Yup. Did you read this? Remember, if the two values are the same, you then go on and run a full md5, so there is no risk. But if they are different you saved yourself the bother/expense of doing it. And statistically, that should be the case in a very large percentage of cases.

The reality of any given set of data probably won't reach that theoretical maximum. For a start, many if not most images don't use the alpha byte, so the range of values is reduced to 

( ( 2**24 * (2**24 - 2) ) / 2**48 ) == 0.999_999_887_92

[download]

but that's still pretty good odds for the effort of reading 4 bytes and comparing two integers.

Of course, if the two pictures being compared are

  1. A 640x480x24-bit color image of a black cat in a coal celler inside the Artic Circle in winter, with no flash.
  2. A 640x480x24-bit color image closeup of a black hole.

Then this quick and simple test may not discriminate between them. But then, will the viewers? :)

More seriously, it's possible that the camera that took the images has a bad cell in the CCD that means that one pixel in the same place on every image is always black (or white or red), and this test would fail to distinquish them if it happens to check that exact pixel. But if the dword compared is (semi-)randomly chosen, that would be pretty unlucky.

Even if you mount the camera on a tripod and use a remote trigger to avoid micro-seismic disturbances, and take two frames one after the other, with 16 mlllion colors to choose from for each pixel, even the slightest variation in the light, or focus, or even the battery charge is likely to cause variations in the pixel colors at identical positions in identical shots by the same camera. In 8-bit color/grey scale images, the variation will be less, but then you are comparing 4 pixels not one. In a strictly B&W image, you would be comparing 32 adjacent pixels.

But when the statistically improbable happens and you get a false positive, that false positive would be caught by the full md5 anyway. The point is to save time by avoiding that full md5 (or similar) where possible--not ditch it all together.

Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
Lingua non convalesco, consenesco et abolesco. -- Rule 1 has a caveat! -- Who broke the cabal?
"Science is about questioning the status quo. Questioning authority".
In the absence of evidence, opinion is indistinguishable from prejudice.

In reply to Re^3: Comparing images by BrowserUk
in thread Comparing images by Anonymous Monk

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