将bitmap文件从32bpp 转化为8bpp

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;

}