Agreed, however the article already acknowledges that approach (read near the end about the flower). Luckily, unlike text, which follows a very constrained set

Agreed, however the article already acknowledges that approach (read near the end about the flower). Luckily, unlike text, which follows a very constrained set of rules, (eg an X will always be two lines criss-crossed), the same doesn't apply to 3D objects, where you can have 2 images of the same object that look entirely different, a simple example being the chair, that comes in all varieties of shapes but still easily identifiable to a human. So this would automatically require human input in respects to identifying the object, you can't create a program that would 'learn' new objects, at least, not yet. Also, the silouhette strategy can only be applied when a shape remains relatively constant, moving the camera a little to the left would render a completely new silouhette. Add that the bot would still need to be told how to answer the arbritrary 'How many legs does the chair that the man is sitting on have?' questions. The fact that so much human input is required just to identify /one/ object in the captcha, the fact that once that object has been compromised it is trivial to switch in another one (which is impossible in text captcha because there are only 26+10 amount of characters that the whole world knows) means that this is a damn effective captcha.