Sample Lighting 2005-04-26 - By Harvey White
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On Tue, 26 Apr 2005 20:42:52 -0000, you wrote:
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>Yes, I know... I'm a newbee: here's another stupid question:
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>I'm thrilled by the fabulous lighting that Carrara provides. No photo studio
could do a job
>like that. And thats what keeps me hangin' on to this software that really
tries me hard
>every now and then.
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>How come that I have to look at the "Sample lighting" thermometer for an
eternity with a
>very simple Global Illumination skylight (just a typo logo). The render itself
take less than
>30 secs, but the sampling about 6 mins.
As I understand it (and I could be wrong)
Normal photorealistic rendering takes the camera as a viewpoint, then
looks out at every possible object within range, it computes what
objects are "in the way" and picks the color coming towards the camera
from the surface characteristics of the first object. It then looks
to see how light might bounce off that object, and looks in that
direction, back towards what might be contributing light that falls on
that object... it keeps doing this until it's told to stop by the
setting on "maximum ray depth" The lighting on any one pixel of any
object is determined also by whatever lights are within range. The
key here is to think of it as bouncing a stone off a pond, with the
stone skipping from place to place. It is only one path....
As I understand GI (global illumination), it attempts to calculate
things differently. Not only does it do the calculations in the
normal photorealistic editing above, but it also takes into account
light coming from the sky from all directions, not just the direction
that would be produced by a surface bounce. It also tries to take
into account the light bouncing from other objects, but not exact
reflections. It's far more accurate, but it involves trying to figure
out where light is coming from, and then (perhaps) replacing each
*object* that is bouncing light off it by a computed light source.
Those calculations are what's taking the time.
Now if you want to take a lot more time to compute an image, add
caustics to this mix. Caustics actually computes the effective path
of light within glass (or any other transparent object), allowing for
both the color and the index of refraction. This is where the classic
"light through colored spheres" picture comes in.
To do a general pointer on an image:
1) the less maximum ray depth, the faster
2) the object and shadow accuracy affect render time (greater accuracy
= slower).
3) Antialiasing (jaggies) takes time, the better takes more time.
4) GI takes time, the higher the photon count and photon map accuracy,
etc., the more time.
5) Caustics takes time, same as GI
6) In animation, Motion blur takes more time because of the need to
compute intermediate frames.
Hope that this helps a bit. The really nice thing about Carrara's
renderers is that you can pick the level of accuracy you want, so
inaccurate is ok for composition, and when you start to look at
lighting effects, you may want to go for more accurate.
Harvey
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>I have set all parameters as low as possible according to the advice in the
manual (yes, I
>read manuals).
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>kindly, jj
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