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

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);
    }
}

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.

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.

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.

If the original problem is just to find the exact duplicates among two bitmaps, then just a bit level comparison will have to do. I don't know C# but in C I would use the following function:

int areEqual (long size, long *a, long *b)
{
    long start = size / 2;
    long i;
    for (i = start; i != size; i++) { if (a[i] != b[i]) return 0 }
    for (i = 0; i != start; i++) { if (a[i] != b[i]) return 0 }
    return 1;
}

I would start looking in the middle because I suspect there is a much better chance of finding unequal bits near the middle of the image than the beginning; of course, this would really depend on the images you are deduping, selecting a random place to start may be best.

If you are trying to find the exact duplicates among hundreds of images then comparing all pairs of them is unnecessary. First compute the MD5 hash of each image and place it in a list of pairs (md5Hash, imageId); then sort the list by the m5Hash. Next, only do pairwise comparisons on the images that have the same md5Hash.