At
one time or another, many computer animation people have worked to
create an illusion of the night sky from earth or of its cousin, a
“star field”, which is an imaginary view of the stars from space.
Whether this was for their own amusement, for visual effects
purposes, or for scientific visualization, these innocents would
approach the problem with the assumption that it was going to be
easy. How hard could it be, its just a bunch of random white dots,
after all. Imagine their surprise when they discovered that doing
excellent starfields is far from trivial.
A
classic traditional technique to create starfields is to create a
cyc, or curved screen, painted black and with very small holes
punched in it. Then behind this screen was a curved light source,
usually florescent tubes. The camera would be at the center of the
implied sphere of the screen and when the room was darkened and the
backlight illuminated, you had a curved space of very bright, very
small light sources which could be photographed with long exposures
when the camera was moving. The result was excellent motion blurred,
perfectly antialiased, very high contrast star fields. But
ultimately there were certain moves that the motion control camera
could not easily do, such as tumble end over end for example, so
there was a need to synthetically generate these elements.
Another
time honored technique which looked excellent was the painting on
glass. Most of the times you saw stars in Close Encounters of the
Third Kind (1977) you were seeing an optical composite of a live
action element or motion control shot with a matte painting on glass.
Since
everyone seems to have to go through the same learning curve, I am
providing notes here for what some of the issues facing 3D technical
directors as they produce their first starfield and I have written it as a letter to my younger self.
September
19, 1983
Oh,
unwary traveler, so proud of your 3D knowledge, your knowledge of
geometric modeling, or of animation whether scripted or procedural,
and of global illumination; do you think to encompass the heavens
with these pathetic tools? Fool, your doom is assured. There are
more things in heaven and on earth than are encompassed in your
philosophy, or so I have heard, and when you approach the field of scientific
visualization you must unlearn what you have learned and embrace the
esoteric wisdom. You must open your eyes in order to see the light.
What
perils await the unwary, the arrogant, the unlettered?
The
first peril is the vast expanse of space. There is the scale of
mortal man, then the scale of the solar system, then the scale of one
single galaxy, and then beyond. These differences in scales are way
beyond what most software packages can handle, so using the 3D
positions of everything in a naive fashion is unlikely to work.
And that renderer you are so proud of. Does it do all its calculations of space in 64 bit floating point or even higher precision? Most renderers, with a few notable exceptions, do the majority of their work using single precision floating point which may be adequate for a giant robot or two, but falls apart in the vast distances of space.
The
second peril involves the issue of filtering of what is very untypical samples. Most scenes render surfaces with various lighting applied. But a great deal of what you wish to render are stars but what are
stars? Stars are huge things, but they are (for all practical
purposes) infinitely bright and infinitely small (on the screen).
The amount of energy concentrated in a single pixel may be immense,
but the pixel next to it may have very little or no energy at all.
And what happens under those circumstances when you move the camera?
Well, it aliases, of course, terribly. Furthermore, if one has
modeled stars very far away and you are using point sampling of one
form or another to simulate area sampling, then if you are not
careful, some of your samples will miss and you will have aliasing
again.
Part
of the solution is to use a good filter and lots of samples and in
the choice of filter lurks another threat since as we know a "good" filter, perhaps a 7x7 sinc for example, is
likely to have negative lobes, and instead of throwing those values
out, you should keep them until the end and even then you should not
throw them away. What then to do with them is a mystery left as an
exercise for the reader. The best solution of course would be to have a display that could absorb light as well as emit it, but we wait in vain for the display manufacturers to come to our aid.
And
what about those overly bright stars? Will you generate glows and
other artifacts? After all we are not just trying to simulate
realistic stars, we are often trying to simulate realistic stars as
the audience has seen them, and expects to see them.
Although
the sky is filled with stars, that is not the only thing that there
is. There are also great fuzzy areas known as nebulae and sometimes
other galaxies. It turns out that if there is any data for that, it
is likely to be volume data. But even if there is no data and you
create your own, volume rendering is the best way to render a nebula
one might argue. Does your renderer of choice do volume rendering?
Review
the following image of the earthling's galaxy.
Do
you notice the great areas of darkness? That of course is the
infamous "space dust", the so-called Interstellar Media or ISM which must surely exist to hide from us the center of our galaxy where no doubt an entity of great evil exists. Surely you do not think it a coincidence that the space dust would hide what is arguably the most spectacular sight in our little neighborhood? Since most
star catalogs do not have the ISM modeled, you may wish to develop a model of ISM in your spare time. If not, the galaxy will
not look right unless you simply leave out the stars that are in
those areas (which may or may not be be in the catalog anyway as they are impossible to view from earth, at least in the visible bands).
Because
you are rendering stars, no doubt you have studied scotopic vision. It goes without saying that whenever the biped mammals have watched the stars
they have, generally, been night adapted. And yet they see color
sometimes, perhaps they see Angry Red Planet Mars or Betelgeuse and they perceive the color red. How then are they seeing color? It may help the
seeker of knowledge to realize that “scotopic” is named for the
Skoptsy sect of religious devotees whose most notable doctrine is of
male castration. (see link below)
Of
course I am sure when you move the camera you will motion blur
everything. Oh yes, what do you plan to do with the speed of light
issue? I am sure you will come up with something.
So,
foolish mortal, you have been warned.
These are just the first of the issues you must address for a proper starfield.
Fools may go where wise people fear to tread.
Sincerely,
A Friend.
___________________________________________________________
Scotopic
Vision
The
Skoptsy
Close
Encounters of the Third Kind (1977) on IMDB
