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Contents [hide] 1 Radiosity Rendering 1.1 Description 1.2 Options 1.3 Examples 1.4 Hints 1.5 Technical Details

Radiosity Rendering

Description

Let's assume you have a scene ready, and that you want to render it with the Radiosity Rendering. The first thing to grasp when doing Radiosity is that no Lamps are necessary, but some meshes with an Emit material property greater than zero are required, since these will be the light sources. You can build the test scene shown in Set-up for Radiosity test., it is rather easy. Just make a big cube for the room, give different materials to the side walls, add a cube and a stretched cube within it, and add a plane with a non-zero Emit value next to the roof, to simulate the area light.

You assign Materials as usual to the input models. The RGB value of the Material defines the Patch colour. The 'Emit' value of a Material defines if a Patch is loaded with energy at the start of the Radiosity simulation. The 'Emit' value is multiplied with the area of a Patch to calculate the initial amount of unshot energy.

Emitting faces: Check the number of 'emitters' on Blender console! If this is zero nothing interesting can happen. You need at least one emitting patch to have light and hence a solution.

Options

Radio

Enables radiosity calculations as part of the render process. This will automatically consider all objects in the scene, and those materials with an Emit: value > 0 will emit light.

When assigning materials be sure that all of them have the Radio toggle on to enable the Shader Panel of the Material subcontext buttons (Radiosity enabled material.). You also need to have the Amb setting of each material at around 0.5.

Hemires

The hemicube resolution; the color-coded images used to find the Elements that are visible from a 'shoot Patch', and thus receive energy. Hemicubes are not stored, but are recalculated each time for every Patch that shoots energy. The 'Hemires' value determines the Radiosity quality and adds significantly to the solving time.

Max Iterations

The maximum number of Radiosity iterations. If set to zero Radiosity will go on until the convergence criterion is met. You are strongly advised to set this to some non-zero number, usually greater than 100.

Mult, Gamma

The colourspace of the Radiosity solution is far more detailed than can be expressed with simple 24 bit RGB values. When Elements are converted to faces, their energy values are converted to an RGB colour using the Mult and Gamma values. With the Mult value you can multiply the energy value, with Gamma you can change the contrast of the energy values.

Convergence

When the amount of unshot energy in an environment is lower than this value, the Radiosity solving stops. The initial unshot energy in an environment is multiplied by the area of the Patches. During each iteration, some of the energy is absorbed, or disappears when the environment is not a closed volume. In Blender's standard coordinate system a typical emitter (as in the example files) has a relatively small area. The convergence value is divided by a factor of 1000 before testing for that reason.

Examples

The rendering will take more time than usual, in the console you will notice a counter going up. The result will be quite poor (Radiosity rendering for coarse meshes (left) and fine meshes (right)., left) because the automatic radiosity render does not do adaptive refinement! Select all meshes, one after the other, and in EditMode subdivide them at least three times. The room, which is much bigger than the other meshes, you can even subdivide four times. Set the Max Iterations a bit higher, 300 or more. Try Rendering again (F12). This time the rendering will take even longer but the results will be much nicer, with soft shadows and colour leakage (Radiosity rendering for coarse meshes (left) and fine meshes (right)., right).

Note: In the Radiosity Rendering Blender acts as for a normal rendering, this means that textures, Curves, Surfaces and even Dupliframed Objects are handled correctly.

Hints

Please note that the light emission is governed by the direction of the normals of a mesh, so the light emitting plane should have a downward pointing normal and the outer cube (the room) should have the normals pointing inside, (flip them!). Switch to the Radiosity sub-context of the Shading Context. The Panels, shown in Radiosity buttons for radiosity rendering., are two: Radio Rendering which governs Radiosity when used as a rendering tool (present case) and Radio Tool, which governs Radiosity as a modelling tool (next section).

Technical Details

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, to speed things up the energy is modelled as if it were radiated by the Patch's centre; the size of the patches should then be small enough to make this a realistic energy distribution. (For example, when a small object is located above the Patch centre, all energy the Patch sends is obscured by this object, even if the patch is larger! This patch should be subdivided in smaller patches).

Elements.

These are the triangles or squares which receive energy. Each Element is associated with a Patch. In fact, Patches are subdivided into many small Elements. When an Element receives energy it absorbs part of it (depending on its colour) and passes the remainder to the Patch, for further radiation. 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 and light gradients.

• 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 (gradients) 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.

• Radiosity for Modelling

The final step can be replacing the input Meshes with the Radiosity solution (button Replace Meshes). At that moment the vertex colours 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 colours:

VColPaint.

This option treats vertex colours as a replacement for the normal RGB value in the Material. You have to add Lamps in order to see the Radiosity colours. In fact, you can use Blender lighting and shadowing as usual, and still have a neat Radiosity 'look' in the rendering.

VColLight.

The vertexcolors are added to the light when rendering. Even without Lamps, you can see the result. With this option, the vertex colours are pre-multiplied by the Material RGB colour. This allows fine-tuning of the amount of 'Radiosity light' in the final rendering. As with everything in Blender, Radiosity settings are stored in a datablock. It is attached to a Scene, and each Scene in Blender can have a different Radiosity 'block'. Use this facility to divide complex environments into Scenes with independent Radiosity solvers.

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