C#中保存GIF文件后透明背景问题的一个解决方法

　　以前在用C#做网站保存缩略图的程序中发现，当保存为GIF文件类型时，原来的透明背景变成了黑色，当时由于赶时间，就统一用白色代替了背景，并用Jpeg格式存储，并没有深究。

　　近来在网上查阅了许多资料，看到了两种解决方法：一种是在显示时设置透明背景色，GIF文件本身并不改变，另一种是不推荐使用的调用API的方法。将后一种I的VB源码用C#重写后，发现其中的调色板设置太少，转换效果不理想。

　　重新到网上搜索关于调色板的资料，发现MSDN上的一篇《对 ASP.NET 图像的颜色量化(Quantization)进行优化》的文章。

http://www.microsoft.com/china/MSDN/library/archives/library/DNAspp/html/colorquant.asp

　　将其中基于调色板量化的代码分离出来，透明背景色的图片保存成功。

　　完整代码如下：

using System;

using System.Collections;

using System.Drawing;

using System.Drawing.Imaging;

using System.Runtime.InteropServices;

namespace WindwoodGif

{

public class GifPalette

{

private static ArrayList _cardPalette;

private Color[] _colors;

private Hashtable _colorMap;

public GifPalette(ArrayList palette)

{

_colorMap = new Hashtable();

_colors = new Color[palette.Count];

palette.CopyTo(_colors);

}

public GifPalette()

{

ArrayList palette = SetPalette();

_colorMap = new Hashtable();

_colors = new Color[palette.Count];

palette.CopyTo(_colors);

}

public Bitmap Quantize(Image source)

{

int height = source.Height;

int width = source.Width;

Rectangle bounds = new Rectangle(0, 0, width, height);

Bitmap copy = new Bitmap(width, height, PixelFormat.Format32bppArgb);

Bitmap output = new Bitmap(width, height, PixelFormat.Format8bppIndexed);

using (Graphics g = Graphics.FromImage(copy))

{

g.PageUnit = GraphicsUnit.Pixel;

g.DrawImageUnscaled(source, bounds);

}

BitmapData sourceData = null;

try

{

sourceData = copy.LockBits(bounds, ImageLockMode.ReadOnly, PixelFormat.Format32bppArgb);

output.Palette = this.GetPalette(output.Palette);

SecondPass(sourceData, output, width, height, bounds);

}

finally

{

copy.UnlockBits(sourceData);

}

return output;

}

private ColorPalette GetPalette(ColorPalette palette)

{

for (int index = 0; index < _colors.Length; index++)

palette.Entries[index] = _colors[index];

return palette;

}

private unsafe void SecondPass(BitmapData sourceData, Bitmap output, int width, int height, Rectangle bounds)

{

BitmapData outputData = null;

try

{

outputData = output.LockBits(bounds, ImageLockMode.WriteOnly, PixelFormat.Format8bppIndexed);

byte* pSourceRow = (byte*)sourceData.Scan0.ToPointer();

Int32* pSourcePixel = (Int32*)pSourceRow;

Int32* pPreviousPixel = pSourcePixel;

byte* pDestinationRow = (byte*)outputData.Scan0.ToPointer();

byte* pDestinationPixel = pDestinationRow;

byte pixelValue = QuantizePixel((Color32*)pSourcePixel);

*pDestinationPixel = pixelValue;

for (int row = 0; row < height; row++)

{

pSourcePixel = (Int32*)pSourceRow;

pDestinationPixel = pDestinationRow;

for (int col = 0; col < width; col++, pSourcePixel++, pDestinationPixel++)

{

if (*pPreviousPixel != *pSourcePixel)

{

pixelValue = QuantizePixel((Color32*)pSourcePixel);

pPreviousPixel = pSourcePixel;

}

*pDestinationPixel = pixelValue;

}

pSourceRow += sourceData.Stride;

pDestinationRow += outputData.Stride;

}

finally

{

output.UnlockBits(outputData);

}

private unsafe byte QuantizePixel(Color32* pixel)

{

byte colorIndex = 0;

int colorHash = pixel->ARGB;

if (_colorMap.ContainsKey(colorHash))

colorIndex = (byte)_colorMap[colorHash];

else

{

if (0 == pixel->Alpha)

{

for (int index = 0; index < _colors.Length; index++)

{

if (0 == _colors[index].A)

{

colorIndex = (byte)index;

break;

}

else

{

int leastDistance = int.MaxValue;

int red = pixel->Red;

int green = pixel->Green;

int blue = pixel->Blue;

for (int index = 0; index < _colors.Length; index++)

{

Color paletteColor = _colors[index];

int redDistance = paletteColor.R - red;

int greenDistance = paletteColor.G - green;

int blueDistance = paletteColor.B - blue;

int distance = (redDistance * redDistance) +

(greenDistance * greenDistance) +

(blueDistance * blueDistance);

if (distance < leastDistance)

{

colorIndex = (byte)index;

leastDistance = distance;

if (0 == distance)

break;

}

_colorMap.Add(colorHash, colorIndex);

}

return colorIndex;

}

[StructLayout(LayoutKind.Explicit)]

public struct Color32

{

[FieldOffset(0)]

public byte Blue;

[FieldOffset(1)]

public byte Green;

[FieldOffset(2)]

public byte Red;

[FieldOffset(3)]

public byte Alpha;

[FieldOffset(0)]

public int ARGB;

public Color Color

{

get { return Color.FromArgb(Alpha, Red, Green, Blue); }

}

public static ArrayList SetPalette()

{

if (null == _cardPalette)

{

_cardPalette = new ArrayList();

Insert the colors into the arraylist

}

return _cardPalette;

}

　　调用方法：

　　先将该类添加到项目中，再在合适的地方调用。例：

Bitmap bitmap = new System.Drawing.Bitmap(width, height); // Image类也可

// ......(图形操作代码)

WindwoodGif.GifPalette gifPalette = new WindwoodGif.GifPalette();

bitmap = gifPalette.Quantize(bitmap);

bitmap.Save(SaveFileName, ImageFormat.Gif);

　　经测试，这种方法能够实现GIF文件的透明背景存储，在WinForm、WebForm均能使用。由于使用了标准256色调色板，内存开销可能较大，转换时间相对较慢，图像质量也有一定影响。此外，代码中使用了非安全代码（指针），在编译时项目属性中要设置允许不安全代码。

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