OpenCVで輪郭検出を行いテキスト領域を抽出するサンプル。 This program is released under the MIT license.

GcaTextDetector.cs

namespace LibTextDetector.TextDetector
{
    using System;
    using System.Collections.Generic;
    using System.Diagnostics;
    using System.Drawing;
    using System.Linq;
    using System.Text;
    using System.Threading.Tasks;
    using Emgu.CV;
    using Emgu.CV.Structure;

    public class GcaTextDetector
    {
        /// <summary>
        /// 指定した画像からテキスト領域のリストを返す。
        /// （輪郭検出を利用しテキスト領域を抽出）
        /// </summary>
        /// <param name="img">画像</param>
        /// <returns>テキスト領域のリスト</returns>
        public static List<Rectangle> DetectText(Image<Bgr, byte> img)
        {
            //// sobel(エッジ抽出)
            Image<Gray, byte> sobel =
                img
                .Convert<Gray, byte>()
                .Sobel(1, 0, 3)
                .AbsDiff(new Gray(0.0))
                .Convert<Gray, byte>()
                .ThresholdBinary(new Gray(100), new Gray(255));
            ////
            Mat se = CvInvoke.GetStructuringElement(
                Emgu.CV.CvEnum.ElementShape.Rectangle,
                new Size(10, 2),
                new Point(-1, -1));
            ////
            sobel = sobel.MorphologyEx(
                Emgu.CV.CvEnum.MorphOp.Dilate,
                se,
                new Point(-1, -1),
                1,
                Emgu.CV.CvEnum.BorderType.Reflect,
                new MCvScalar(255));
            //// 輪郭検出
            var contours = new Emgu.CV.Util.VectorOfVectorOfPoint();
            var m = new Mat();
            CvInvoke.FindContours(
                sobel,
                contours,
                m,
                Emgu.CV.CvEnum.RetrType.External,
                Emgu.CV.CvEnum.ChainApproxMethod.ChainApproxSimple);
            //// テキストラインの矩形領域のリスト
            var list = new List<Rectangle>();
            for (int i = 0; i < contours.Size; i++)
            {
                var brect = CvInvoke.BoundingRectangle(contours[i]);
                //// 矩形領域を条件に従い抽出する
                double ar = brect.Width / brect.Height;
                if (ar > 2 && brect.Width > 25 && brect.Height > 5 && brect.Height < 100)
                {
                    //// 領域の縦横比が1:2。２文字以上？
                    //// 領域の幅が > 25 pixel
                    //// 5 pixel < 領域の高さ < 100
                    list.Add(brect);
                }
            }

            //// X > Y でソート
            IOrderedEnumerable<Rectangle> sortList = list.OrderBy(rec => rec.Y).ThenBy(rec => rec.X);
            List<Rectangle> result = new List<Rectangle>();

            foreach (var aaa in sortList)
            {
                result.Add(aaa);
            }

            return result;
        }
    }
}

GcaVideoEffector.cs

namespace LibTextDetector.TextDetector
{
    using Emgu.CV;
    using Emgu.CV.Structure;
    using LibTextDetector.Ocr;
    using System;
    using System.Collections.Generic;
    using System.Diagnostics;
    using System.Drawing;
    using System.Linq;
    using System.Text;
    using System.Threading.Tasks;

    public class GcaVideoEffector
    {
        public static List<string> GetImageToTexts(string imagePath, string outImagePath)
        {
            Image<Bgr, byte> img = new Image<Bgr, byte>(imagePath);
            var regions = GcaTextDetector.DetectText(img);

            var ocr = new GcaOcr();
            var result = ocr.GetText(img, regions);

            var outImage = TextDetectedImage(img, regions);
            outImage.Save(outImagePath);

            return result;
        }

        public static double Process(string inputVideoPath, string outputVideoPath)
        {
            Stopwatch sw = new Stopwatch();
            double max = 0.0f;

            //// @"data\Eschalon_ Book I_001.mp4"
            var capture = new VideoCapture(inputVideoPath);

            var fps = capture.GetCaptureProperty(Emgu.CV.CvEnum.CapProp.Fps);
            System.Drawing.Size size = new System.Drawing.Size(capture.Width, capture.Height);
            int fourcc = VideoWriter.Fourcc('H', '2', '6', '4');

            Backend[] backends = CvInvoke.WriterBackends;
            int backend_idx = 0;
            foreach (Backend be in backends)
            {
                if (be.Name.Equals("MSMF"))
                {
                    backend_idx = be.ID;
                    break;
                }
            }

            var writer = new VideoWriter(outputVideoPath, backend_idx, fourcc, fps, size, true);

            var ocr = new GcaOcr();
            try
            {
                while (true)
                {
                    var m = new Mat();
                    capture.Read(m);
                    if (!m.IsEmpty)
                    {
                        var img = m.ToImage<Bgr, byte>();
                        sw.Start();
                        var regions = GcaTextDetector.DetectText(img);
                        sw.Stop();
                        max = Math.Max(max, sw.ElapsedMilliseconds);
                        sw.Reset();
                        List<string> textList = ocr.GetText(img, regions);

                        Image<Bgr, byte> imgout = img.CopyBlank();
                        foreach (var r in regions)
                        {
                            CvInvoke.Rectangle(img, r, new MCvScalar(0, 0, 255), 2);
                            CvInvoke.Rectangle(imgout, r, new MCvScalar(0, 255, 255), -1);
                        }

                        imgout._And(img);
                        writer.Write(img.Mat);
                    }
                    else
                    {
                        break;
                    }
                }
            }
            finally
            {
                writer.Dispose();
            }

            return max;
        }

        /// <summary>
        /// テキスト検出結果のイメージを返す。
        /// </summary>
        /// <param name="img">元イメージ</param>
        /// <param name="regions">テキスト領域のリスト</param>
        /// <returns>テキスト検出結果のイメージ</returns>
        private static Image<Bgr, byte> TextDetectedImage(Image<Bgr, byte> img, List<Rectangle> regions)
        {
            Image<Bgr, byte> imgout = img.CopyBlank();
            foreach (var r in regions)
            {
                CvInvoke.Rectangle(img, r, new MCvScalar(0, 0, 255), 2);
                CvInvoke.Rectangle(imgout, r, new MCvScalar(0, 255, 255), -1);
            }

            imgout._And(img);

            return imgout;
        }
    }
}