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Radiosity (x)
Most rendering models, including
ray-tracing, assume a simplified spatial model, highly optimised for
the light that enters our 'eye' in order to draw the image. You can
add reflection and shadows to this model to achieve a more realistic
result. Still, there's an important aspect missing! When a
surface has a reflective light component, it not only shows up in
our image, it also shines light at surfaces in its neighbourhood.
And vice-versa. In fact, light bounces around in an environment
until all light energy is absorbed (or has escaped!).
Re-irradiated light carries information about the object which has
reirradiated it, notably colour. Hence not only the shadows
are 'less black' because of re-irradiated light, but also they tend
to show the colour of the nearest, brightly illuminated, object.
A phenomenon often referred to as 'colour leaking'
(Figure 1).
Figure 1. Radiosity example
In
closed environments, light energy is generated by 'emitters' and is
accounted for by reflection or absorption of the surfaces in the
environment. The rate at which energy leaves a surface is called the
'radiosity' of a surface. Unlike conventional rendering methods,
radiosity methods first calculate all light interactions in an
environment in a view-independent way. Then, different views can be
rendered in real-time.
In Blender, Radiosity is more of a
modelling tool than a rendering tool. It is the integration of an
external tool and still has all the properties (and limits) of external
tools.
You can run a radiosity solution
of your scene. The output of such Radiosity solution is a new Mesh Object with vertex
colors. These can be retouched with the VertexPaint option or
rendered using the Material properties "VertexCol" (light color) or
"VColPaint" (material color). Even new Textures can be applied, and
extra lamps and shadows added.
Currently the Radiosity
system doesn't account for animated Radiosity solutions. It is meant
basically for static environments, real time (architectural)
walkthroughs or just for fun to experiment with a simulation driven
lighting system.
The Blender Radiosity method
First, some theory! You can skip to next section if you like, and get
back here if questions arise.
During the late eighties and
early nineties radiosity was a hot topic in 3D computer graphics.
Many different methods were developed, the most successful of these solutions
were based on the "progressive refinement" method with an "adaptive
subdivision" scheme. And this is what Blender uses.
To be able to get
the most out of the Blender Radiosity method, it is important to
understand the following principles:
Finite Element Method
Many computer graphics or
simulation methods assume a simplification of reality with 'finite
elements'. For a visually attractive (and even scientifically proven)
solution, it is not always necessary to dive into a molecular level
of detail. Instead, you can reduce your problem to a finite number
of representative and well-described elements. It is a common fact
that such systems quickly converge into a stable and reliable
solution.
The Radiosity method is a typical example of a finite
element method inasmuch as every face is considered a 'finite element'
and its light emission considered as a whole.
Patches and Elements
In the radiosity universe, we
distinguish between two types of 3D faces:
Patches.
These are triangles or squares which are able to send energy.
For a
fast solution it is important to have as few of these patches as
possible. But, because of the approximations taken the energy is only
distributed from the
Patch's center, the size should be small enough to make a realistic
energy distribution. (For example, when a small object is located
above the Patch center, all energy the Patch sends is obscured
by this object).
Elements These are the triangles or
squares which receive energy. Each Element is associated to
a Patch. In fact, Patches are subdivided into many small Elements.
When an element receives energy it absorbs part of it (depending on
the Patch color) and passes the remainder to the Patch. Since the
Elements are also the faces that we display, it is important to have
them as small as possible, to express subtle shadow boundaries.
Progressive Refinement
This method starts with
examining all available Patches. The Patch with the most 'unshot'
energy is selected to shoot all its energy to the environment. The
Elements in the environment receive this energy, and add this to the
'unshot' energy of their associated Patches. Then the process
starts again for the Patch NOW having the most unshot
energy. This continues for all the Patches until no energy is
received anymore, or until the 'unshot' energy has converged below a
certain value.
The hemicube method
The calculation of how much energy
each Patch gives to an Element is done through the use of
'hemicubes'. Exactly located at the Patch's center, a hemicube
(literally 'half a cube')
consist of 5 small images of the environment. For each pixel in
these images, a certain visible Element is color-coded, and the
transmitted amount of energy can be calculated. Especially with the
use of specialized hardware the hemicube method can be accelerated
significantly. In Blender, however, hemicube calculations are
done "in software".
This method is in fact a simplification and
optimisation of the 'real' radiosity formula (form factor
differentiation). For this reason the resolution of the hemicube
(the number of pixels of its images) is approximate and its
careful setting is important to prevent
aliasing artefacts.
Adaptive subdivision
Since the size of the patches and elements in a Mesh
defines the quality of the Radiosity solution, automatic subdivision
schemes have been developed to define the optimal size of Patches
and Elements. Blender has two automatic subdivision
methods:
1. Subdivide-shoot Patches. By shooting energy to the
environment, and comparing the hemicube values with the actual
mathematical 'form factor' value, errors can be detected that
indicate a need for further subdivision of the Patch. The
results are smaller Patches and a longer solving time, but a higher
realism of the solution.
2. Subdivide-shoot Elements. By
shooting energy to the environment, and detecting high energy
changes (frequencies) inside a Patch, the Elements of this Patch are
subdivided one extra level. The results are smaller Elements and a
longer solving time and maybe more aliasing, but a higher level
of detail.
Display and Post Processing
Subdividing Elements in
Blender is 'balanced', that means each Element differs a maximum of
'1' subdivide level with its neighbours. This is important for a
pleasant and correct display of the Radiosity solution with Gouraud
shaded faces. Usually after solving, the solution consists of
thousands of small Elements. By filtering these and removing
'doubles', the number of Elements can be reduced significantly
without destroying the quality of the Radiosity solution.
Blender stores the energy values in 'floating point' values.
This makes settings for dramatic lighting situations possible, by
changing the standard multiplying and gamma values.
Rendering and integration in the Blender environment
The final step can be replacing the input Meshes
with the Radiosity solution (button Replace Meshes). At that
moment the vertex colors are converted from a 'floating point' value
to a 24 bits RGB value. The old Mesh Objects are deleted and
replaced with one or more new Mesh Objects. You can then delete the
Radiosity data with Free Data. The new Objects get a default
Material that allows immediate rendering. Two settings in a Material
are important for working with vertex colors:
VColPaint
This option treats vertex colors as a replacement for
the normal RGB value in the Material. You have to add Lamps in order
to see the radiosity colors. In fact, you can use Blender lighting
and shadowing as usual, and still have a neat radiosity 'look' in
the rendering.
VertexCol
It would have been better to call this option
"VertexLight". The vertexcolors are added to the light
when rendering. Even without Lamps, you can see the result. With
this option, the vertex colors are pre-multiplied by the Material
RGB color. This allows fine-tuning of the amount of 'radiosity
light' in the final rendering.
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