Motion Blur parameters?

Motion Blur parameters?

Post by Alex I. Sega » Fri, 07 Apr 1995 04:00:00



I'm wondering, which are dependencies between Shutter and MotionBegin parameters?
Does Shutter allow setting reduced influence of Motion transformations as _RC_
declares it? I mean that Shutter 0.0 0.25 is expected to reduce motion tails twice
vs Shutter 0.0 0.5. And what about MotionBegin parameters? I saw several .RIBs
generated by Pixar Typestry, they all had equal Shutter - MotionBegin values?
Is there some deep sense? Maybe there are some implementation-dependent things?

Alex
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Motion Blur parameters?

Post by Antoine Du » Fri, 07 Apr 1995 04:00:00



: I'm wondering, which are dependencies between Shutter and MotionBegin parameters?
: Does Shutter allow setting reduced influence of Motion transformations as _RC_
: declares it? I mean that Shutter 0.0 0.25 is expected to reduce motion tails twice
: vs Shutter 0.0 0.5. And what about MotionBegin parameters? I saw several .RIBs
: generated by Pixar Typestry, they all had equal Shutter - MotionBegin values?
: Is there some deep sense? Maybe there are some implementation-dependent things?

  A current limitation of PRMan's motion blur is that it can only motion
blur between two clearly defined positions or shapes.  Therefore, The
MotionBegin statements all have to be MotionBegin [0 1], and the shutter
values also have to be 0 and 1.
 You are correct that you should be able to tailor the motion blur with
the Shutter statement.  The motion blocks define positions and shapes at
various times, and you should be able to interpolate between them with
the Shutter values.  Alas, PRMan didn't completely implement the spec in
this regard.

-- Antoine
_______________________________________________________________________________


_______________________________________________________________________________

 
 
 

Motion Blur parameters?

Post by coo.. » Sat, 08 Apr 1995 04:00:00


 I don't use renderman, but always loved that lightspeed effect in Star Wars where the
stars stretch out then streak past the windshield.  

  I wonder if anybody knows how to create an effect where motion blur is combined
with some form of ray-traced refraction shifting whereby objects either to directly in
front or behind are unaffected, but objects rendered at greater and greater divergence
from forward or behind become severely elongated.  Sort of like taking an imaginary
sphere the world is projected on and stretching it severly towards the rear.

Remember in SW, the stars strretch out BEFORE they begin to move.  Anyhow, if someone
came up with a renderman shader to do this, the next SW film would have some friggen
spectacular jump to lightspeed effects.  I figure with a "hyperspace" shader, you could
show the streaking effect from any angle, not just dead ahead with the old optical
effect.  The streaking stars could be placed randomly through 3D so when you move

through them they shower past in perspective.

 
 
 

Motion Blur parameters?

Post by Larry Gri » Sat, 08 Apr 1995 04:00:00




Quote:

>Shutter open/close times must correspond to MotionBegin values.  Currently, the two
>publicly available renderers that support this feature (rendrib and prman) limit
>you to two samples in a MotionBegin/End block, so don't bother giving them more...

You're right about PRMan -- two samples, and they must match
everywhere (all MotionBegin statements and the Shutter command).
However Antoine seemed to imply that the times had to be 0 and 1,
which is incorrect.  Any numerical values can be used (obviously with
opentime <= closetime).

BMRT is almost as restrictive, but not quite.  Rendrib is also limited
to two times, but the shutter times don't have to match the motion
times.  Also, all transformations for any one primitive must match the
same two times, but different primitives may have different motion
times.  And the camera doesn't have to have the same two times as the
geometry.  I haven't particularly found a use for this flexibility,
but it just happens naturally the way I implemented keeping track of
the motion.

Another important thing to note is that PRMan will blur both
transformations and certain geometric parameters.  BMRT will only blur
transformations, not geometry.  (You can *ask* it to blur geometry,
but it will use the first primitive in the motion block.)

        -- lg

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Motion Blur parameters?

Post by Michael B. Johns » Sat, 08 Apr 1995 04:00:00



>>I'm wondering, which are dependencies between Shutter and MotionBegin parameters?
>>Does Shutter allow setting reduced influence of Motion transformations as _RC_
>>declares it? I mean that Shutter 0.0 0.25 is expected to reduce motion tails twice
>>vs Shutter 0.0 0.5. And what about MotionBegin parameters? I saw several .RIBs
>>generated by Pixar Typestry, they all had equal Shutter - MotionBegin values?
>>Is there some deep sense? Maybe there are some implementation-dependent things?

Shutter open/close times must correspond to MotionBegin values.  Currently, the two
publicly available renderers that support this feature (rendrib and prman) limit
you to two samples in a MotionBegin/End block, so don't bother giving them more...

--

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-->  MIT Media Lab,  Computer Graphics & Animation Group
-->  20 Ames St. E15-023G, Cambridge, MA 02139

 
 
 

Motion Blur parameters?

Post by Francisco X DeJes » Tue, 11 Apr 1995 04:00:00



> I don't use renderman, but always loved that lightspeed effect in Star Wars where the
>stars stretch out then streak past the windshield.  

>  I wonder if anybody knows how to create an effect where motion blur is combined
>with some form of ray-traced refraction shifting whereby objects either to directly in
>front or behind are unaffected, but objects rendered at greater and greater divergence
>from forward or behind become severely elongated.  Sort of like taking an imaginary
>sphere the world is projected on and stretching it severly towards the rear.

[...]

While not Renderman-specific, isn't the simplest approach using geometry
for the stars and scaling them towards the camera?
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Motion Blur parameters?

Post by Michael B. Johns » Wed, 12 Apr 1995 04:00:00


hmm... this might not answer the question, but it might help the thought
process...

When thinking about how to make a certain kind of image, I always find
it useful to think about if something is (for want of better terms)
an *artifact* or an *effect*.  

For example, I spent a signficant portion of time (3 + 10 months)
working on Mike McKenna & Bob Sabiston's animation "Grinning Evil Death",
and I did all the motion blur *effects* and caused the depth of field
*artifacts*, using my recursive camera compositor.  

Here's what I mean: Dave Sturman, for his qualifying exam in the Ph.D.
program here at the Media Lab, did some interesting depth of field
experiments.  When I built my recursive camera code (a "useful fiction"
for building a digital multi-plane camera), I used Dave's code to let
me specify normal 35mm camera parameters to my software so I could use
my still photographer's intuition to relatively position plates and
do rack focus.  When it came time to do motion blur, though, I designed
a bunch of filters that I thought "looked right" and tweaked various
things to generate image sequences that I thought worked.  The depth
of field effects were artifacts, the motion blur was an effect.

On the other hand, I recently designed and executed two shots for a really
nice short film called "Dizzy Horse", by Gary Cohen.  In that piece, there's a
ring flying at, through, and away from the camera, where the camera is doing
a follow focus on the ring as it moves through a forest.  In that piece,
the depth of field is an effect, but the motion blur is an artifact.

What I mean is this: in my animation system, the ring is moving continuously,
over the course of 7 seconds.  I set up a camera to shoot the ring for
two shots, each 3 seconds, where the camera was sampling 24 times per second,
and its shutter was open 1/48 of a second.  This spit out the appropriate
RIB file which prman rendered (with the appropriate motion blur).  The
motion blur here was an artifact.  On the other hand, the depth of field
(i.e. the follow focus) was done for me by Greg Zinn at DuArt using some images
I had shot on 35mm (scanned to PhotoCD) by blurring them using Alias
Eclipse on an SGI using start and end points I gave him.  I then took
those images off tape and composited them with the motion blurred rendered
ring.

In this case, the depth of field was an effect, but the motion blur was
an artifact.

This is all a long-winded way of saying that you might want to think
about how to do your SW streaky stars; and think about if you have
to do it as an effect, or if you can figure out some process whereby
it's an artifact (which is always easier, I think).  Of course, one
man's artifact is another's effect, but getting the right perspective on that
can be really useful...  

For example, I would think (off the top of
my head, probably wrong) is that the streaky stars were originally done
as a slit scan shot with an old analog optical printer.  One easy way
to do them digitally is by compositing two shots of the same scene; one
rendered strobe and one rendered motion blurred.  That's a popular
party-shooting strategy (check out the rave pics off my home page),
where you shoot with a strobe but leave the shutter open for a second
or two.

In all cases, I doubt that this is a custom shader issue, I would think it
would be a matter of rendering with a variety of shutter speeds and
compositing.

Anyway...
--

-->  http://wave.www.media.mit.edu/people/wave/
-->  MIT Media Lab,  Computer Graphics & Animation Group
-->  20 Ames St. E15-023G, Cambridge, MA 02139

 
 
 

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