Find an image in an image C#

Question

Hi

I have a scan of a document (a form actually) filled with some handwritten infos.

I have a bitmap of the form empty.

how can i "cancel" the printed form to extract the handwritting only.

I use C#... Thanks Jonathan

Answer 1

What you want to do is subtract the empty form image from the image of the form with handwriting in it. This will give you a reasonable image of the handwriting alone.

Please note that this will not register the images. Registration will line them up so that they are at identical orientations to give the subtraction the best chance of success. If your images are poorly aligned you'll have to look into image registration.

Here's a snippet of code I wrote a while back to do something similar (this code highlights differences in red):

        Bitmap b1 = new Bitmap(fname1);
        Bitmap b2 = new Bitmap(fname2);

        if (b1.Height != b2.Height || b1.Width != b2.Width) {
           MessageBox.Show("Input files are not the same dimensions!");
           Application.Exit();
        }

        totalPixels = b1.Height * b1.Width * 4;

        Bitmap outImg = new Bitmap(b1.Width, b1.Height, System.Drawing.Imaging.PixelFormat.Format32bppRgb);

        BitmapData b1Data = b1.LockBits(new Rectangle(0, 0, b1.Width, b1.Height), System.Drawing.Imaging.ImageLockMode.ReadOnly, System.Drawing.Imaging.PixelFormat.Format32bppRgb);
        BitmapData b2Data = b2.LockBits(new Rectangle(0, 0, b1.Width, b1.Height), System.Drawing.Imaging.ImageLockMode.ReadOnly, System.Drawing.Imaging.PixelFormat.Format32bppRgb);
        BitmapData oData = outImg.LockBits(new Rectangle(0, 0, b1.Width, b1.Height), System.Drawing.Imaging.ImageLockMode.WriteOnly, System.Drawing.Imaging.PixelFormat.Format32bppRgb);

        byte[] cur1 = new byte[b1Data.Stride * b1Data.Height];
        byte[] cur2 = new byte[b2Data.Stride * b2Data.Height];
        byte[] curOut = new byte[b2Data.Stride * b2Data.Height];

        Marshal.Copy(b1Data.Scan0, cur1, 0, b1Data.Stride * b1Data.Height);
        Marshal.Copy(b2Data.Scan0, cur2, 0, b2Data.Stride * b2Data.Height);

        for (int i = 0; i < b1Data.Stride * b1Data.Height; i += 4) {
           byte temp1 = cur1[i], temp2 = cur2[i], first = 0, second = 0;
           curOut[i] = 0;
           first = (byte) ((temp1 > temp2) ? temp1 - temp2 : temp2 - temp1);

           temp1 = cur1[i + 1];
           temp2 = cur2[i + 1];
           curOut[i + 1] = 0;
           second = (byte) ((temp1 > temp2) ? temp1 - temp2 : temp2 - temp1);

           temp1 = cur1[i + 2];
           temp2 = cur2[i + 2];
           curOut[i + 2] = (byte) ((temp1 > temp2) ? temp1 - temp2 : temp2 - temp1);
           curOut[i + 2] = (byte) ((first + second + curOut[i + 2]) * 255);

           curPixel = i;
        }

        Marshal.Copy(curOut, 0, oData.Scan0, b2Data.Stride * b2Data.Height);

        b1.UnlockBits(b1Data);
        b2.UnlockBits(b2Data);
        outImg.UnlockBits(oData);

        outImg.Save(outfile);

Answer 2

As an alternative (and possibly much faster method) could you not just store the rectangle psoitions of where the "fields" are going to be, then simply extract the pixels for each rectangles?