called, if not how should I approach this problem?</p> <p>Edit: For example, let's say I have the below page:</p> <p><img src="https://i.stack.imgur.com/DER5l.png" alt="Initial Image"></p> <p>Note: This is for illustrative purposes only, but the actual image does indeed have a black rectangle at each corner of the page, maybe this will help.</p> <p>When I print this out, and scan it back into my scanner, it looks like this:</p> <p><img src="https://i.stack.imgur.com/G6slz.png" alt="Scanned Image"></p> <p>I need to deskew this image so that my box is in the same place each time. In the real world, there are a lot of boxes, they are smaller and close together, so the accuracy is important.</p> <p>My current method for this is a massive ineffective pain-in-the-ass:</p> <pre><code>using AForge.Imaging; using AForge.Imaging.Filters; using System.Drawing; using System.Drawing.Imaging; using System.IO; using System.Windows.Media.Imaging; public static BitmapImage DeskewBitmap(BitmapImage skewedBitmap) { //Using a memory stream to minimise disk IO var memoryStream = BitmapImageToMemoryStream(skewedBitmap); var bitmap = MemoryStreamToBitmap(memoryStream); var skewAngle = CalculateSkewAngle(bitmap); //Aforge needs a Bppp indexed image for the deskewing process var bitmapConvertedToBbppIndexed = ConvertBitmapToBbppIndexed(bitmap); var rotatedImage = DeskewBitmap(skewAngle, bitmapConvertedToBbppIndexed); //I need to convert the image back to a non indexed format to put it back into a BitmapImage object var imageConvertedToNonIndexed = ConvertImageToNonIndexed(rotatedImage); var imageAsMemoryStream = BitmapToMemoryStream(imageConvertedToNonIndexed); var memoryStreamAsBitmapImage = MemoryStreamToBitmapImage(imageAsMemoryStream); return memoryStreamAsBitmapImage; } private static Bitmap ConvertImageToNonIndexed(Bitmap rotatedImage) { var imageConvertedToNonIndexed = rotatedImage.Clone( new Rectangle(0, 0, rotatedImage.Width, rotatedImage.Height), PixelFormat.Format32bppArgb); return imageConvertedToNonIndexed; } private static Bitmap DeskewBitmap(double skewAngle, Bitmap bitmapConvertedToBbppIndexed) { var rotationFilter = new RotateBilinear(-skewAngle) { FillColor = Color.White }; var rotatedImage = rotationFilter.Apply(bitmapConvertedToBbppIndexed); return rotatedImage; } private static double CalculateSkewAngle(Bitmap bitmapConvertedToBbppIndexed) { var documentSkewChecker = new DocumentSkewChecker(); double skewAngle = documentSkewChecker.GetSkewAngle(bitmapConvertedToBbppIndexed); return skewAngle; } private static Bitmap ConvertBitmapToBbppIndexed(Bitmap bitmap) { var bitmapConvertedToBbppIndexed = bitmap.Clone( new Rectangle(0, 0, bitmap.Width, bitmap.Height), PixelFormat.Format8bppIndexed); return bitmapConvertedToBbppIndexed; } private static BitmapImage ResizeBitmap(BitmapImage originalBitmap, int desiredWidth, int desiredHeight) { var ms = BitmapImageToMemoryStream(originalBitmap); ms.Position = 0; var result = new BitmapImage(); result.BeginInit(); result.DecodePixelHeight = desiredHeight; result.DecodePixelWidth = desiredWidth; result.StreamSource = ms; result.CacheOption = BitmapCacheOption.OnLoad; result.EndInit(); result.Freeze(); return result; } private static MemoryStream BitmapImageToMemoryStream(BitmapImage image) { var ms = new MemoryStream(); var encoder = new JpegBitmapEncoder(); encoder.Frames.Add(BitmapFrame.Create(image)); encoder.Save(ms); return ms; } private static BitmapImage MemoryStreamToBitmapImage(MemoryStream ms) { ms.Position = 0; var bitmap = new BitmapImage(); bitmap.BeginInit(); bitmap.StreamSource = ms; bitmap.CacheOption = BitmapCacheOption.OnLoad; bitmap.EndInit(); bitmap.Freeze(); return bitmap; } private static Bitmap MemoryStreamToBitmap(MemoryStream ms) { return new Bitmap(ms); } private static MemoryStream BitmapToMemoryStream(Bitmap image) { var memoryStream = new MemoryStream(); image.Save(memoryStream, ImageFormat.Bmp); return memoryStream; } </code></pre> <p>In retrospect, a couple more questions:</p> <ol> <li>Am I using AForge correctly?</li> <li>Is AForge the best library to use for this task?</li> <li>How could my current approach to this be improved to get more accurate results?</li> </ol>
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    1. COThis is the problem of using image-processing tools as a black-box. There are many ways to deskew, and it is important to know the approach being used specially when it "isnt' great". Otherwise, how do you know whether another deskew black-box has any chance to produce better results than your current black-box ? Letponica also has a black-box deskew method, but at http://tpgit.github.com/Leptonica/skew_8c.html you can read what it does. And there are many other ways to achieve this too.
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    2. CO@mmgp I do agree, and I wish I had the time to learn the ins and outs of the the hough transform algorithm, and C++ but sadly I have a deadline, so black boxing is my only option right now! Thankyou for the link, I will check it out.
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    3. COJohn, Could you put up a link to images that you are having problems deskewing or include them in the question? This will make it easier for people to answer.
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