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

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  1. POWhat is the fastest way I can compare two equal-size bitmaps to determine whether they are identical?
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    <p>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.</p> <p>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?</p> <p>The language I'm using is C# by the way - and yes, I am already using the .LockBits method. =)</p> <p><strong>Edit</strong>: 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:</p> <pre><code>CompareByInts (Marc Gravell) : 1107ms CompareByMD5 (Skilldrick) : 4222ms CompareByMask (GrayWizardX) : 949ms </code></pre> <p>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.</p> <p>Thanks everyone. =)</p> <p><strong>Edit 2</strong>: Forgot to turn optimizations on - doing that gives GrayWizardX's answer even more of a boost:</p> <pre><code>CompareByInts (Marc Gravell) : 944ms CompareByMD5 (Skilldrick) : 4275ms CompareByMask (GrayWizardX) : 630ms CompareByMemCmp (Erik) : 105ms </code></pre> <p>Interesting that the MD5 method didn't improve at all.</p> <p><strong>Edit 3</strong>: Posted my answer (MemCmp) which blew the other methods out of the water. o.O</p>
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    1. COHave you tested this with the "full size" bitmaps you are going to run through in production? Has it proven itself slow? Have you profiled your code on your production bitmaps to determine where it is slowing down?
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    2. COYes - the problem is that the worst case - scanning both bitmaps and determining that they are identical - is also the most common case.
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    3. CODefine "identical" in the context of bitmaps. Do you mean, binary identical, file-based (both are .png files, and the contents are identical)? Do you mean pixel-identical (could be different file formats, but the pixels are the same)? Do you mean perceptually identical (slightly different red channels are OK since the eyes of humans aren't that good to discern differences in the red channel anyway?
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      1. USLasse Vågsæther Karlsen
 

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