07 February 2012

Mod 2 -- I guess you had to be there-- and I mean right there.

I was walking through my unglamorous corner of San Francisco and saw this sight:

Unremarkable, right? Maybe at first and second glance. But it reminded me of a mistake that beginning students often make with cast shadows: Having the shadow take on a simple shape, like this triangle, that falls onto the ground intact without respect to any incidental objects it may hit, such as the weeds. As in this D-winning student work, where the (totally unrealistic) shape of the cast shadow just powers through the second flower pot as if it wasn't really there, or...

...was flat artwork like this trompe l'oeil chalk art.

So how could this be happening in real life? How could the shadow just act as if the weeds weren't there? The short answer is that it was totally dependent on my having a particular position, and thus a particular line of sight. Similar to the way that this sidewalk art...

...only looks right from one precise position in space. (The very same position where last night the cunning chalk artist set up a projector on a tripod to show him exactly what to draw.)

The shorter lesson of this is: distrust a shadow that's a simpler shape than the object what done made it.

The more involved answer is that I happened to be standing where two shadow-edges with two totally different directions appeared continuous. I'm betting it could only be seen thus from that one position, among the infinite others possible.

The weeds, by being very robustly 3D and--with their fine leaves--effectively semi-transparent, were showing me something that you don't normally see except in fog or airborne dust: the passage of a cast shadow through 3D space. I just happened to be standing where the sloping plane of the tent-like volume of the cast shadow falling through the weeds was (1) raked by my eyeline, (2) was foreshortened from my end-on vantage point as to appear to be a line and not a plane, and (3) just happened to appear continuous with the edge of the cast shadow of the old house's gutter on the sidewalk, despite being at a radically different angle in space.

Just a step and a half to the left, and the view was quite different, and befit the real complexity of the situation:


P.S.: If you are really into this kind of thing, you will understand why the sloping sidewalk was necessary for the creation of that first scene above: The slope disrupted the normal parallel relationship that the gutter and its shadow would have on level ground.

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