image processing to improve tesseract OCR accuracy

Question

I\'ve been using tesseract to convert documents into text. The quality of the documents ranges wildly, and I\'m looking for tips on what sort of image processing might impr

Answer 1

Three points to improve the readability of the image:

1. Resize the image with variable height and width(multiply 0.5 and 1 and 2 with image height and width).

2. Convert the image to Gray scale format(Black and white).

3. Remove the noise pixels and make more clear(Filter the image).

Refer below code :

Resize

public Bitmap Resize(Bitmap bmp, int newWidth, int newHeight)
        {
         
                Bitmap temp = (Bitmap)bmp;
            
                Bitmap bmap = new Bitmap(newWidth, newHeight, temp.PixelFormat);
             
                double nWidthFactor = (double)temp.Width / (double)newWidth;
                double nHeightFactor = (double)temp.Height / (double)newHeight;

                double fx, fy, nx, ny;
                int cx, cy, fr_x, fr_y;
                Color color1 = new Color();
                Color color2 = new Color();
                Color color3 = new Color();
                Color color4 = new Color();
                byte nRed, nGreen, nBlue;

                byte bp1, bp2;

                for (int x = 0; x < bmap.Width; ++x)
                {
                    for (int y = 0; y < bmap.Height; ++y)
                    {

                        fr_x = (int)Math.Floor(x * nWidthFactor);
                        fr_y = (int)Math.Floor(y * nHeightFactor);
                        cx = fr_x + 1;
                        if (cx >= temp.Width) cx = fr_x;
                        cy = fr_y + 1;
                        if (cy >= temp.Height) cy = fr_y;
                        fx = x * nWidthFactor - fr_x;
                        fy = y * nHeightFactor - fr_y;
                        nx = 1.0 - fx;
                        ny = 1.0 - fy;

                        color1 = temp.GetPixel(fr_x, fr_y);
                        color2 = temp.GetPixel(cx, fr_y);
                        color3 = temp.GetPixel(fr_x, cy);
                        color4 = temp.GetPixel(cx, cy);

                        // Blue
                        bp1 = (byte)(nx * color1.B + fx * color2.B);

                        bp2 = (byte)(nx * color3.B + fx * color4.B);

                        nBlue = (byte)(ny * (double)(bp1) + fy * (double)(bp2));

                        // Green
                        bp1 = (byte)(nx * color1.G + fx * color2.G);

                        bp2 = (byte)(nx * color3.G + fx * color4.G);

                        nGreen = (byte)(ny * (double)(bp1) + fy * (double)(bp2));

                        // Red
                        bp1 = (byte)(nx * color1.R + fx * color2.R);

                        bp2 = (byte)(nx * color3.R + fx * color4.R);

                        nRed = (byte)(ny * (double)(bp1) + fy * (double)(bp2));

                        bmap.SetPixel(x, y, System.Drawing.Color.FromArgb
                (255, nRed, nGreen, nBlue));
                    }
                }

       

                bmap = SetGrayscale(bmap);
                bmap = RemoveNoise(bmap);

                return bmap;
            
        }

SetGrayscale

public Bitmap SetGrayscale(Bitmap img)
            {
    
                Bitmap temp = (Bitmap)img;
                Bitmap bmap = (Bitmap)temp.Clone();
                Color c;
                for (int i = 0; i < bmap.Width; i++)
                {
                    for (int j = 0; j < bmap.Height; j++)
                    {
                        c = bmap.GetPixel(i, j);
                        byte gray = (byte)(.299 * c.R + .587 * c.G + .114 * c.B);
    
                        bmap.SetPixel(i, j, Color.FromArgb(gray, gray, gray));
                    }
                }
                return (Bitmap)bmap.Clone();
    
            }

RemoveNoise

public Bitmap RemoveNoise(Bitmap bmap)
            {
    
                for (var x = 0; x < bmap.Width; x++)
                {
                    for (var y = 0; y < bmap.Height; y++)
                    {
                        var pixel = bmap.GetPixel(x, y);
                        if (pixel.R < 162 && pixel.G < 162 && pixel.B < 162)
                            bmap.SetPixel(x, y, Color.Black);
                        else if (pixel.R > 162 && pixel.G > 162 && pixel.B > 162)
                            bmap.SetPixel(x, y, Color.White);
                    }
                }
    
                return bmap;
            }

INPUT IMAGE

OUTPUT IMAGE