转载: http://www.codeproject.com/Answers/228635/how-to-convert-32bpp-into-8bpp-bitmap#answer1
public static Bitmap ColorToGrayscale(Bitmap bmp) { int w = bmp.Width, h = bmp.Height, r, ic, oc, bmpStride, outputStride, bytesPerPixel; PixelFormat pfIn = bmp.PixelFormat; ColorPalette palette; Bitmap output; BitmapData bmpData, outputData; //Create the new bitmap output = new Bitmap(w, h, PixelFormat.Format8bppIndexed); //Build a grayscale color Palette palette = output.Palette; for (int i = 0; i < 256; i++) { Color tmp = Color.FromArgb(255, i, i, i); palette.Entries[ i ] = Color.FromArgb(255, i, i, i); } output.Palette = palette; //No need to convert formats if already in 8 bit if (pfIn == PixelFormat.Format8bppIndexed) { output = (Bitmap)bmp.Clone(); //Make sure the palette is a grayscale palette and not some other //8-bit indexed palette output.Palette = palette; return output; } //Get the number of bytes per pixel switch (pfIn) { case PixelFormat.Format24bppRgb: bytesPerPixel = 3; break; case PixelFormat.Format32bppArgb: bytesPerPixel = 4; break; case PixelFormat.Format32bppRgb: bytesPerPixel = 4; break; default: throw new InvalidOperationException("Image format not supported"); } //Lock the images bmpData = bmp.LockBits(new Rectangle(0, 0, w, h), ImageLockMode.ReadOnly, pfIn); outputData = output.LockBits(new Rectangle(0, 0, w, h), ImageLockMode.WriteOnly, PixelFormat.Format8bppIndexed); bmpStride = bmpData.Stride; outputStride = outputData.Stride; //Traverse each pixel of the image unsafe { byte* bmpPtr = (byte*)bmpData.Scan0.ToPointer(), outputPtr = (byte*)outputData.Scan0.ToPointer(); if (bytesPerPixel == 3) { //Convert the pixel to it's luminance using the formula: // L = .299*R + .587*G + .114*B //Note that ic is the input column and oc is the output column for (r = 0; r < h; r++) for (ic = oc = 0; oc < w; ic += 3, ++oc) outputPtr[r * outputStride + oc] = (byte)(int) (0.299f * bmpPtr[r * bmpStride + ic] + 0.587f * bmpPtr[r * bmpStride + ic + 1] + 0.114f * bmpPtr[r * bmpStride + ic + 2]); } else //bytesPerPixel == 4 { //Convert the pixel to it's luminance using the formula: // L = alpha * (.299*R + .587*G + .114*B) //Note that ic is the input column and oc is the output column for (r = 0; r < h; r++) for (ic = oc = 0; oc < w; ic += 4, ++oc) outputPtr[r * outputStride + oc] = (byte)(int) ((bmpPtr[r * bmpStride + ic] / 255.0f) * (0.299f * bmpPtr[r * bmpStride + ic + 1] + 0.587f * bmpPtr[r * bmpStride + ic + 2] + 0.114f * bmpPtr[r * bmpStride + ic + 3])); } } //Unlock the images bmp.UnlockBits(bmpData); output.UnlockBits(outputData); return output; }