Cat on the stairs

Late in the spring semester, Danielle sent me a copy of this picture.  Is the cat going up or down the stairs?

This picture, originally posted on 9gag.com, went viral.  Some of the places that picked it up included Business insider, CNet (with a real professor!!!! to explain the illusion), ABC news, USA Today, and even [my favorite] National Geographic.

What's the answer?

Well, some places focused on architecture.  Here's a picture emphatically claiming a solution based on overhang . . . but there's a problem with this approach . . .

Because the person who posted this didn't think about "stair nosing".  What is nosing? You can see more at this site, but basically it's a lip that sticks up at the edge of a stair.  It's a safety feature, designed to keep people from sliding off the edge of the stair.

Since we don't know if those lines are overhangs or nosing, another possibility is to try using perspective.  

Let's assume that the clearing we see behind the cat is a doorway with vertical edges.  If that's true (it's a reasonable assumption, but of course we don't know this for sure), then the fact that the image lines in the picture converge downward tell us something important.

When we look UP a staircase, vertical lines appear to converge upward, as in this photo.

But when we look DOWN a stairway, the images of vertical lines converge downward.

So, based on that doorway alone, we'd say the photo is looking DOWN the stairs, so this cat is walking UP.

Still, we have to take all of our assumptions with a grain of salt.  The photo appears to have been doctored just a tad, as the following photos indicate.