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VRay Rendering Surface Interfaces
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General
In this tutorial we will discuss the rendering of refractive surface
interfaces with VRay. A typical example of this is the rendering of a
liquid in a glass container. The problem is that we have two separate
objects, which share the exact same surface boundary.
From a modeling point of view, it is very difficult to ensure that the
two objects have the exact same boundary geometry. While this can be
achieved (although with difficulty) for static scenes, things become very
problematic in animation.
From a rendering point of view, if the two surfaces match exactly, the
renderer cannot distinguish between them accurately, which may lead to
errors in the rendering.
Considering these two issues, it is obvious that we will need to separate
the surfaces, or to model the surface interface separately. We have the
following three options:
- The liquid and the container do not touch and have a gap between them.
This approach is simple to implement and does not require any special
support from the renderer. However, it does not produce a reallistic result.
This is because the gap between the two surfaces changes the rendering
drastically and makes the liquid look like a solid block.
- The liquid overlaps the container. This approach potentially produces
much more realistic results, but it requires special support from the
renderer. This is because the renderer must keep track of which surfaces
have been intersected along a ray path, so that it can compute the correct
index of refraction. Without this built-in support, this method cannot be
used. Luckily, VRay supports the necessary framework for this, which makes
this approach the preferred one.
- The third approach is to model the interface separately. In general,
this is difficult to implement - especially in animations. Further on, with
this approach introduces a third object in the scene with a separate
material, which makes things more complicated. Also, it will not work very
well with things like fog in the glass - since in that case, VRay cannot
make the connection between the glass/liquid material, and the surface
interface material, and cannot apply the correct fogging value.
Below, we will discuss the first two methods only; the third method is
more difficult and we do not recommend it with VRay.
Initial rendering
1.1. Open the starting scene, which can be found
here.
1.2. Assign VRay as the current renderer.
1.3. To make the previews of the scene faster, go to the
Image sampler rollout and set the
Image sampler type to
Fixed.
Next, we want to get some fast GI:
1.4. In the
Indirect illumination rollout, turn
GI on and set both
the Primary and
Secondary GI engines to Light cache.
1.5. Turn off the
Refractive GI caustics. We will add photon-mapped caustics later on
for the final image.
1.6. In the
Light cache rollout, set the light cache Sample
size to 0.04 for less noisy light cache
samples.
1.7. Set the light cache Filter mode to
Fixed and set the Filter
size to 0.08 since we will display the
light cache directly.
1.8. Turn off the
Store direct light option for the light cache, since we want to
compute direct lighting separately.
1.9. Set the light cache Subdivs to
500.
1.10. Optionally, turn on the frame stamp from the
System rollout.
1.11. Render:
This shows the container that we will fill with liquid in the next
section.
Rendering the liquid
Now that we have the container, we have to add the liquid.
2.1. Unhide the "liquid inside" object. If you look at the from the
Front viewport, you will notice that the object
is slightly inside the glass container, without touching it:
2.2. Render:
Although the distance between the container and the liquid is really
small, the rendering does not look realistic - it looks like we have a
glass with a solid block inside it. To avoid this, we will make the
liquid to overlap the container a little bit.
2.3. Hide the "liquid inside" object and unhide the "liquid overlapped"
object. In the Front viewport you can notice that
this object overlaps the container a little bit:
2.4. Render:
Now it looks a lot better; the liquid really seems to be touching the
glass.
Final rendering
For the final rendering we would like to improve the antialiasing, and to
add some caustics.
3.1. Turn Caustics on
from the
Caustics rollout.
3.2. Set the Max. density to
0.2 - we want to limit the caustics photon
density, since this allows us to shoot more caustics photons for a smoother
caustics effect.
3.3. Set the Search distance to
2.0. Typically values of 5 to 10 times the
Max. density parameter work well.
3.4. Set the Max. photons parameter to 0 -
this will cause VRay to consider all photons within the search distance
from the shaded point.
3.5. Render:
Rendering now takes a little more time, since VRay needs to compute
the caustics as well.
We have caustics now, but they appear quite noisy. We can reduce the
noise by increasing the Caustics subdivs
for the light.
3.6. Select the VRayLight object.
3.7. Right-click in the viewport and select "VRay properties..." to
bring up the VRay
Light settings dialog.
3.8. Set the Caustics subdivs to
4000.
3.9. Render:
Caustics look better now and these are the settings that we will use
for the final rendering. If you want even smoother caustics, further
increase the Caustics subdivs of the
light and/or increase the Search distance
parameter.
3.10. In order to avoid recomputing the caustics every time for the next
renderings, save the caustics photon map to file, set the
Caustics mode to From
file, and select the saved caustics map with the
Browse button.
Now we need to improve the antialiasing and to reduce the noise from
the area light.
3.11. In the
Image sampler rollout, set the Image sampler
type to Adaptive QMC.
3.12. In the
QMC Sampler rollout, set the
Noise threshold to 0.002.
3.13. Set the Global subdivs multiplier to
8.0 - this will reduce the noise from the area
light.
3.14. Render:
This is our final image.
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