Monday, August 24, 2009

Fun with Morphing

In my first post I mentioned that Thompson anticipated the developing of image morphing. Just for fun I decided to explore the application of morphing programs to Thompson’s images. I googled morphing software and looked at a few of the many offerings, some free, some not. One nice-looking non-free option is Fantamorph, which has a gallery of their own and customers creations, some quite impressive. Several are relevant to the topic of this blog:






For my experiment in Thomson-inspired morphing, I chose two images at random:






First I thought I'd better check to make sure Thompson hadn't “cooked the books” by misrepresenting their shapes. I googled up some images of Polyprion (a.k.a Atlantic wreckfish) and Scorpaena; he appears to have done them justice, though perhaps exaggerated the differences a bit:



Not wanting to spend any money on the exercise, I downloaded Free Morphing, which was fairly simple to learn. You place lines on one image, reposition them in the second, and it then interpolates between them.
I tried it on the images I had pulled up. Not surprisingly, it did something reasonable with the two fish. I wanted to put a video of it into the blog, but it didn't save in any blog-ready formats, so I needed some additional software. I found CamStudio which lets you capture screen events as video. The result is below (click triangle to play):

video 

Of course, we already knew that you could morph shapes as disparate as Professor McGonagall and a cat into each other, so the "success" of this little experiment tells us nothing of interest scientifically. Furthermore, the program does not present us with an explicit grid describing the deformation, so we cannot even tell if the deformation looks like Thompson’s. 
To address this I tried another approach, which was to morph the grid and carry the fish along for the ride; that also produced plausible looking results:


video
Hower, as before, it doesn’t really demonstrate anything. The problem is, with morphing I  always end up at my target image, there is no suspense. It doesn't let me evaluate whether some transformation will get me there; it gives a tranformation that does. It would be nice to be able to define a transformation, say as a grid distortion; apply it to fish A, and then compare the resulting distorted fish A' to fish B to see how well they match. Maybe next time I'll try PhotoShop...
What this exercise did clarify for me is that a critical point of Thompson’s transformation images is that the deformations be in some sense simple, or regular. In mathematical terms this means it should be possible to describe them with few parameters (numbers). If I allow every point of interest in image A to move to an arbitrary new position in image B, I could describe the transformation with 2*n numbers (x,y displacements of each point), where n is the number of points (called fiducial points in the trade). For a fish, I could probably get by with n on the order of 10 or so fiducial points, so with 20 numbers I could get as cubist as I wanted. To the extent that the deformation is regular, I should be able to get by with many fewer numbers than that. I can describe a scaling transformation with just one number, a scale factor; for a two-dimensional rotation or shear I might need 4.
 
How many numbers do I need to describe Thompson's transformations? Closer to 1 or to 20?  Thompson’s diagrams seem to be saying look how regular Nature’s transformations are! But is it true? Certainly growth is fairly regular, unless you have teenage children. It is not a simple zoom (or dilation or scaling) transformation, because some aspects of the organism scale according to different powers of the overall size. For example, the weight of an organism is proportional to its volume, and so scales as the cube of its overall size, but the compression strength of bone is proportional to its cross-sectional area, and so scales as the square of the linear dimensions. This means that if you simply scale a cat to the size of a lion, its bones will be too thin for its weight, and they will fracture. To compensate, bones have to get thicker faster as the body grows. This general principle is called allometric scaling, and has been known for a long time.

Thompson’s transformations are not so much about growth, however, but about form; not the ontogenic transformations but the phylogenetic. Are the deformations between related species describable with few parameters, and if so, is that interesting? Maybe all this time these transformations have been some kind of conjurer’s trick, a clever iconic image with no real significance. How many parameters would I need to describe the warping of Polyprion into Scorpaena? The horizontal lines get squished towards the back; one number could probably describe the extent of the squishing. Thompson curves the verticals, which would burn extra parameters, since curves are mathematically more expensive to describe than lines. However, I didn't curve them, since curves are also more expensive to do in Free Morphing, which only gives you lines to work with; you would need to approximate a curve with a series of lines, which gets tedious rather quickly. However my lines seemed to work almost as well as Thompson's curves. So maybe you could turn Polyprion into Scorpaena with a few numbers. Some of Thompson's other transformations are more complex, and start to seem like special pleading.  Given that he lets himself choose from a rather open-ended family of tranformations, you would also technically require one or two parameters to specify the type of transformation; it starts to look like an exercise in minimum length encoding or Kolmogorov complexity, i.e., the shortest way to describe something complicated.

If we accept the point that many morphological transformations between species are simple in the sense of being describable with a small number of parameters, is this biologically interesting? After all, Nature is not using a warping algorithm or specifying parameters. Or is She? The transcription factors mentioned in the last post, the regulators of genes, bind with a strength determined by the sequence of the DNA they are binding to. Change the sequence, change the binding strength, change how hard this regulator turns that gene on or off, and therefore change, or modulate, all the downstream effects of that gene. If that gene says grow the front, or the back, of the critter this much, then you change the shape. So maybe there's something here after all...

2 comments:

  1. Stanley-Unless there are exceptional pressures to adapt to a particular niche, the morphogenetic differences among species may not arise from processes much more complicated than those that make variations in color. I am left to wonder whether the variation of forms, aside from whatever functional purpose they may have, is interesting in itself or only to our human eye. I am reminded of the artist Andy Goldsworthy, who shapes organic materials with natural processes (eg..woven leaves circling in a stream's current, ground iron ore thrown into the wind) while he films their evanescent play. You might want to see the documentary on his work, "Rivers and Tides."
    Thanks for this blog. I am looking forward to following the development of your thoughts .... Bob H

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  2. Bob, I devoted an entire post to responding to your comments. -Stan

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