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Monday, June 20, 2005

Machine vision: spindles

[Audio Version]

Following is another in my series of ad hoc journal entries I've been keeping of my thoughts on machine vision.

Maybe I'm just grasping at straws, but I recently realized one can separate out a new kind of primitive visual element. Every day, we're surrounded by thin, linear structures. Power lines, picture frames, pin-striped shirts, and trunks of tall trees are all great examples of what I mean. A line drawing is often nothing more than these thin, linear structures, and most written human languages are predominated by them.

The first word that comes to mind when I think about these things is "spindles".

On one hand, it seems hard to imagine that we have some built-in way to recognize and deal with spindles as a primitive kind of shape like we might with, say, basic geometric shapes (e.g., squares and circles) or features like edges or regions. But something about them seems tempting from the perspective of machine vision goals. Spindly structures in an image are obviously at least two dimensional, technically, yet ask a human to draw them and he'll most likely just draw thin lines. They're not just edges; not simply where one surface ends and a new one begins. They have their own colors and hence thickness.

Perhaps what makes spindles interesting to me is that it seems as though one could come up with a practical way of segregating spindles out of an image that may be easier than picking out, say, broad regions based on color blobs, texture spans, or edges. Finding blobs is hard in large part because it's hard to describe in simple terms what a given blob's shape is. A few blurry pixels along an otherwise sharp edge can bring a basic region growing technique to its knees and leave the researcher frustrated into hand adjusting cutoff thresholds to get the results he desires.

But spindles might be easier. Characterizing and recognizing a thin structure should be easier than an arbitrarily shaped blob. Even if the spindle is curved, branching, or somewhat jagged, it may still be easier than dealing with blobs. What's more, it's possible to compare the various spindles in an image to search for patterns that might give hints about 3D structures. Looking down a brick wall and you might pick out the horizontal white mortar lines as spindles and note that they all have a common vanishing point and thus hypothesize a 3D interpretation.

Spindles seem to come in two basic 3D flavors: colored edges and floating structures. The distinction, from a low level perspective, seems to be in whether what's on either side of a spindle is the same color or pattern or not. An overhead power line divides the sky, which is the same on both sides. A picture frame provides an enhancement of the boundary between a picture and the wall. Perhaps the similarity of the colors and textures on either side of a spindle also provide some basic suggestions about whether a given spindle is attached to one or both sides or is otherwise free-floating. The concept of "generic views" would say that it seems hard to imagine the frame around a picture might be floating in space in such a way that it would exactly line up with the picture, so the most plausible explanation is that it's no coincidence that the picture frame is actually in the same place as the picture. Whether it's attached to the wall or floating in space is a different question. So spindles can be helpful 3D cues.

I don't know whether to suggest that the human visual system sees spindles as a somehow separate sort of primitive, but it seems plausible. The very fact that printed characters in most all human languages are composed of spindles seems suggestive. Maybe it's because it's economical to write in strokes instead of blobs, but maybe it's more fundamental than that. It's also interesting that we have little trouble understanding technical "some assembly required" line drawings, even when they have no color, shading, or other 3D visual cues.

Perhaps spindles provide a way to explain how it is that a line drawing of a circle can be interpreted just as easily as a hollow hoop or a solid disk. That is, perhaps spindles are considered interchangeable with edges by our visual systems. Yet perhaps it's also that spindles stand out better than edges do.

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