MacroLab3D

Does it looks bulge or hollow to you? This image is bulge or hollow at the same time and your brain randomly choosing between the two. You can switch by looking at it for some time with your side vision and back. This scheme will help you to understand what to expect: http://i.imgur.com/2nx1MGE.jpg Our brain confused because of a lighting of the GIF, particularly because of the absence of shadows. We can see it both ways in random order or by will. Which is pretty crazy experience.

If you see only one version, perhaps 3D stereo pairs will help. Open this image in Cross View 3D format, sit back in front of your screen (farther is easier to focus), cross your eyes until you see 3 images. That image in the middle will be in 3D, and showing you actual foamy bulging version. In case if you prefer SBS/Parallel.

Some people can see even mix of both, when left side is indent and right is a foam.