[Dragaera] The Enclouding

[Michael Wojcik]

Jerry Friedman wrote:
> --- On Sat, 10/11/08, Michael Wojcik <mwojcik at newsguy.com> wrote:
> 
>> In a similar vein, consider someone wearing a fencing mask
>> - they can
>> see out quite clearly, but it's hard to see his or her
>> face. That's
>> partly because the layer diffracts both incident and
>> reflected light,
>> so the light the fencer sees by is diffracted once, but the
>> light the
>> audience sees reflected from the fencer's face is
>> diffracted twice.
>> But it's also because the fencer is much closer to the
>> diffraction
>> grating, so whatever the angle of diffraction, the rays
>> reaching the
>> fencer's eyes have diverged much less than those
>> reaching the eyes of the audience.
> 
> I don't think there's much diffraction going on in a
> fencing mask.  The spacing is too big.

Diffraction occurs any time a wave encounters an obstacle. Wavelength
separation, optical distortion, and similar effects - what you're
probably thinking of as diffraction - would require spacing close to
the wavelength, true; but shading and scattering are also diffraction
effects.

> What's going on is that the mask shades
> the fencer's face while the rest of the room (including
> the mask) is well lit,

Yes, that's a diffraction effect. Some of the light incident on the
mask from outside is reflected back; some of it is reflected at
various other angles (scattered). Some of the light that makes it
through the mask and reflects off the fencer's face is scattered. The
result of those various diffraction effects is "shading".

> and the tiny visible bits of
> the fencer's face are hard to resolve from the bits
> of mesh next to them.

To some extent, but the human visual system is actually quite good at
dealing with that sort of noise.

I still believe that, possibly second to luminance, divergence is the
major factor here. Just trace some rays reflected off facial features
back through the mask and I think you'll see what I mean. If you're
close to the scattering layer, there's still a decent probability of
encountering rays reflected from various parts of the object. As you
move further from the scattering layer, rays reflected off parts of
the object's surface that aren't close to your normal to that surface
have to reflect at a progressively greater angle to *their* normals.
As that's less probable, you get fewer and fewer of those rays.

-- 
Michael Wojcik
Micro Focus
Rhetoric & Writing, Michigan State University