Sunday, April 22, 2012

Image Comparison – What is the quick way to identify whether 2 images are identical or different?

In this blog post, i will show you the quickest way to identify whether 2 images are identical or different using C#.

As a first step in determining whether 2 images are identical or not, we use the file size information for basic comparison. However what happens when 2 different images have same file size. In this case you cannot determine the difference using file size.

Following are several approaches in comparing the images.
  1. Comparing hash of the image
  2. Comparing byte by byte
  3. Comparing pixel by pixel etc..,
In the above approaches, 1st one comparing the hash of the images is the fastest technique to determine whether the images are different.

Image Hash Comparison:

Step 1: Find the hash of the given image
Using any hashing technique supported in C#, compute the hash of the image by providing image bytes as argument. This will create a 32 bytes hash.

Code Snippet:
HMACSHA1 hmacSha1 = new HMACSHA1();
byte[] imageHash = hmacSha1.ComputeHash(ImageBytes);

Note:  Here ComputeHash() method will always generate 32 bytes of hash irrespective of the size of the image.

Step 2: Compare the calculated 32 bytes hash (byte by byte) and if any byte differs you can stop the comparison and return that the images are not equal.

Code Snippet:
private bool IsImageEqual(byte[] imageHash1, byte[] imageHash2)
bool isImageEqual = true;
//Compare the hash values
for (int j = 0; j < imageHash1.Length && j < imageHash2.Length; j++)
if (imageHash1[j] != imageHash2[j])
isImageEqual = false;
return isImageEqual;

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