What is the fastest way I can compare two equal-size bitmaps to determine whether they are identical?

Question

I am trying to write a function to determine whether two equal-size bitmaps are identical or not. The function I have right now simply compares a pixel at a time in each bitmap, returning false at the first non-equal pixel. While this works, and works well for small bitmaps, in production I'm going to be using this in a tight loop and on larger images, so I need a better way. Does anyone have any recommendations? The language I'm using is C# by the way - and yes, I am already using the .LockBits method. =) Edit: I've coded up implementations of some of the suggestions given, and here are the benchmarks. The setup: two identical (worst-case) bitmaps, 100x100 in size, with 10,000 iterations each. Here are the results: ``` CompareByInts (Marc Gravell) : 1107ms CompareByMD5 (Skilldrick) : 4222ms CompareByMask (GrayWizardX) : 949ms ``` In CompareByInts and CompareByMask I'm using pointers to access the memory directly; in the MD5 method I'm using Marshal.Copy to retrieve a byte array and p…

Accepted Answer

Edit 8-31-12: per Joey's comment below, be mindful of the format of the bitmaps you compare. They may contain padding on the strides that render the bitmaps unequal, despite being equivalent pixel-wise. See this question for more details.

Reading this answer to a question regarding comparing byte arrays has yielded a MUCH FASTER method: using P/Invoke and the memcmp API call in msvcrt. Here's the code:

```
[DllImport("msvcrt.dll")]
private static extern int memcmp(IntPtr b1, IntPtr b2, long count);

public static bool CompareMemCmp(Bitmap b1, Bitmap b2)
{
    if ((b1 == null) != (b2 == null)) return false;
    if (b1.Size != b2.Size) return false;

var bd1 = b1.LockBits(new Rectangle(new Point(0, 0), b1.Size), ImageLockMode.ReadOnly, PixelFormat.Format32bppArgb);
    var bd2 = b2.LockBits(new Rectangle(new Point(0, 0), b2.Size), ImageLockMode.ReadOnly, PixelFormat.Format32bppArgb);

try
    {
        IntPtr bd1scan0 = bd1.Scan0;
        IntPtr bd2scan0 = bd2.Scan0;

int stride = bd1.Stride;
        int len = stride * b1.Height;

return memcmp(bd1scan0, bd2scan0, len) == 0;
    }
    finally
    {
        b1.UnlockBits(bd1);
        b2.UnlockBits(bd2);
    }
}
```

What is the fastest way I can compare two equal-size bitmaps to determine whether they are identical?

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