(Almost) JPEG Decoder in C#

jpeg.cs

using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.Drawing;
using System.Drawing.Imaging;
using System.IO;
using System.Runtime.InteropServices;

namespace Jpeg
{

	#region Extensions

	public static class Extensions
	{
		public static void Swap<T>(this IList<T> arr, int i1, int i2)
		{
			Debug.Assert(i1 > 0 && i1 < arr.Count);
			Debug.Assert(i2 > 0 && i2 < arr.Count);

			T tempT = arr[i1];
			arr[i1] = arr[i2];
			arr[i2] = tempT;
		}

		public static Int16 ReadInt16BE(this BinaryReader reader)
		{
			byte[] temp = reader.ReadBytes(2);
			return (short)(
				temp[0] << 8 |
				temp[1]
			);
		}
	}

	#endregion

	#region DCT

	static class DCT
	{
		const int Side = 8;
		const int SideSquared = Side * Side;

		private static readonly double[] Dct = GenerateDct();
		private static readonly double[] DctT = Transpose(GenerateDct());

		private static double[] GenerateDct()
		{
			const int Size2 = Side * Side;

			double[] result = new double[Size2];
			for (int y = 0, o = 0; y < Side; y++)
				for (int x = 0; x < Side; x++)
					result[o++] =
						Math.Sqrt(y == 0 ? .125 : .250) *
						Math.Cos(((2 * x + 1) * y * Math.PI) * .0625);

			return result;
		}

		private static double[] Transpose(double[] m)
		{
			Debug.Assert(m != null && m.Length == SideSquared);

			for (int y = 0; y < Side; y++)
				for (int x = y + 1; x < Side; x++)
					m.Swap(y * Side + x, x * Side + y);

			return m;
		}

		private static double[] MatrixMultiply(double[] m1, double[] m2)
		{
			Debug.Assert(m1 != null && m1.Length == SideSquared);
			Debug.Assert(m2 != null && m1.Length == SideSquared);

			double[] result = new double[m1.Length];
			for (int y = 0; y < Side; y++)
				for (int x = 0; x < Side; x++)
				{
					double sum = 0;
					for (int k = 0; k < Side; k++)
						sum += m1[y * Side + k] * m2[k * Side + x];
					result[y * Side + x] = sum;
				}

			return result;
		}

		public static double[] ToDouble(int[] m)
		{
			double[] r = new double[m.Length];
			for (int i = 0; i < m.Length; i++)
				r[i] = (double)m[i];
			return r;
		}

		public static int[] ToInt(double[] m)
		{
			int[] r = new int[m.Length];
			for (int i = 0; i < m.Length; i++)
				r[i] = (int)Math.Round(m[i]);
			return r;
		}

		public static int[] DoDct(int[] m)
		{
			double[] source = ToDouble(m);
			source = MatrixMultiply(Dct, source);
			source = MatrixMultiply(source, DctT);
			return ToInt(source);
		}

		public static int[] DoIdct(int[] m)
		{
			double[] source = ToDouble(m);
			source = MatrixMultiply(DctT, source);
			source = MatrixMultiply(source, Dct);
			return ToInt(source);
		}

#if false
		public static void Transpose (double[] m)
		{
			Debug.Assert (m != null);
			
			int d = (int)Math.Sqrt (m.Length);
			Debug.Assert (d * d == m.Length);
			
			int o1 = 0, ot = -1;
			for (int oy = 0; oy < d; oy++) {
				o1 += 1 + oy;
				ot += 1 + d;
				for (int o2 = ot; o2 < d*d; o2 += d)
					m.Swap(d1, d2);
			}
		}
#endif

	}

	#endregion

	#region BitReader

	public class BitReader
	: IDisposable
	{
		private int _cnt = 0;
		private uint _buf = 0;
		private Stream _stream;

		public BitReader(byte[] data) { _stream = new MemoryStream(data); }
		public BitReader(Stream stream) { _stream = stream; }

		private void EnsureData(int bitCount)
		{
			int todo = ((bitCount - _cnt) + 7) / 8;
			for (int i = 0; i < todo; i++)
				_buf = (_buf << 8) | (uint)_stream.ReadByte();
			_cnt += (todo > 0) ? todo * 8 : 0;
		}

		public int Peek(int bitCount)
		{
			EnsureData(bitCount);
			int mask = ((1 << _cnt) - 1) ^ ((1 << (_cnt - bitCount)) - 1);
			return (int)(_buf & mask) >> (_cnt - bitCount);
		}

		public int Read(int bitCount)
		{
			EnsureData(bitCount);
			int mask = ((1 << _cnt) - 1) ^ ((1 << (_cnt - bitCount)) - 1);
			int val = (int)(_buf & mask) >> (_cnt - bitCount);
			_cnt -= bitCount;
			return val;
		}

		public void Skip(int bitCount)
		{
			EnsureData(bitCount);
			_cnt -= bitCount;
		}

		#region IDisposable Members

		public void Dispose()
		{
			if (_stream == null)
				return;

			_stream.Dispose();
		}

		#endregion
	}

	#endregion

	#region Jpeg

	public class Jpeg
	{

		#region Variables

		private int _xres, _yres;
		private byte[][] _dqt = new byte[4][];
		private Huff[][] _dht = new Huff[4][];
		private byte[] _scandata;

		#endregion

		#region Properties

		public int Width { get { return _xres; } }
		public int Height { get { return _yres; } }

		#endregion

		#region File

		public Jpeg(string filename)
		{
			if (String.IsNullOrEmpty(filename))
				throw new ArgumentNullException("filename");

			using (var stream = File.OpenRead(filename))
			using (var reader = new BinaryReader(stream))
				ParseFile(reader);
		}

		private void ParseFile(BinaryReader reader)
		{
			while (reader.PeekChar() >= 0)
			{
				if (reader.ReadByte() != 0xff)
					throw new Exception("Cannot find next marker");

				switch (reader.ReadByte())
				{
					// SOF0
					case 0xc0: ParseSof0(reader); break;
					// DHT
					case 0xc4: ParseDht(reader); break;
					// SOI
					case 0xd8: break;
					// EOI
					case 0xd9: return;
					// SOS
					case 0xda: ParseSos(reader); break;
					// DQT
					case 0xdb: ParseDqt(reader); break;
					// APP0-APP15
					case 0xe0: ParseApp(reader, 0); break;
					case 0xe1: ParseApp(reader, 1); break;
					case 0xe2: ParseApp(reader, 2); break;
					case 0xe3: ParseApp(reader, 3); break;
					case 0xe4: ParseApp(reader, 4); break;
					case 0xe5: ParseApp(reader, 5); break;
					case 0xe6: ParseApp(reader, 6); break;
					case 0xe7: ParseApp(reader, 7); break;
					case 0xe8: ParseApp(reader, 8); break;
					case 0xe9: ParseApp(reader, 9); break;
					case 0xea: ParseApp(reader, 10); break;
					case 0xeb: ParseApp(reader, 11); break;
					case 0xec: ParseApp(reader, 12); break;
					case 0xed: ParseApp(reader, 13); break;
					case 0xee: ParseApp(reader, 14); break;
					case 0xef: ParseApp(reader, 15); break;

					default:
						throw new Exception("Unknown marker");
				}
			}
		}

		#region AppN

		private void ParseApp(BinaryReader reader, int app)
		{
			int length = reader.ReadInt16BE();
			reader.BaseStream.Seek(length - 2, SeekOrigin.Current);
		}

		#endregion

		#region SofN

		private void ParseSof0(BinaryReader reader)
		{
			int length = reader.ReadInt16BE();
			if (reader.ReadByte() != 8) throw new NotImplementedException();

			_yres = reader.ReadInt16BE();
			_xres = reader.ReadInt16BE();
			if (reader.ReadByte() != 3) throw new NotImplementedException();

			for (int i = 0; i < 3; i++)
			{
				reader.ReadByte();
				int samp = reader.ReadByte();
				if (i == 0 && samp != 0x22) throw new NotImplementedException();
				if (i != 0 && samp != 0x11) throw new NotImplementedException();
				reader.ReadByte();
			}
		}

		#endregion

		#region Dqt

		private readonly byte[] NaturalOrder = new byte[] {
			 0,  1,  8, 16,  9,  2,  3, 10,
			17, 24, 32, 25, 18, 11,  4,  5,
			12, 19, 26, 33, 40, 48, 41, 34,
			27, 20, 13,  6,  7, 14, 21, 28,
			35, 42, 49, 56, 57, 50, 43, 36,
			29, 22, 15, 23, 30, 37, 44, 51,
			58, 59, 52, 45, 38, 31, 39, 46,
			53, 60, 61, 54, 47, 55, 62, 63,
		};

		private void ParseDqt(BinaryReader reader)
		{
			reader.BaseStream.Seek(2, SeekOrigin.Current);
			byte ident = reader.ReadByte();

			if ((ident & 0xf0) >> 4 != 0)
				throw new NotImplementedException();

			if (_dqt[ident] == null)
				_dqt[ident] = new byte[64];

			for (int i = 0; i < 64; i++)
				_dqt[ident][NaturalOrder[i]] = reader.ReadByte();
		}

		#endregion

		#region Dht

		private struct Huff { public int b, v; }

		private void ParseDht(BinaryReader reader)
		{
			int length = reader.ReadInt16BE();
			int select = reader.ReadByte();
			select = ((select & 0xf0) >> 3) | (select & 0x0f);

			if (_dht[select] == null)
				_dht[select] = new Huff[0x10000];

			byte[] lens = reader.ReadBytes(16);

			int code = 0;
			for (int i = 1; i <= 16; i++)
			{
				for (int j = 0; j < lens[i - 1]; j++)
				{
					byte val = reader.ReadByte();

					// Fill table
					int x = 16 - i;
					int lo = code << x;
					int hi = code << x | ((1 << x) - 1);
					for (int k = lo; k <= hi; k++)
						_dht[select][k] = new Huff() { b = i, v = val };

					code += 1;
				}
				code <<= 1;
			}
		}

		#endregion

		#region Sos

		private void ParseSos(BinaryReader reader)
		{
			int length = reader.ReadInt16BE();
			if (reader.ReadByte() != 3) throw new NotImplementedException();

			for (int i = 0; i < 3; i++)
			{
				reader.ReadByte();
				int samp = reader.ReadByte();
				if (i == 0 && samp != 0x00) throw new NotImplementedException();
				if (i != 0 && samp != 0x11) throw new NotImplementedException();
			}

			if (reader.ReadByte() != 0) throw new NotImplementedException();
			if (reader.ReadByte() != 63) throw new NotImplementedException();
			reader.ReadByte();

			ReadScanData(reader);
		}

		#endregion

		#endregion

		#region Scan

		private void ReadScanData(BinaryReader reader)
		{
			List<byte> scandata = new List<byte>(1024);
			while (true)
			{
				byte b = reader.ReadByte();

				if (b == 0xff)
				{
					if (reader.ReadByte() == 0x00)
						scandata.Add(0xff);
					else
					{
						reader.BaseStream.Seek(-2, SeekOrigin.Current);
						break;
					}
				}
				else scandata.Add(b);
			}

			// Add two padding bytes for huffman decoding
			scandata.Add(0);
			scandata.Add(0);

			_scandata = scandata.ToArray();
		}

		public byte Clamp(int n)
		{
			if (n < 0x00) return 0x00;
			if (n > 0xff) return 0xff;
			return (byte)n;
		}

		private static void ScaleX2(byte[] src, byte[] dst, int w, int h)
		{
			Debug.Assert(src.Length == w * h);
			Debug.Assert(dst.Length == w * h * 4);

			int sl = src.Length;
			int w2 = w * 2;

			for (int sy = 0, dy = 0; sy < sl; sy += w, dy += w2)
				for (int sx = 0, dx = 0; sx < sy + w; sx++, dx += 2)
					dst[dx + 0] =
					dst[dx + 2] =
					dst[dx + w2 + 0] =
					dst[dx + w2 + 2] = src[sx];
		}

		const int BPP = 3;
		public void YCbCrToRgb(int[][] src, byte[] dst, int mx, int my, int stride)
		{
			int blk = (my * 16) * stride + (mx * 16 * BPP);
			for (int bl = 0; bl < 4; bl++)
			{
				int ro = (bl & 2) != 0 ? 8 : 0;
				int co = (bl & 1) != 0 ? 8 : 0;

				for (int y = 0, yy = 0, cy = 0; y < 8; y++, yy += 8, cy += 4)
				{
					int off = blk + ((ro + y) * stride) + (co * BPP);
					for (int x = 0; x < 8; x++)
					{
						int ty = src[bl][yy + x] + 128;
						int tcb = src[5][cy + (x / 2)];
						int tcr = src[4][cy + (x / 2)];
						int r = (int)Math.Round(ty + (1.402 * tcr));
						int g = (int)Math.Round(ty - (0.344 * tcb) - (0.714 * tcr));
						int b = (int)Math.Round(ty + (1.772 * tcb));
						dst[off++] = Clamp(r);
						dst[off++] = Clamp(g);
						dst[off++] = Clamp(b);
					}
				}
			}
		}

		public void YCbCrToRgb(byte[] img, int off, int y, int cb, int cr)
		{
			int r = (int)(y + (1.402 * cr));
			int g = (int)(y - (0.344 * cb) - (0.714 * cr));
			int b = (int)(y + (1.772 * cb));

			img[off + 0] = Clamp(b);
			img[off + 1] = Clamp(g);
			img[off + 2] = Clamp(r);
		}

		public byte[] DecodeScan()
		{
			// Calculate the size of the image in mcu's
			int xMcu = (_xres + 15) / 16;
			int yMcu = (_yres + 15) / 16;
			int mcu = xMcu * yMcu;

			int stride = ((xMcu * 16 * BPP) + 3) & ~0x03;
			byte[] img = new byte[yMcu * 16 * stride];
			using (MemoryStream ms = new MemoryStream(_scandata))
			{
				BitReader reader = new BitReader(ms);

				int dY = 0, dCb = 0, dCr = 0;
				int[][] block = new int[6][];
				for (int y = 0; y < yMcu; y++)
					for (int x = 0; x < xMcu; x++)
					{
						for (int i = 0; i < 4; i++)
						{
							block[i] = DecodeBlock(reader, false);
							block[i][0] += dY; dY = block[i][0];
							DequantBlock(block[i], false);
						}

						block[4] = DecodeBlock(reader, true);
						block[4][0] += dCb; dCb = block[4][0];
						DequantBlock(block[4], true);

						block[5] = DecodeBlock(reader, true);
						block[5][0] += dCr; dCr = block[5][0];
						DequantBlock(block[5], true);

						for (int i = 0; i < 6; i++)
							block[i] = DCT.DoIdct(block[i]);

						YCbCrToRgb(block, img, x, y, stride);
					}
			}

			return img;
		}

		private void DequantBlock(int[] block, bool chroma)
		{
			byte[] dqt = _dqt[chroma ? 1 : 0];

			for (int i = 0; i < 8 * 8; i++)
				block[i] *= dqt[i];
		}

		private static int DecodeNumber(int num, int bits)
		{
			return num < (1 << (bits - 1))
				? num - ((1 << bits) - 1)
				: num;
		}

		private int[] DecodeBlock(BitReader reader, bool chroma)
		{
			Huff h;
			int tab = chroma ? 1 : 0;

			var result = new int[64];

			// Read DC value
			h = _dht[0 + tab][reader.Peek(16)];
			reader.Skip(h.b);
			result[0] = DecodeNumber(reader.Read(h.v), h.v);

			// Read AC values
			for (int i = 1; i < 64; i++)
			{
				h = _dht[2 + tab][reader.Peek(16)];
				reader.Skip(h.b);

				switch (h.v)
				{
					case 0x00: i = 63; break;
					case 0xf0: i += 16; continue;
					default:
						i += (h.v & 0xf0) >> 4;
						result[NaturalOrder[i]] = DecodeNumber(reader.Read(h.v & 0x0f), h.v & 0x0f);
						break;
				}
			}

			return result;
		}

		#endregion

	}

	#endregion

	#region Program

	static class Program
	{
		static void Usage()
		{
			Console.WriteLine("jpegdecode <in.jpg> <out.png>");
			Console.WriteLine(" - probably crashes");
		}

		static void Main(string[] args)
		{
			if (args.Length != 2)
			{
				Usage();
				return;
			}

			var jpeg = new Jpeg(args[0]);
			byte[] img = jpeg.DecodeScan();
			var bmp = new Bitmap((jpeg.Width + 15) & ~0xf, (jpeg.Height + 15) & ~0xf, PixelFormat.Format24bppRgb);
			var dat = bmp.LockBits(new Rectangle(Point.Empty, bmp.Size), ImageLockMode.ReadWrite, PixelFormat.Format24bppRgb);
			Marshal.Copy(img, 0, dat.Scan0, img.Length);
			bmp.UnlockBits(dat);
			bmp.Save(args[1], ImageFormat.Png);
		}
	}

	#endregion

}

Hi Benedict, this was just something I did when I was bored to learn how jpeg works. If you want to decode jpeg in .net I suggest taking a look at system.drawing.image.

I just done the code in compression using this code:
try
{
//Create an ImageCodecInfo-object for the codec information
ImageCodecInfo jpegCodec = null;

            //Set quality factor for compression
            EncoderParameter imageQualitysParameter = new EncoderParameter(System.Drawing.Imaging.Encoder.Quality, imageQuality);

            //List all avaible codecs (system wide)
            ImageCodecInfo[] alleCodecs = ImageCodecInfo.GetImageEncoders();

            EncoderParameters codecParameter = new EncoderParameters(1);
            codecParameter.Param[0] = imageQualitysParameter;

            //Find and choose JPEG codec
            for (int i = 0; i < alleCodecs.Length; i++)
            {
                if (alleCodecs[i].MimeType == "image/jpeg")
                {
                    jpegCodec = alleCodecs[i];
                    break;
                }
            }

            //Save compressed image
            sourceImage.Save(savePath, jpegCodec, codecParameter);
            return;
        }
        catch (Exception e)
        {
            throw e;
        }

My problem is I need to decompress it. can you help me I am stuck searching in google.

Also how can I use your code to decompress my compressed image. Can your code help me decompress my compress image with this codes?