If you look closely at this picture, you see Kenny, a friend of mine. The photo looks a little odd, but Kenny is clearly recognizable. But if you get far enough away (we might be talking 15 feet or more, unfortunately) , take off your glasses, or squint (or some combination of them) you'll see my parrot, Cirrus.
Note: I changed the size of the photograph to make it easier to see. If you want to see the original 600 by 900 image, clicking on the picture will take you there.
The effect might be easier to see in the photograph thumbnail. At the very least, you won't have to back off as far.
What I've done is create a hybrid photograph. The image of Kenny has had most of the "low frequency information" stripped out. In other words, I used the high pass filter on it. A second photograph, the image of Cirrus, was blurred — removing all the "high frequency information." The two images were then merged. When you're close up, you see the high frequency image, but the low frequency information is too spread out for you to notice, other than to think the photograph is odd. When you're far away and the high frequency image blurs with distance, the low frequency image becomes much easier to see.
I'm still learning to do this trick. Artists have used this in the past; it may be part of what makes the Mona Lisa smile so mysterious.
I found out about it in the New Scientist article "Hybrid images: Now you see them…", which, unfortunately, you can only read if you have a subscription. But in the article, I found a web site by the scientists studying these images. If you want to learn more, I especially recommend the PDF "Hybrid Images."
I've learned a couple things in playing around. The best images use objects with similar profiles. For example, once I decided to use Cirrus, I had to search for a human with a face oriented in a similar direction. I couldn't get the eyes to match up, since African grey parrots are prey while humans are alpha predators, but Kenny's face was pointed in about the same direction and his goatee matched nicely with the beak. Images where a frown turns into a smile, a son turns into a father, a daughter into a mother, or a homeless person turns into Jesus strike me as likely uses of this technique. The metaphorical associations go with the images, giving the trick a purpose.
I think that's why I'm so impressed with the bicycle that turns into a motorcycle on the scientists web site. The photographic trick brings to mind the childish fantasy that a bike is a motorcycle.
Beyond that, I'm not sure there's much purpose for this trick for the casual photographer or artist. But for Drs.Oliva, Schyns, and Toralba, they are using the high frequency/low frequency images to tell them about how the human eye processes visual information. I'm glad they pointed this out. I've had fun playing with it.
To create the high pass image, I used Adobe Photo Elements high pass filter, which can be found under Filter > Others. I haven't been able to find something that does that in Corel Paint Shop Pro, but that doesn't mean there isn't something. For the low pass, I used the Adobe Photo Elements Filter > Blur > Gaussian blur. I then created a new image and put the two images on different layers, merging the layers after playing with the transparencies. Now you can do it! I haven't decided if color or black and white images work best. If you look through the many images on the scientists' web site, you can decide for yourself what you think does or doesn't work as well.
That is so incredibly cool!