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Image subtraction caution

Asked by Walter Roberson on 8 Dec 2011

This is not a MATLAB-specific issue, but it is a good cautionary tale about how one can be mislead by the results of image subtraction, an operation often encountered in image procesing:

The news headline may sound absurd and crackpot, but the article itself is neither of those; the fact that a lot of people are misinterpreting the results should warn us all to be careful both in what we do and in how we describe it to others.

UFO hunters say there is alien spaceship parked next to Mercury

What looks to UFO hunters like a massive alien spaceship parked next to planet Mercury appeared on a sequence of images of a Coronal Mass Ejection (CME) from the sun taken by a NASA telescope. [...]
According to Daily Mail, the image has been analyzed by the United States Naval Research Laboratory. Experts explain that the "alien ship" really is the "ghost" of the image of Mercury taken the previous day. According to the experts, to ensure that the CME being tracked stands out clearly visible, astronomers compare new images taken with previous ones and eliminate or subtract images that appear twice as "interfering background light."
Far and stationary objects such as stars are easy to identify and eliminate, but moving objects such as near planets are difficult to identify for subtraction. [...]
      "When [this averaging process] is done between the previous day and the current day and there is a feature like a planet, this introduces dark artifacts in the background where the planet was on the previous day, which then show up as bright areas in the enhanced image." [...]
"The scientists have not managed to subtract the image of Mercury. The technical guys are saying the problem is that when you try to subtract something that’s moving the pixels blend into each. It’s imaging processing that they haven’t got their heads around

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Walter Roberson

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Answer by Image Analyst on 8 Dec 2011

"It’s imaging processing that they haven’t got their heads around..." The astronomer is talking about the UFO hunters. Most of them should know better since they are the ones who usually debunk this stuff, but then again there are always idiots like SiniXster who said in the article that there is "absolutely no explanation for the nearly Mercury-size mystery object other than that it's a spaceship." You'd think after a lifetime of making stupid assumptions and claims like that that they'd shut up to avoid further embarrassment. Maybe the astronomers will have to use Photoshop's bandaid tool next time to avoid such nonsensical speculation.

At least they had negative values. Many people subtract two uint8's and get clipping at 0 and never even realize it. Or add and get clipping at 255.

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Image Analyst

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