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 pass that as an argument to MD5.ComputeHash. CompareByMask is only slightly faster, but given the context I think any improvement is useful.

Thanks everyone. =)

Edit 2: Forgot to turn optimizations on - doing that gives GrayWizardX's answer even more of a boost:

CompareByInts   (Marc Gravell) :    944ms
CompareByMD5    (Skilldrick)   :   4275ms
CompareByMask   (GrayWizardX)  :    630ms
CompareByMemCmp (Erik)         :    105ms

Interesting that the MD5 method didn't improve at all.

Edit 3: Posted my answer (MemCmp) which blew the other methods out of the water. o.O

Answer 1

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.

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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);
    }
}

Answer 2

If you are trying to determine if they are 100% equal, you can invert one and add it to the other if its zero they are identical. Extending this using unsafe code, take 64 bits at a time as a long and do the math that way, any differences can cause an immediate fail.

If the images are not 100% identical (comparing png to jpeg), or if you are not looking for a 100% match then you have some more work ahead of you.

Good luck.

Answer 3

Well, you're using .LockBits, so presumably you're using unsafe code. Rather than treating each row origin (Scan0 + y * Stride) as a byte*, consider treating it as an int*; int arithmetic is pretty quick, and you only have to do 1/4 as much work. And for images in ARGB you might still be talking in pixels, making the math simple.

Answer 4

Could you take a hash of each and compare? It would be slightly probabilistic, but practically not.

Thanks to Ram, here's a sample implementation of this technique.