在.net下，如果你加载了一副8位的灰度图像，然后想向其中绘制一些线条、或者填充一些矩形、椭圆等，都需要通过Grahpics.FromImage创建Grahphics对象，而此时会出现：无法从带有索引像素格式的图像创建graphics对象 这个错误，让我们的后续工作无法完成。本文叙述了一种另外的方法来实现它。
我们通过Reflector发编译.net framework的相关函数后发现，FromImage的实现过程如下：
public static Graphics FromImage(Image image) { if (image == null) { throw new ArgumentNullException("image"); } if ((image.PixelFormat & PixelFormat.Indexed) != PixelFormat.Undefined) { throw new Exception(SR.GetString("GdiplusCannotCreateGraphicsFromIndexedPixelFormat")); } IntPtr zero = IntPtr.Zero; int status = SafeNativeMethods.Gdip.GdipGetImageGraphicsContext(new HandleRef(image, image.nativeImage), out zero); if (status != 0) { throw SafeNativeMethods.Gdip.StatusException(status); } return new Graphics(zero) { backingImage = image }; }
而在MSDN中，对GdipGetImageGraphicsContext函数的描述有如下部分：
This constructor also fails if the image uses one of the following pixel formats:
- PixelFormatUndefined
- PixelFormatDontCare
- PixelFormat1bppIndexed
- PixelFormat4bppIndexed
- PixelFormat8bppIndexed
- PixelFormat16bppGrayScale
- PixelFormat16bppARGB1555
因此，.net是判断当图像为索引模式时，直接返回错误，而不是通过判断GdipGetImageGraphicsContext的返回值来实现的。
针对这个事实，我们其实觉得也无可厚非，Graphics对象是用来干什么的，是用来向对应的Image中添加线条，路径、实体图形、图像数据等的，而普通的索引图像，其矩阵的内容并不是实际的颜色值，而只是个索引，真正的颜色值在调色板中，因此，一些绘制的过程用在索引图像上存在着众多的不适。
但是有个特列，那就是灰度图像，严格的说，灰度图像完全符合索引图像的格式，可以认为是索引图像的一种特例。但是我也可以认为他不属于索引图像一类：即他的图像数据总的值可以认为就是其颜色值，我们可以抛开其调色板中的数据。所以在photoshop中把索引模式和灰度模式作为两个模式来对待。
真是有这个特殊性，一些画线、填充路径等等的过程应该可以在灰度图像中予以实现，单GDI+为了规避过多的判断，未对该模式进行特殊处理。
但是，在一些特殊的场合，对灰度进行上述操作很有用途和意义。比如：在高级的图像设计中，有着选区的概念，而选区的实质上就是一副灰度图像，如果我们创建一个椭圆选区，设计上就是在灰度图像上填充了一个椭圆。如果能借助GDI+提供的优质的抗锯齿填充模式加上丰富自由的填充函数，那么就可以创建出多种多样的选区了。可.net的一个无法创建Graphics让我们此路不通。
有没有办法呢，其实也是有的，熟悉GDI+平板化API的人还知道有GdipCreateFromHDC函数，该函数可以从HDC中创建Graphics。因此我的想法就是利用GDI的方式创建位图对象吗，然后从GDI的HDC中创建对应的Graphics。经过实践，这种方法是可以行的。
为此，我用GDI结合GDI+的方式创建了一个GrayBitmap类，该类的主要代码如下：
unsafe class GrayBitmap { #region GDIAPI private const int DIB_RGB_COLORS = 0; private const int BI_RGB = 0; [StructLayout(LayoutKind.Sequential, Pack = 1)] private struct RGBQUAD { internal byte Blue; internal byte Green; internal byte Red; internal byte Reserved; } [StructLayout(LayoutKind.Sequential, Pack = 1)] private struct BITMAPINFOHEADER { internal uint Size; internal int Width; internal int Height; internal ushort Planes; internal ushort BitCount; internal uint Compression; internal uint SizeImage; internal int XPelsPerMeter; internal int YPelsPerMeter; internal uint ClrUsed; internal uint ClrImportant; } [StructLayout(LayoutKind.Sequential, Pack = 1)] private struct BITMAPINFO { internal BITMAPINFOHEADER Header; [MarshalAs(UnmanagedType.ByValArray, SizeConst = 256)] internal RGBQUAD[] Palette; } [StructLayout(LayoutKind.Sequential)] internal struct LOGPALETTE { internal ushort PalVersion; internal ushort PalNumEntries; [MarshalAs(UnmanagedType.ByValArray, SizeConst = 0x400)] internal byte[] PalPalEntry; } [DllImport("User32.dll", SetLastError = true)] private extern static IntPtr GetDC(IntPtr Hwnd); [DllImport("User32.dll", SetLastError = true)] private extern static int ReleaseDC(IntPtr Hwnd, IntPtr Hdc); [DllImport("Gdi32.dll", SetLastError = true)] private extern static IntPtr CreateCompatibleDC(IntPtr Hdc); [DllImport("Gdi32.dll", SetLastError = true)] private static extern uint SetDIBColorTable(IntPtr Hdc, int un1, int un2, RGBQUAD[] pcRGBQUAD); [DllImport("Gdi32.dll", SetLastError = true)] private static extern IntPtr CreateDIBSection(IntPtr Hdc, ref BITMAPINFO BmpInfo, uint iUsage, out byte* ppvBits, IntPtr hSection, uint dwOffset); [DllImport("Gdi32.dll", SetLastError = true)] private extern static Boolean DeleteDC(IntPtr Hdc); [DllImport("Gdi32.dll", SetLastError = true)] private extern static IntPtr SelectObject(IntPtr Hdc, IntPtr Object); [DllImport("Gdi32.dll", SetLastError = true)] private static extern bool DeleteObject(IntPtr Object); #endregion #region PrivateVariable private int m_Width = 0; private int m_Height = 0; private int m_Stride = 0; private IntPtr m_Hdc = IntPtr.Zero; private Graphics m_Graphics = null; private IntPtr m_Handle = IntPtr.Zero; private byte* m_Pointer = null; private Bitmap m_Bitmap = null; private bool Disposed = false; #endregion #region Property public int Width { get { return m_Width; } } public int Height { get { return m_Height; } } public int Stride { get { return m_Stride; } } public IntPtr Handle { get { return m_Handle; } } public IntPtr Hdc { get { return m_Hdc; } } public Graphics Graphics { get { return m_Graphics; } } public byte* Pointer { get { return m_Pointer; } } public Bitmap Bitmap { get { return m_Bitmap; } } #endregion #region Constructor public GrayBitmap(int Width, int Height) { AllocateBitmap(Width, Height); } public GrayBitmap(string FileName) { Bitmap Bmp = (Bitmap)Bitmap.FromFile(FileName); if (IsGrayBitmap(Bmp) == false) { Bmp.Dispose(); throw new Exception("Wrong PixelFormat"); } else { AllocateBitmap(Bmp.Width, Bmp.Height); BitmapData BmpData = new BitmapData(); BmpData.Scan0 = (IntPtr)m_Pointer; BmpData.Stride = m_Stride; // 把Image对象的数据拷贝到DIBSECITON中去 Bmp.LockBits(new Rectangle(0, 0, Bmp.Width, Bmp.Height), ImageLockMode.ReadWrite | ImageLockMode.UserInputBuffer, Bmp.PixelFormat, BmpData); Bmp.UnlockBits(BmpData); Bmp.Dispose(); } } public GrayBitmap(Bitmap Bmp) { if (IsGrayBitmap(Bmp) == false) throw new Exception("Wrong PixelFormat"); else { AllocateBitmap(Bmp.Width, Bmp.Height); BitmapData BmpData = new BitmapData(); BmpData.Scan0 = (IntPtr)m_Pointer; BmpData.Stride = m_Stride; // 把Image对象的数据拷贝到DIBSECITON中去 Bmp.LockBits(new Rectangle(0, 0, Bmp.Width, Bmp.Height), ImageLockMode.ReadWrite | ImageLockMode.UserInputBuffer, Bmp.PixelFormat, BmpData); Bmp.UnlockBits(BmpData); } } ~GrayBitmap() { Dispose(); } #endregion #region PublicMethod public static GrayBitmap FromFile(string FileName) { GrayBitmap Bmp = new GrayBitmap(FileName); return Bmp; } public void Dispose() { Dispose(true); } protected void Dispose(bool Suppress = true) { if (Disposed == false) { Disposed = true; if (m_Hdc != IntPtr.Zero) DeleteDC(m_Hdc); m_Hdc = IntPtr.Zero; if (m_Graphics != null) m_Graphics.Dispose(); m_Graphics = null; if (m_Bitmap != null) m_Bitmap.Dispose(); m_Bitmap = null; if (m_Handle != IntPtr.Zero) DeleteObject(m_Handle); m_Handle = IntPtr.Zero; m_Width = 0; m_Height = 0; m_Stride = 0; m_Pointer = null; if (Suppress == true) GC.SuppressFinalize(this); } } #endregion #region PrivateMethod private void AllocateBitmap(int Width, int Height) { if (Width <= 0) throw new ArgumentOutOfRangeException("Width", Width, "Width must be >=0"); if (Height <= 0) throw new ArgumentOutOfRangeException("Height", Height, "Height must be >=0"); BITMAPINFO BmpInfo = new BITMAPINFO(); BmpInfo.Header.Size = (uint)sizeof(BITMAPINFOHEADER); BmpInfo.Header.Width = Width; BmpInfo.Header.Height = -Height; // 为了和GDI对象的坐标系统（起点坐标在左上角），建立一个倒序的DIB BmpInfo.Header.BitCount = (ushort)8; ; BmpInfo.Header.Planes = 1; BmpInfo.Header.Compression = BI_RGB; // 创建DIBSection必须用不压缩的格式 BmpInfo.Header.XPelsPerMeter = 0; // CreateDIBSection does not use the BITMAPINFOHEADER parameters biXPelsPerMeter or biYPelsPerMeter and will not provide resolution information in the BITMAPINFO structure. BmpInfo.Header.YPelsPerMeter = 0; BmpInfo.Header.ClrUsed = 0; BmpInfo.Header.SizeImage = 0; BmpInfo.Header.ClrImportant = 0; BmpInfo.Header.SizeImage = 0; BmpInfo.Palette = new RGBQUAD[256]; for (int X = 0; X <256 ; X++) // for (byte X=0;X<=255;X++) 用这个代码试试，呵呵 { BmpInfo.Palette[X].Red = (byte)X; BmpInfo.Palette[X].Green = (byte)X; BmpInfo.Palette[X].Blue = (byte)X; BmpInfo.Palette[X].Reserved = 255; } IntPtr ScreecDC = GetDC(IntPtr.Zero); m_Hdc = CreateCompatibleDC(ScreecDC); ReleaseDC(IntPtr.Zero, ScreecDC); m_Handle = CreateDIBSection(Hdc, ref BmpInfo, DIB_RGB_COLORS, out m_Pointer, IntPtr.Zero, 0); if (m_Handle == IntPtr.Zero) { DeleteDC(m_Hdc); m_Hdc = IntPtr.Zero; throw new OutOfMemoryException("CreateDIBSection function failed,this may be caused by user input too large size of image."); } else { SelectObject(m_Hdc, m_Handle); SetDIBColorTable(m_Hdc, 0, 256, BmpInfo.Palette); m_Width = Width; m_Height = Height; m_Stride = (int)((m_Width + 3) & 0XFFFFFFFC); m_Graphics = Graphics.FromHdc(m_Hdc); m_Bitmap = new Bitmap(m_Width, m_Height, m_Stride, PixelFormat.Format8bppIndexed, (IntPtr)m_Pointer); ColorPalette Pal = m_Bitmap.Palette; for (int X = 0; X < 256; X++) Pal.Entries[X] = Color.FromArgb(255, X, X, X); // 设置灰度图像的调色板 m_Bitmap.Palette = Pal; } } private bool IsGrayBitmap(Bitmap Bmp) { bool IsGray; if (Bmp.PixelFormat == PixelFormat.Format8bppIndexed) { IsGray = true; if (Bmp.Palette.Entries.Length != 256) IsGray = false; else { for (int X = 0; X < Bmp.Palette.Entries.Length; X++) { if (Bmp.Palette.Entries[X].R != Bmp.Palette.Entries[X].G || Bmp.Palette.Entries[X].R != Bmp.Palette.Entries[X].B || Bmp.Palette.Entries[X].B != Bmp.Palette.Entries[X].G) { IsGray = false; break; } } } } else { IsGray = false; } return IsGray; } #endregion }
正如上面所述，我们用GDI的方式（CreateDIBSection）创建灰度图像，然后从HDC中创建Graphics，从而可以顺利的调用Graphics的任何绘制函数了。
比如填充椭圆：
SolidBrush SB = new SolidBrush(Color.FromArgb(255, 255, 255, 255)); Bmp.Graphics.SmoothingMode = System.Drawing.Drawing2D.SmoothingMode.AntiAlias; Bmp.Graphics.FillEllipse(SB, new Rectangle(100, 100, 200, 300)); SB.Dispose(); Canvas.Invalidate();
心细的朋友可以在测试中会发现，通过这种方式绘制的颜色可能和指定的颜色有所不同，比如上面我们要求绘制白色的椭圆，但是实际绘制的颜色是RGB(252,252,252)的，但是并不是所有的颜色都有误差，引起这个的原因估计还是GDI+的内部的一些机制上的问题吧。
工程完整代码：http://files.cnblogs.com/Imageshop/GrayModeBitmap.rar
希望朋友们喜欢我的文章。
***************************作者： laviewpbt 时间： 2013.7.13 联系QQ: 33184777 转载请保留本行信息*************************