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glReadPixels

C#里声明glReadPixels的形式如下：

 1         /// <summary>
 2         /// Reads a block of pixels from the frame buffer.
 3         /// </summary>
 4         /// <param name="x">Top-Left X value.</param>
 5         /// <param name="y">Top-Left Y value.</param>
 6         /// <param name="width">Width of block to read.</param>
 7         /// <param name="height">Height of block to read.</param>
 8         /// <param name="format">Specifies the format of the pixel data. The following symbolic values are accepted: OpenGL.COLOR_INDEX, OpenGL.STENCIL_INDEX, OpenGL.DEPTH_COMPONENT, OpenGL.RED, OpenGL.GREEN, OpenGL.BLUE, OpenGL.ALPHA, OpenGL.RGB, OpenGL.RGBA, OpenGL.LUMINANCE and OpenGL.LUMINANCE_ALPHA.</param>
 9         /// <param name="type">Specifies the data type of the pixel data.Must be one of OpenGL.UNSIGNED_BYTE, OpenGL.BYTE, OpenGL.BITMAP, OpenGL.UNSIGNED_SHORT, OpenGL.SHORT, OpenGL.UNSIGNED_INT, OpenGL.INT or OpenGL.FLOAT.</param>
10         /// <param name="pixels">Storage for the pixel data received.</param>
11         [DllImport(Win32.OpenGL32, EntryPoint = "glReadPixels", SetLastError = true)]
12         public static extern void ReadPixels(int x, int y, int width, int height, uint format, uint type, IntPtr pixels);

这个函数的功能是：将指定范围(x, y, width, height)的像素值读入指定的内存处(pixels)。能把像素信息读到内存里，就可以保存到文件了。

其中(x, y)是以窗口左下角为原点(0, 0)的。而Windows窗口是以左上角为原点的。所以用的时候要注意转换一下。

数据结构

为方便起见，我先定义一个描述像素的数据结构。

 1     struct Pixel
 2     {
 3         public byte r;
 4         public byte g;
 5         public byte b;
 6         public byte a;
 7 
 8         public Pixel(byte r, byte g, byte b, byte a)
 9         {
10             this.r = r; this.g = g; this.b = b; this.a = a;
11         }
12 
13         public Color ToColor()
14         {
15             return Color.FromArgb(a, r, g, b);
16         }
17 
18         public override string ToString()
19         {
20             return string.Format("{0}, {1}, {2}, {3}", r, g, b, a);
21         }
22     }

struct Pixel

为了使用非托管数组，还需要用到 UnmanagedArray<T> 。关于这个类型详情见（C#+无unsafe的非托管大数组(large unmanaged array in c# without 'unsafe' keyword)）。

方法一：Bitmap.SetPixel()

最直接的方法是用Bitmap.SetPixel()来一个一个地指定图片的像素值。

 1         /// <summary>
 2         /// 把OpenGL渲染的内容保存到图片文件。
 3         /// </summary>
 4         /// <param name="x">左下角坐标为(0, 0)</param>
 5         /// <param name="y">左下角坐标为(0, 0)</param>
 6         /// <param name="width">宽度</param>
 7         /// <param name="height">高度</param>
 8         /// <param name="filename"></param>
 9         public static void Save2Picture(int x, int y, int width, int height, string filename)
10         {
11             var pdata = new UnmanagedArray<Pixel>(width * height);
12             GL.ReadPixels(x, y, width, height, GL.GL_RGBA, GL.GL_UNSIGNED_BYTE, pdata.Header);
13             var bitmap = new Bitmap(width, height);
14             int index = 0;
15             for (int j = height - 1; j >= 0; j--)
16             {
17                 for (int i = 0; i < width; i++)
18                 {
19                     Pixel v = pdata[index++];
20                     Color c = v.ToColor();
21                     bitmap.SetPixel(i, j, c);
22                 }
23             }
24 
25             bitmap.Save(filename);
26         }

方法二：Marshal.Copy

方法一用到的SetPixel()速度是很慢的。

先把读到的内容写入一个byte[]，然后再用Marshal.Copy()复制到Bitmap。这个方法的思路是（非托管数组->托管数组->bmpData.Scan0）

 1         /// <summary>
 2         /// 把OpenGL渲染的内容保存到图片文件。
 3         /// </summary>
 4         /// <param name="x">左下角坐标为(0, 0)</param>
 5         /// <param name="y">左下角坐标为(0, 0)</param>
 6         /// <param name="width">宽度</param>
 7         /// <param name="height">高度</param>
 8         /// <param name="filename"></param>
 9         public static void Save2Picture(int x, int y, int width, int height, string filename)
10         {
11             var pdata = new UnmanagedArray<Pixel>(width * height);
12             GL.ReadPixels(x, y, width, height, GL.GL_RGBA, GL.GL_UNSIGNED_BYTE, pdata.Header);
13             var format = System.Drawing.Imaging.PixelFormat.Format32bppArgb;
14             var lockMode = System.Drawing.Imaging.ImageLockMode.WriteOnly;
15             var bitmap = new Bitmap(width, height, format);
16             var bitmapRect = new Rectangle(0, 0, bitmap.Width, bitmap.Height);
17             System.Drawing.Imaging.BitmapData bmpData = bitmap.LockBits(bitmapRect, lockMode, format);
18             {
19                 int length = Math.Abs(bmpData.Stride) * bitmap.Height;
20                 byte[] bitmapBytes = new byte[length];
21                 int index = 0;
22                 for (int j = height - 1; j >= 0; j--)
23                 {
24                     for (int i = 0; i < width; i++)
25                     {
26                         Pixel v = pdata[index++];
27                         bitmapBytes[j * bmpData.Stride + i * 4 + 0] = v.b;
28                         bitmapBytes[j * bmpData.Stride + i * 4 + 1] = v.g;
29                         bitmapBytes[j * bmpData.Stride + i * 4 + 2] = v.r;
30                         bitmapBytes[j * bmpData.Stride + i * 4 + 3] = v.a;
31                     }
32                 }
33 
34                 System.Runtime.InteropServices.Marshal.Copy(bitmapBytes, 0, bmpData.Scan0, length);
35             }
36             bitmap.UnlockBits(bmpData);
37 
38             bitmap.Save(filename);
39         }

方法三：直接写入bmpData.Scan0

上一个方法里，通过托管数组byte[]进行过渡，是为了使用Marshal.Copy()这个method。但是我明明可以直接操作bmpData.Scan0啊，何必弄个byte[]。

 1         /// <summary>
 2         /// 把OpenGL渲染的内容保存到图片文件。
 3         /// </summary>
 4         /// <param name="x">左下角坐标为(0, 0)</param>
 5         /// <param name="y">左下角坐标为(0, 0)</param>
 6         /// <param name="width">宽度</param>
 7         /// <param name="height">高度</param>
 8         /// <param name="filename"></param>
 9         public static void Save2Picture(int x, int y, int width, int height, string filename)
10         {
11             var pdata = new UnmanagedArray<Pixel>(width * height);
12             GL.ReadPixels(x, y, width, height, GL.GL_RGBA, GL.GL_UNSIGNED_BYTE, pdata.Header);
13             var format = System.Drawing.Imaging.PixelFormat.Format32bppArgb;
14             var lockMode = System.Drawing.Imaging.ImageLockMode.WriteOnly;
15             var bitmap = new Bitmap(width, height, format);
16             Rectangle bitmapRect = new Rectangle(0, 0, bitmap.Width, bitmap.Height);
17             System.Drawing.Imaging.BitmapData bmpData = bitmap.LockBits(bitmapRect, lockMode, format);
18             unsafe
19             {
20                 var array = (byte*)bmpData.Scan0.ToPointer();
21                 int index = 0;
22                 for (int j = height - 1; j >= 0; j--)
23                 {
24                     for (int i = 0; i < width; i++)
25                     {
26                         Pixel v = pdata[index++];
27                         array[j * bmpData.Stride + i * 4 + 0] = v.b;
28                         array[j * bmpData.Stride + i * 4 + 1] = v.g;
29                         array[j * bmpData.Stride + i * 4 + 2] = v.r;
30                         array[j * bmpData.Stride + i * 4 + 3] = v.a;
31                     }
32                 }
33             }
34             bitmap.UnlockBits(bmpData);
35 
36             bitmap.Save(filename);
37         }

方法四：ReadPixels直接搞定

在上面的方法里，思路是（非托管数组->非托管数组）。

显然这个转换步骤也是多余的，直接让ReadPixels写入bmpData.Scan0的位置不就好了嘛。

 1         /// <summary>
 2         /// 把OpenGL渲染的内容保存到图片文件。
 3         /// </summary>
 4         /// <param name="x">左下角坐标为(0, 0)</param>
 5         /// <param name="y">左下角坐标为(0, 0)</param>
 6         /// <param name="width">宽度</param>
 7         /// <param name="height">高度</param>
 8         /// <param name="filename"></param>
 9         public static void Save2Picture(int x, int y, int width, int height, string filename)
10         {
11             var format = System.Drawing.Imaging.PixelFormat.Format32bppArgb;
12             var lockMode = System.Drawing.Imaging.ImageLockMode.WriteOnly;
13             var bitmap = new Bitmap(width, height, format);
14             var bitmapRect = new Rectangle(0, 0, bitmap.Width, bitmap.Height);
15             System.Drawing.Imaging.BitmapData bmpData = bitmap.LockBits(bitmapRect, lockMode, format);
16             GL.ReadPixels(x, y, width, height, GL.GL_BGRA, GL.GL_UNSIGNED_BYTE, bmpData.Scan0);
17             bitmap.UnlockBits(bmpData);
18             bitmap.RotateFlip(RotateFlipType.Rotate180FlipX);
19 
20             bitmap.Save(filename);
21         }