GDI+ 彩色到黑白

彩色图转换为灰度图公式很简单：
Y=0.3RED+0.59GREEN+0.11 Blue

用GDI+实现的方式由两种：
1. 直接用上述公式修改象素点
2. 用ColorMatrix。

下面是用ColorMatrix实现示例：

using namespace Gdiplus;
    Image img(wszFileName);
    Graphics graphics(GetDC()->GetSafeHdc());
    ColorMatrix cm= {0.3f, 0.3f, 0.3f, 0, 0,
        0.59f,0.59f,0.59f,0, 0,
        0.11f,0.11f,0.11f,0, 0,
        0,    0,    0,    1, 0,
        0,    0,    0,    0, 1};
    ImageAttributes ia;
    ia.SetColorMatrix(&cm);

    float x = (float)img.GetWidth();
    float y = (float)img.GetHeight();
    graphics.DrawImage(&img, 
        RectF(0.0f,0.0f,x,y,
        0.0f,0.0f,x,y, 
        UnitPixel,
        &ia);

遍历像素点，修改颜色，貌似比上面方法快

public static bool Invert0(Bitmap b)
{
BitmapData bmData = b.LockBits(new Rectangle(0, 0, b.Width, b.Height), ImageLockMode.ReadWrite, PixelFormat.Format24bppRgb);
int stride = bmData.Stride;
System.IntPtr Scan0 = bmData.Scan0;
unsafe
{
byte * p = (byte *)(void *)Scan0;
int nOffset = stride - b.Width*3;
int nWidth = b.Width * 3;
int h=b.Height;

for(int y=0;y<h;++y)
{
for(int x=0; x < nWidth; ++x )
{
p[0] = (byte)(255-p[0]);
++p;
}
p += nOffset;
}
}
b.UnlockBits(bmData);
return true;
}