Traditionally, to simulate the illumination that comes
through the window, area lights are placed in the window. This method is
enhanced by using portal light which obtains its intensity and color from the
environment outside the window.
The portal light helps in a great deal reducing Final Gathering's noise and
since it's a light source it can cast shadow.
Here's a scene with a
room. I have set up an area lights for each window and a portal light is
connected to each light.
This is the spherical environment texture used for IBL as a
primary environment for Mental ray in HDR format. It's a very high resolution
texture. ( 6250x3125)
Using the default setting, this is the initial render
result.
RC
0.3 info : type
number per eye ray
RC
0.3 info : eye rays 2446614 1.00
RC
0.3 info : transparent rays 308765 0.13
RC
0.3 info : shadow rays 93660841 38.28
RC
0.3 info : wallclock 0:14:19.38 for rendering
RC 0.3 info : allocated 1463 MB, max resident 1476
MB
It took around 14 min to render. That's quite slow and
the Final Gathering was even on.
Let's turn off Lookup
Using FG Rays option and render again.
RC
0.3 info : type number per eye ray
RC
0.3 info : eye rays 2610573 1.00
RC
0.3 info : transparent rays 308765 0.12
RC
0.3 info : shadow rays 98771344 37.84
RC
0.3 info : environment rays 100692186 38.57
RC
0.3 info : wallclock 0:01:43.34 for rendering
RC 0.3 info : allocated 1449 MB, max resident 1462
MB
The render time's reduced dramatically. It only took 1m 43s.
So Let's stay away from using Lookup
Using FG Rays feature for now.
Now look at the wall on the left portion of the image.
You can see very fine grainy noise on the wall.
This is from using shadow threshold of 0.005 which is on
by default.
Let's set it to 0 and render again.
The render time's reduced again. It took 1m 35s and
visually there's no grainy noise.
Let's try to use a custom environment for the Portal
light Instead of using the high-res image with all the small and sharp details.
I scaled down (780x390) and blurred the env texture like below and use it as a
custom environment texture.
Be sure to flip the image horizontally since the custom environment node is flipped to the opposite of IBL.
Turn on Use Custom Environment and create and connect the
new Env.
The below is the render result.
RC 0.7
info : type number per eye ray
RC 0.7
info : eye rays 1631469 1.00
RC 0.7
info : transparent rays 308898 0.19
RC 0.7
info : shadow rays 84620960 51.87
RC 0.7
info : environment rays 84620960 51.87
RC 0.3
info : wallclock 0:01:16.66 for
rendering
RC 0.3
info : allocated 1473 MB, max resident 1486 MB
Without losing no noticeable quality, now the render time is
reduced to 1m 16s.
I think the render time's low enough and now I'll turn on FG
and render.
Here's the settings for FG which is moderate.
Accuracy : 128
Point Density : 0.5
Point Interpolation : 40
Secondary Diffuse Bounces : 2
RCFG 0.9 info : depth
#finalgather points
RCFG 0.9 info :
0 55670
RCFG 0.9 info : ray type number
RCFG 0.9 info :
eye 80909
RCFG 0.9 info :
transparency 1213
RCFG 0.9 info :
shadow 32187748
RCFG 0.9 info :
environment 32187748
RCFG 0.9 info :
finalgather 14031127
RCFG 0.9 info : balancing updated finalgather map
(55670 points)
RCFG 0.9 info : wallclock 0:00:56.67 for computing finalgather points
RCFG 0.9 info : allocated 1526 MB, max resident 1734
MB
RC 0.3
info : rendering statistics
RC 0.3
info : type number per eye ray
RC 0.3
info : eye rays 1455672 1.00
RC 0.3
info : transparent rays 308898 0.21
RC 0.3
info : shadow rays 73369984 50.40
RC 0.3
info : environment rays 73369984 50.40
RC 0.3
info : fg points
interpolated 1146406 0.79
RC 0.3
info : on average 73.51 finalgather points used per interpolation
RC 0.3
info : wallclock 0:02:05.18 for
rendering
RC 0.3
info : allocated 1527 MB, max resident 1734 MB
It took 2m
05s. This is a very acceptable result considering the initial render took 14min
even without FG.
I turn on
Lookup Using FG Rays option now and render again to get the same quality but
the render time's reduced by 9 seconds.
RCFG 0.9 info : depth
#finalgather points
RCFG 0.9 info :
0 55670
RCFG 0.9 info : ray type number
RCFG 0.9 info :
eye 80909
RCFG 0.9 info :
transparency 1213
RCFG 0.9 info :
shadow 32187748
RCFG 0.9 info :
finalgather 14031127
RCFG 0.9 info : balancing updated finalgather map
(55670 points)
RCFG 0.9 info : wallclock 0:00:53.93 for computing finalgather points
RCFG 0.9 info : allocated 1528 MB, max resident 1734
MB
RC 0.3
info : rendering statistics
RC 0.3
info : type number per eye ray
RC 0.3
info : eye rays 1455554 1.00
RC 0.3
info : transparent rays 308898 0.21
RC 0.3
info : shadow rays 73362432 50.40
RC 0.3
info : fg points
interpolated 1146288 0.79
RC 0.3
info : on average 73.51 finalgather points used per interpolation
RC 0.3
info : wallclock 0:01:56.23 for
rendering
RC 0.3
info : allocated 1528 MB, max resident 1734 MB
From my
tests, I found that Mental ray's default IBL environment doesn't work well with
the portal light especially when a very high-resolution texture is used.
I strongly
recommend using a custom environment for the portal light.
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