Cup or Bub?


The convex/concave riddle in rock art photography


By Jan J. Brouwer (rockartuk)


This page is part of the

British Rock Art Collection



The "flip-over" animation of the Ormaig rosettes, 1999

We were aware of the fact that you always have to sit next to a person while looking at rock art photos. When you sit face to face to each other you may be trapped in what we call the 'convex/concave riddle'; cups become bulbs and the motif seems to lie on the rock instead of being carved into the rock surface. Light is playing around with our brains and since we have 'old' brains we took the effect for granted and were always seated nicely on one side of the table when we went through our photos.


In the beginning of November 2002, our member Maurice started a discussion about this subject in the Rock Art Forum. He wrote:

"Sometimes your mind takes you away from reality. Especially when you look at 2D pictures from cups and rings. Take for instance the photograph 'Duncroisk face' at the What's New page.

If I look at the picture I see boulders and on top placed rings in stead of cups and rings. If you see cups, try to stand on your hands and take again a look at the picture......Weird phenomenon eh!"

After some replies of other members, Maurice came up with an example from the Micro-world. He wrote:


        


This is an example in which I saw the optical illusion the first time. This is a electromicroscopical photo of a membrame which you will find lots of in every living cell. During a laborious                     process with freeze drying tissue and cover it with a layer of gold you can make electromicroscopical pictures of the inside of a cell. This is when I started to flip the pictures over and saw              that it depends on how you look at the picture it will turn convex or concave. But the weird thing is is that sometimes my fellow students would see the same picture just the other way                  around as I would see it. So I think it also depends on the imagination of the person in which direction he or she thinks where the light is coming from.Deep Freudian Psychology this                         message is turning in!


Loraine (from Spain) contributed an amazing Macro effect; a wall in Peru!




But between these great Micro/Macro examples there was still the rock art riddle as seen in the following photos of Duncroisk:
 
 



February 2003: The convex/concave riddle solved!


In February 2003, the riddle seems to be solved. We came in contact with John H. Krantz of Hanover College at Hanover, Indiana (USA) who is an expert on the subject. In his very interesting e-lecture "Vision and Art, The Effects of Shadow ", he gives the explanation we were looking for:

Part of the way our perception of shadow works is by our expectation of the direction of the sun.  The following diagram illustrates this point.

shadow2.gif (1705 bytes)

        A shadow is on the far side of a mound and the near side of an indentation.  However, to be able to interpret these differences, our brain must make some assumptions about the direction                    of the sun.

                One basic assumption is that the sun comes from above.  This assumption helps the view make a coherent interpretation of an image.  If the assumption is violated, then our                                               perceptions can be tricked as in these examples.


Enjoy the mysterious world of light and shadow. It is an 'eye-opener' in this fast world of 'wysiwyg' (what you see is what you get). Now we know that our eyes and brain are subordinated to the laws and wonders of nature. You might run into familiar animated images!
 
Thanks to Maurice who started this discussion and wrote:

"So I think it also depends on the imagination of the person in which direction he or she thinks where the light is coming from"

He therefore deserves our non-existing but very honourable "BRAC - good thinking award".
 
John Krantz wrote:

It is my honor to link to your neat observations.  I think all of you dealing with this fun problem get the "Good Thinking Award".  It has been a real pleasure to have had this contact with you.
Tell Maurice that that is a great example of his own. Again, thanks for all of this,
John.


We at the "British Rock Art Collection" thank John Krantz by dedicating this page to him!


Thanks John!




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