使用LockBits时的AccessViolation

Question

我想使用LockBits方法更快地比较相似的图像，如下所示

using System;
using System.Drawing;
using System.Drawing.Imaging;
using System.Runtime.InteropServices;

public class CompareImages {

 public static void Main ( String[] args ) {

 Bitmap bm1 = new Bitmap ( "PB270029.JPG" );  
 Console.WriteLine ( bm1.PixelFormat.ToString() );

 int width = bm1.Width;
 int height = bm1.Height;
 Console.WriteLine ( "width = " + width + "  height = " + height );

 Rectangle rect1 = new Rectangle ( 0, 0, width, height );
 BitmapData bm1Data = bm1.LockBits ( rect1, ImageLockMode.ReadOnly, bm1.PixelFormat );

 Console.WriteLine ( "stride = " + bm1Data.Stride );

 IntPtr bm1Ptr = bm1Data.Scan0;

 int bytes = Math.Abs(bm1Data.Stride) * height;
 Console.WriteLine ( "bytes = " + bytes );

 byte[] rgbValues1 = new byte [ bytes ];
 Marshal.Copy ( bm1Ptr, rgbValues1, 0, bytes );

 Console.WriteLine ( "After 1st Marshal.Copy ..." );

 Bitmap bm2 = new Bitmap ( "PA050164.JPG" ); 
 Rectangle rect2 = new Rectangle ( 0, 0, bm2.Width, bm2.Height );
 BitmapData bm2Data = bm2.LockBits ( rect2, ImageLockMode.ReadOnly, bm2.PixelFormat );

 IntPtr bm2Ptr = bm2Data.Scan0;
 byte[] rgbValues2 = new byte [ Math.Abs(bm2Data.Stride) * bm2.Height ];
 Marshal.Copy ( bm2Ptr, rgbValues2, 0, rgbValues2.Length );

 }

}

但在第二次Marshal.Copy期间发生了AccessViolationException：

C:\CompareImages>csc CompareImages.cs
Microsoft (R) Visual C# 2010 Compiler version 4.0.30319.1
Copyright (C) Microsoft Corporation. All rights reserved.


C:\CompareImages>CompareImages.exe
Format24bppRgb
width = 3648   height = 2736
stride = 10944
bytes = 29942784
After 1st Marshal.Copy ...

Unhandled Exception: System.AccessViolationException: Attempted to read or write
 protected memory. This is often an indication that other memory is corrupt.
   at System.Runtime.InteropServices.Marshal.CopyToManaged(IntPtr source, Object
 destination, Int32 startIndex, Int32 length)
   at CompareImages.Main(String[] args)

我的程序出了什么问题？

谢谢。

Answer 1

我已经调查了几个小时类似的问题，我想我已经找到了可能是你的问题所在。我猜您的位图可能以略有不同的格式存储。位图可以向前或向后存储。向后存储时，Stride将为负值。但是，Scan0将始终指向扫描的第一行，即第一个像素不是数组中的第一个字节。

因此，在向后扫描位图中，Scan0 +Abs(步长)-1是数组中的最后一个字节。Scan0 + Stride将始终是第二行的开始，因此如果stride为负，它将向后工作，而正将向前工作。

如果以负步长执行Marshal.Copy(bm2Ptr，rgbValues2，0，rgbValues2.Length)，这将复制进入访问冲突区域之前的最后一条扫描线。下面的代码将把任何位图转换成向后扫描的字节just because that's what I was working with。我猜你现在已经修复/解决了你的问题，但希望这能帮助其他人。

    private byte[] BitmapToByteArray2(Bitmap bmp)
    {
        // Lock the bitmap's bits.  
        Rectangle rect = new Rectangle(0, 0, bmp.Width, bmp.Height);
        System.Drawing.Imaging.BitmapData bmpData =
            bmp.LockBits(rect, ImageLockMode.ReadOnly,
            bmp.PixelFormat);

        int absStride = Math.Abs(bmpData.Stride);
        int bytes = absStride * bmp.Height;

        // Declare an array to hold the bytes of the bitmap.
        byte[] rgbValues = new byte[bytes];

        for (int i = 0; i < bmp.Height; i++)
        {
            IntPtr pointer = new IntPtr(bmpData.Scan0.ToInt32() + (bmpData.Stride * i));
            System.Runtime.InteropServices.Marshal.Copy(pointer, rgbValues, absStride * (bmp.Height - i - 1), absStride);
        }

        // Unlock the bits.
        bmp.UnlockBits(bmpData);

        return rgbValues;
    }

Answer 2

我测试了代码，对我来说它是有效的。

首先，我使用了一些随机的图片，然后是一张更大的全白图片，和你的图片大小一样。也许你可以提供更多关于这两张图片的信息:它们是否具有相同的大小或像素格式？

我看到的唯一可能的错误是在复制之后没有调用UnlockBits。

This article很好地解释了LockBit函数。

Answer 3

它是为我而运行的。我试图使图像文件无效，但这抛出了一个不同的异常。它确实需要清理一下。

using (Bitmap bm1 = new Bitmap("PB270029.JPG"))
{
    Console.WriteLine(bm1.PixelFormat.ToString());

    int width = bm1.Width;
    int height = bm1.Height;
    Console.WriteLine("width = " + width + "  height = " + height);

    Rectangle rect1 = new Rectangle(0, 0, width, height);
    BitmapData bm1Data = bm1.LockBits(rect1, ImageLockMode.ReadOnly, bm1.PixelFormat);
    try
    {
        Console.WriteLine("stride = " + bm1Data.Stride);

        IntPtr bm1Ptr = bm1Data.Scan0;

        int bytes = Math.Abs(bm1Data.Stride) * height;
        Console.WriteLine("bytes = " + bytes);

        byte[] rgbValues1 = new byte[bytes];
        Marshal.Copy(bm1Ptr, rgbValues1, 0, bytes);

        Console.WriteLine("After 1st Marshal.Copy ...");
    }
    finally
    {
        bm1.UnlockBits(bm1Data);
    }
}

using (Bitmap bm2 = new Bitmap("PA050164.JPG"))
{
    Rectangle rect2 = new Rectangle(0, 0, bm2.Width, bm2.Height);
    BitmapData bm2Data = bm2.LockBits(rect2, ImageLockMode.ReadOnly, bm2.PixelFormat);
    try
    {
        IntPtr bm2Ptr = bm2Data.Scan0;
        byte[] rgbValues2 = new byte[Math.Abs(bm2Data.Stride) * bm2.Height];
        Marshal.Copy(bm2Ptr, rgbValues2, 0, rgbValues2.Length);
    }
    finally
    {
        bm2.UnlockBits(bm2Data);
    }
}