Question of the Day
by JS
What is an object?
UPDATE: This is such a general question, I thought it might be worth providing a specific example. In the dominant paradigm of computer vision research, an object is an array of pixels with a particular label. The main research challenge is identifying some model that explains the variations in pixel arrays for each kind of label. As an example, if you have a thousand images of bikes, the actual values of the pixels in each image will vary quite a bit even though all the labels are the same. Using this kind of model, you’d like to be able to determine if an arbitrary array of pixels in some new image contains some kind of bike.
Now “labelled array of pixels” isn’t a very rich notion of objects, and it usually depends on having some population of already labelled pixel arrays around. Simplicity aside, having this kind of model would be incredibly useful for many applications.
Other disciplines where the notion of “object” comes up quite a bit have developed somewhat more comprehensive definitions. The challenge of today’s question is to come up with some common sense definition of your own.
Comments
My first definition was to say “An object is a connected set of points in space”. But this quickly run into problems. What about two objects that touch other? So, a more refined version would be “An object is a set of points in (3D) space such that each point translates by the same amount when a force is applied to the center of mass”. Now, this definition involves the concepts of force and mass, and may not be very useful when recognizing objects in a static images. But it can be useful for a robot that can interact with objects. Roughly, when a collection of points moves together in a certain way, we can say that the collection comprises an object.
Do you see any problems with this definition?
I like this definition, but I can think of a few edge cases to consider.
1. A slinky is an object, but all the points of the slinky don’t move by the same amount when force is applied to its center of mass. In fact, when force is applied anywhere on the slinky, the motion of the rest of the slinky is highly unpredictable (and entertaining!).
2. Some objects are fixed and rather immovable. The bench I sit on at the bus stop is a slab of concrete that probably weighs a good 750 lbs. Not manipulable by any human agent, but it certainly is an object. If we can only move an object in our imagination, is it then only an imaginary object?
Interesting points. So my definition applies to rigid bodies, but not deformable bodies. Another tricky situation is when one object is glued to another. For example, if you glue a pencil to your book, we will still call them two objects. Why? Because we have prior knowledge that these were two distinct objects. What happens when a child always encounters a book and pencil in the glued state. Will the child still think of them as two separate objects? Similarly for your example of a fixed bench.