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This task shows how to tune your lighting parameters and shooting quality
using a special (specular) case of global illumination called
caustics.
Caustics and global illumination are to be set independently from each
other since they are used in different ways and thus, require different
parameters. |
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Open the
CornellBox.CATProduct document. |
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Before starting to define indirect illumination parameters, make sure that
direct lighting parameters (intensity, attenuation, etc.) have been
correctly set by generating quick renders to check the result. |
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Access the light properties (by
right-clicking the light item in the specification tree then selecting
Properties) then access the Indirect Illumination
tab:
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Emit Photons is
already selected for the current sample document but note that it
is deactivated by default. |
What are caustics?
Caustics can be defined as small localized lighting effects
created by
specular light interactions (reflection or refraction) onto a
diffuse surface.
A typical example of caustics would be the light pattern cast on
the bottom of a swimming pool by the wave pattern on the
(refractive) water surface.
The two pictures below show examples of
caustics: |
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Although caustics are a
kind of global illumination, they are treated separately to allow
finer control and improve performance. |
How does it work?
Caustics and global illumination have identical photon map
methods but different parameters. Refer to
Using Global Illumination
for detailed information. |
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In the Intensity Factor box,
indicate the multiplication factor of the light intensity associated to
each photon, using either the slider or by entering a value directly in
the box.
By default, direct and indirect lighting have the same
multiplication factor which is equal to 1. |
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In the Caustic Photons box,
indicate the number of photons to be stored in the photon map. This value
controls the rendering quality of caustics.
Note that too small values tend to
create blurry caustics. On the contrary, high values provide a
better result but increase the duration of the rendering phase. |
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Caustics generally require a
lower number of photons than global illumination. |
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Click
OK to validate your lighting parameters then click Create
Shooting
to define the shooting parameters in the Indirect Illumination
tab:
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This tab lets you define caustics parameters for
rendering purpose. |
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In the Caustics area, click Active
to activate caustics and activate the other two boxes.
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In the Photons box, indicate for
each pixel the number of photons to be searched in a specified area
(defined in the Maximum radius box) in order to compute the
local intensity of caustic brightness.
The default value is 100, the highest value is
1000.
Note that increasing the number of photons makes
caustics look less noisy. However the higher the value, the
blurrier the result and the longer the rendering phase. |
As for
lighting properties, a number of photons lower than the one
required for global illumination is needed. |
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Enter the Maximum radius (in
millimeters) to set the limits of the area in which the photons are
searched to compute the local intensity of caustics` brightness.
A null value means that the radius is computed from the scene
extent. |
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Click
Do not use ambient in the Material area if you do
not want the material ambient coefficient to be taken into account for
indirect illumination.
The ambient coefficient corresponds to the light intensity
diffused in any direction by the object, even if not lit by any
light source. This parameter affects the whole object, including
the shadowed area. |
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Click
OK to validate your parameters then render the shooting using
Render Shooting
(you can also right-click the Shooting item in the specification tree
then select Render Single Frame) to check the result.
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When setting your parameters, it is recommended
to start with low values since the less photons, the quicker the
rendering phase (but the lower the quality). You can then increase the
values progressively to tune the result.
The following are samples illustrating the various results you can
obtain for caustics according to the values you defined (we have kept a
null maximum radius and a realistic falloff for each of the four
examples):
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Intensity Factor = 10
Caustic Photons = 60 000
Photons = 100 |
Intensity Factor = 3
Caustic Photons = 60 000
Photons = 100 |
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Intensity Factor = 3
Caustic Photons = 200 000
Photons = 500 |
Intensity Factor = 1
Caustic Photons = 200 000
Photons = 500 |
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