缩略图生成算法

抄的，避免GetThumbNail GIF透明信息丢失的问题，

private unsafe Bitmap GetThumbnail(Bitmap src, int width, int height)
            {
                // preserve source-image aspect ratio 

                float src_t = (float) src.Height/(float) src.Width;

                if (src_t > ((float) height/(float) width))
                    width = (int) (height/src_t);
                else
                    height = (int) (width*src_t);

                // end of preserve source-image aspect ratio 


                if (src.PixelFormat == PixelFormat.Format8bppIndexed)
                {
                    // do it yourself 

                    Bitmap dst = new Bitmap(width, height, src.PixelFormat);
                    dst.Palette = src.Palette;

                    BitmapData dstbd =
                        dst.LockBits
                            (
                            new Rectangle(0, 0, dst.Width, dst.Height),
                            ImageLockMode.WriteOnly,
                            dst.PixelFormat
                            );
                    try
                    {
                        BitmapData srcbd =
                            src.LockBits
                                (
                                new Rectangle(0, 0, src.Width, src.Height),
                                ImageLockMode.ReadOnly,
                                src.PixelFormat
                                );
                        try
                        {
                            byte* srcp;
                            byte* dstp;

                            float m = (float) src.Width/(float) dst.Width;


                            for (int r = 0; r < dst.Height; ++r)
                            {
                                dstp = (byte*) dstbd.Scan0;
                                dstp += dstbd.Stride*r;

                                for (int c = 0; c < dst.Width; ++c)
                                {
                                    srcp = (byte*) srcbd.Scan0;
                                    srcp += srcbd.Stride*(int) (r*m);
                                    srcp += (int) (c*m);

                                    *dstp++ = *srcp;
                                }
                            }

                        }
                        finally
                        {
                            src.UnlockBits(srcbd);
                        }
                    }
                    finally
                    {
                        dst.UnlockBits(dstbd);
                    }

                    return dst;
                }
                else
                {
                    // rely on microsoft guy 

                    return (Bitmap) src.GetThumbnailImage(width, height, null, IntPtr.Zero);
                }
            }