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Rendering an animation with moving objects
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Search Keywords:
animation, moving object, VRaySphereFade, compositing
Note: the scene we use for this tutorial is based on the
Sponza Atrium scene, modeled by Marco Dabrovic (http://www.rna.hr)
and is one of the models for the
CGTechniques Radiosity
competition.
General
In this tutorial we will render an animation with moving objects and GI
with VRay. The goal is to make the process as efficient as possible. We can
always use a high-quality GI solution to render an animation without any
special tricks, however this will typically take very long. We would like to
optimize this process as much as possible.
Typically, we have a small number of moving objects (f.e. a few
characters) in some more complex environment. Our main optimization idea
uses this fact: we can split the animation into a background part, and
animation part.
Since the background does not move, we can render it using a walk-through
animation method, for example as pointed out in
this tutorial. Then we can render out the moving objects, and composite
these for the final animation.
Compositing basics
Before we go into details on rendering the animation, we will cover some
compositing basics.
Our task is this: given an object in a 3d scene, we would like to
composite it over a given background, taking into account as many effects as
possible (shadows, reflections, GI etc).
This is typically done with the help of two layers with the 3d object,
which are then composited over the background. We will call one layer a
mask layer, and it determines which parts of the
background are altered by the 3d object, and which parts remain the same.
The mask layer is multiplied by the background. We will call the second
layer object layer, and it determines what color
must be added to the masked background in order to get the final image. The
object layer is added to the masked background.
So, in general, the compositing formula is this:
final_image = background_layer * mask_layer + object_layer
Our background layer is given, and we don't have to worry about it. It
may be a photograph, live footage, or a prerendered image. We must determine
only the mask layer and the object layer. We will compute these using three
separate renderings of the 3d object:
- a render without the object; we will call this "pure" render;
- a render with the object, but with a perfectly black material applied to
the object; we will call this "black" render;
- a render with the object with a normal material; we will call this
"normal" render.
Given these three renders, we can compute the mask layer and the object
layer in this way:
mask_layer = black / pure;
object_layer = normal - black;
Then, we can use the mask and object layers to composite the final image.
Compositing in 3dsmax
Now, let us apply all this theory in practice. We will use 3dsmax for
compositing, but this can be done in any compositing program that supports
the necessary image operators (addition, difference, multiplication and
division) - for example, Digital Fusion.
Unfortunately, 3dsmax does not have the necessary compositing operators
built-in, so we are going to use an additional plugin, the
VRayCompTex texture map, which you can download
here (extract the .dlt file in the \plugins\vrayplugins\
folder of 3dsmax).
This is the background plate that we will use:
We are going to add a teapot in the middle of this rendering.
1.1. Open the starting scene, which can be found
here. The scene is not exactly the same as the one that was used for the
background plate, but it is close enough for the composition. In general,
you should try to match the plate as best as possible. If the background
plate is a rendered 3d scene, then it would be best to use the same scene.
1.2. Render the scene and save it as normal.png,
with 48 bits per pixel (for increased precision during the compositing later
on).
1.3. Open the material editor, create a new VRayLightMtl and assign a
pure black color to the material - or you can use the
black mtl material in the ME. Apply the material to the teapot in the
scene.
1.4. Render the scene and save it as black.png
with 48 bits per pixel (for increased precision).
1.5. Hide the teapot.
1.6. Render the scene and save it as pure.png
(again, with 48 bits per pixel).
This is all the work we have to do with the 3d scene. Now we are going to
composite the final result.
1.7. Open 3dsmax with an empty scene.
1.8. Open the Material Editor and put the four images (normal.png,
black.png, pure.png and the background) into four separate bitmap textures;
set the mapping mode of the bitmaps to Screen
environment.
Now we will create the mask and object layers.
1.9. Create a VRayCompTex and name it
object.
1.10. Drag the normal bitmap onto the
Source A bitmap slot in the
VRayCompTex (choose the
Instance method when prompted).
1.11. Drag the black bitmap onto the
Source B bitmap slot in the
VRayCompTex (again, choose the
Instance method).
1.12. Set the Operator of the VRayCompTex
to Difference. This is what you should get:
Now we have our object layer. Next, we need to create the mask layer.
1.13. Create a new VRayCompTex in an unused slot of the material editor
and name it mask.
1.14. Drag the black bitmap onto the
Source A bitmap slot in the
VRayCompTex (choose the
Instance method when prompted).
1.15. Drag the pure bitmap onto the
Source B bitmap slot in the
VRayCompTex (again, choose the
Instance method).
1.16. Set the Operator of the
VRayCompTex to Divide.
This is what you should get:
Now we have the mask and object layers for the final compositing. We need
to combine these with the background to get the final image.
1.17. Create a new VRayCompTex in an unused
slot of the material editor and name it masked bg.
1.18. Drag and instance the mask texure into
the Source A slot of the VRayCompTex map.
1.19. Drag and instance the background bitmap
into the Source B slot of the VRayCompTex
map.
1.20. Set the Operator of the
VRayCompTex to Multiply.
Now that we have the masked background map, we need to add the object map
to it.
1.21. Create a new VRayCompTex in an unused
slot of the material editor and name it final.
1.22. Drag and instance the masked bg map into
the Source A slot of the
VRayCompTex.
1.23. Drag and instance the object map into
the Source B slot of the
VRayCompTex.
1.24. Set the Operator of the
VRayCompTex to Add.
This is our final composite texture map. Here is a schematic view of how
we have connected the four input images:
Now we need to render that map in order to get the final image.
1.25. Open the Render scene dialog and in
the Common Tab set the resolution to 600x600
(the resolution of all our images in this section).
1.26. Set the current renderer to Default scanline.
1.27. In the Renderer tab, turn off the
Antialiasing option.
1.28. Turn off the
Filter maps option.
1.29. Open the Environment dialog and
assing the final map as an environment map.
1.30. Render the scene:
Here is the final scene for the composite that you should get if you
followed all the steps correctly.
The beauty of using this method is that it can composite properly all
effects (reflections, shadows, GI etc), without having to worry about any
matte/shadow stuff or anything in 3dsmax.
The VRaySphereFade plugin
atmospheric
We will use the same workflow as defined above, to render our entire
sequence. However, we have to render our animation three times. With more
heavy scenes, this means a lot of rendering. The correct calculation of GI
effects requires all of the scene geometry, so we can't just remove it to
speed things up.
Most of the effects that we are interested in (shadows, reflections, GI)
appear near the animated object in the scene. Ideally we would like to
render only those parts, and skip re-rendering the entire image.
This is where the VRaySphereFade atmospheric
comes into play. It allows you to render only that part of the scene, which
is located near the animated object. The rest of the scene is shown as a
solid grey color. However GI, reflections, shadows etc. are still computed
using the entire scene, so that they produce the correct effect. You can
download the plugin here (extract the .dlr file to the \plugins\vrayplugins\
folder of 3dsmax).
Here is how we would use the VRaySphereFade
plugin in the scene from the previous part.
2.1. Open the starting scene which can be found
here.
2.3. Create a spherical gizmo object around the teapot. Typically, you
will surround each animated object with a gizmo linked to that object:
2.2. Open the Environment dialog and add the
VRaySphereFade atmospheric.
2.4. Click the Pick button in the
VRaySphereFade parameters and then click the
newly created gizmo.
2.5. Go to the VRay
System rollout and check the Optimized
atmospheric evaluation option - this will speed up a lot the
rendering of the areas masked by the atmospheric.
2.6. Render the scene:
You can see how only the parts of the scene inside the gizmo are visible
to the camera. Note the grey color - because we are computing our mask layer
through division, it is not a good idea to have pure black colors in the
"normal" image.
Proceed as in the previous step to create the pure, black and normal
renders, and to composite them onto the background. The result is this:
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Normal layer |
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Black layer |
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Pure layer |
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Object layer =
Normal - Black |
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Mask layer =
Black / Pure |
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| Final composite over the background plate |
It is quite close to the result in the previous section, and it was a lot
faster to calculate. Here are comparisons between the two composites that we
got, as well as a rendering of the original scene with the teapot inside it:
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| Original rendering of the scene with the teapot |
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| Full-scene compositing |
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| Compositing with VRaySphereFade |
Using the VRaySphereFade plugin, we can
control how much an object will affect its surroundings. For GI effects, it
is usually enough to limit the influence not far away from the object.
However, some effects (like long shadows, or reflections on surfaces at
grazing angles) may require larger spheres of influence. An alternative in
that case would be to add more gizmos to capture those effects, or to render
them in a separate pass and composite them separately.
Workflow
Now we have all the necessary information to render our animation. This
is done in 4 steps:
a) Render the background as a walk-through animation, without the
animated objects.
b) Create and link gizmos to the animated objects, create a
VRaySphereFade atmospheric and add those gizmos
to the atmospheric.
c) Render the animation with the moving objects normal, black, and hidden
respectively into three separate animations.
d) Composite the background animation out of the four other animations.
Rendering the layers
a) First, we need to render the background template which will serve as a
basis on the moving characters. We will do this similarly to how it is
described in the
Walk-through animation tutorial. We will calculate the light cache
first.
3.1. Open the starting background
scene.
We have rendered all animations at a resolution of 900 x 500 pixels. This
may take quite a while (it took more than two days to render the three
character animations when preparing this tutorial), so you may want to use
lower resolution, for example, 450 x 250.
3.1a. In the Common tab of the Render scene dialog, set the rendering
resolution to 450 x 250 pixels.
3.2. Turn off Default
lights in the
Global switches rollout.
3.3. Set the Antialiasing to
Adaptive QMC with the default settings of
1/4.
3.4. Turn on GI
and set the primary and secondary engines to Light cache.
3.5. Set the light cache Subdivs to
4000, Sample size
to 4.0 and Scale
to World.
3.6. Switch the lightcache Mode to
Fly-through and for display purposes turn
on Show calc. phase.
3.7. Turn on Use light
cache for glossy rays and set Interp. samples
to 5.
3.8. In the GI Environment group of the
Environment rollout, turn on the
Override MAX's option and set the
Multiplier to 2.0.
3.9. Render any frame of the animation to calculate the ligth cache.
3.10. Save the light cache and change the Mode
to From file with the just saved file.
Once we have the light cache ready, we need to calculate the irradiance
map.
3.11. Change the Primary GI engine to
Irradiance map with High
preset.
3.12. Set the Noise threshold in the
QMC Sampler rollout to 0.002.
3.13. Turn on the
Don't render final image option in the
Global switches rollout.
3.13. Set the irradiance map Mode to
Multiframe incremental.
3.14. Render every 10th frame of the animation
to calculate the irradiance map.
3.15. Change the irradiance map Mode to
From file with the just saved file.
3.16. Turn off the
Don't render final image in the
Global switches rollout.
3.17. Render every 1st frame of the final
background animation with the saved GI solutions and save the animation as a
48bit .png sequence.
Now we have a ready background template that will be populated with
the moving objects.
Here is the
end scene for stage a).
b) Now we will use sphere gizmos and VRaySphereFade
atmospheric to isolate the moving objects in the scene, so we can composite
them later with the ready background.
3.18. Continue with the background scene from part a) and unhide the four
biped characters in the scene.
3.19. 
 Create four
sphere gizmos with radius of about 80.
3.20.  Align
each of them to each Biped COM object (Bip01,
Bip02, Bip03,
Bip04).
3.21. Open the Environment dialog and add
the VRaySphereFade atmospheric.
3.22. Add the four gizmos in the list.
Here is the
end scene after the gizmos setup.
c) For the rendering of the three animations we will use QMC GI as a
primary GI engine to avoid flickering problems on and around the characters.
3.23. Change the Primary GI engine to
QMC GI with 25 subdivs.
3.24. Change the light cache Subdivs to
3000.
3.25. Change the light cache Mode to
Single frame.
3.26. Go to the
System rollout and turn on the
Optimized atmospheric evaluation option -
this will accelerate the calculation considerably.
First, we will render the normal animation.
3.27. Render the whole sequence to 48-bit .png files.
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| Frame 145 |
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| Frame 268 |
Second, we will render the black animation.
3.28. Apply a black VRayLight material to the four biped characters and
render the entire sequence to 48-bit .png files.
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| Frame 145 |
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| Frame 268 |
Third, we will render the pure animation.
3.29. Hide all the biped characters and render the whole sequence again.
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| Frame 145 |
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| Frame 268 |
Now we have all the necessary animations to composite the final
animation.
Final compositing
In this part we will amalgamate the normal, black and pure animations
with the help of the VRCompTex map.
4.1. Open a new scene in max and open the Material editor.
4.2.  Load the
normal, black and pure animation sequences in three different slots and name
them "normal", "black"
and "pure".
4.3. Load also the backgroud animation that we rendered before.
4.3. Change the display type of all four textures to
Environment with Screen mapping.
4.4. Make
four VRayCompTex maps into four unused slots.
4.5. Drag-drop the normal map into
Source A, the black
map into Source B, change
Operator to Subtract
in the first VRayCompTex map and name it
object.
4.6. Drag-drop the black map into
Source A, the pure
map into Source B, change
Operator to Divide
in the second VRayCompTex map and name it
mask.
4.7. Drag-drop the mask map into
Source A, the background
map into Source B, change
Operator to Multiply
in the third VRayCompTex map and name it
masked bg.
4.8. Drag-drop the masked bg map into
Source A, the object
map into Source B, change
Operator to Add in
the fourth VRayCompTex map and name it
final.
We have just followed the scheme that we described earlier in the
Compositing with 3dsmax section.
4.9. Drag-drop the final map into the
Environment map slot of the
Environment dialog of 3dsmax.
4.10. Change the renderer to Scanline.
4.11. In the Renderer Tab of the
Render scene dialog, switch
off Antialiasing and
Filter Maps.
4.12. Render the final animation.
Here is the
compositing scene.
Here is a link to the final rendered animation at the original
900x500 resolution (5 MB), and
here is a link to a smaller version (2 MB).
Let is look at one frame of the animation:
You can notice that the character shadow for the metallic biped is
cut off where its sphere gizmo ends. This is because we used somewhat
smaller gizmos when we rendered the animation, in order to save render
time. To make the full shadow visible, you can either enlarge the gizmo
for that character, or use another gizmo near the ground, specifically
for the purpose of capturing the shadow. If we have lights that cast
long shadows, or the shadows are far away from the object that casts
them, it might be better to render the shadows in a separate pass and
composite them additionally. The same is true for reflections of the
animated objects.
Conclusion
We did a lot of extra work to render out our animation - setting up
gizmos, rendering four different animations and compositing them. Why was
this necessary and do we really have to go through all these troubles to
render any animation?
If we are going to composite the animation against live-action footage,
then we don't really have a choice other than to use compositing, in which
case there is no way around rendering all the different passes.
However, when we are rendering a fully CG animation, the only thing that
we save is render time - which is the single point of the whole exercise. Of
course, if we had a large render farm then we could have simply rendered out
each frame of the animation as it is, with high enough settings to obtain a
stable GI solution from frame to frame. If the scene environment is simple
enough, this would be the best and fastest thing to do. However, if we don't
have a render farm or the scene is quite detailed, then we can drastically
reduce the render times by avoiding re-rendering of the entire scene's
background just because we have one or two moving characters in there.
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