AccessViolation when using LockBits

Question

I would like to compare the similar images more faster using LockBits method as follows

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 );

 }

}

but during the second Marshal.Copy the AccessViolationException is occurred:

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)

What is wrong in my program ?

Thanks.

Answer 1

I've been looking into a similar issue for a few hours now, and I think I've found what might be your problem. I'm guessing your bitmaps might be stored in slightly different formats. Bitmaps can be stored either forwards or backwards. Stride will be negative when stored backwards. However, Scan0 will always point to the first line of the scan, ie, the first pixel NOT the first byte in the array.

Therefore, in a backwards scan bitmap, Scan0 + Abs(Stride) - 1 is the last byte in the array. Scan0 + Stride will always be the start of the second line, so if stride is negative it works backwards and positive will work forwards.

If you do Marshal.Copy(bm2Ptr, rgbValues2, 0, rgbValues2.Length) with a negative stride, this will copy the last scanline before entering access violation territory. The following code will convert any bitmap into a backwards scan byte[] (just because that's what I was working with). I'm guessing you've fixed / worked round your problems by now, but hopefully this helps others.

    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;
    }