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December 30, 2007

Human vision vs. computer vision, another example

Last time we had an image that looked blank and yet Pixcavator was able to detect objects that spelled “Hello!”. The reason was that humans can only recognize about 30 levels of gray while standard image formats have 256. Since the text was in virtually white (254) on white (255) background, it was invisible. This was supposed to illustrate the ability of computer vision to succeed where humans fail.

This last example was based on the failure to see small variations of luminosity. Now we have an almost identical example except it is based on the fact that humans can’t detect subtle variation of color (under fixed luminosity). The “blank” cyan rectangle below contains my initials “PS” which is found and displayed by Pixcavator on the right. The text written in color (0,254,255) – in the RGB model - that is only slightly different from the background (0,255,254). It is invisible to us but not to the computer. Once again my point is that image enhancement tools have nothing to do with computer vision.

 invisible writing computer vision

To verify that the image on the left contains the text you do need Photoshop again. This time Auto Contrast won’t help, but Smart Fix does work.

There will be another post on this topic.

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