Surface scattering models
Surface scattering models (表面拡散モデル) では、シーン内のサーフェスに相互作用する光の挙動を記述しています。
それらは、マテリアル内で起こるメゾスコピック散乱プロセスを見えるようにするために便宜的に要約しています。
これは、Mitsuba のマテリアルシステムのある中心的なコンポーネントを記述しており、レンダラーの他の部分とそれ自身とがサーフェス相互作用の間で起こる事と関わりがあります。これに関する詳細は、Dipole Subsurface と Participating Media を参照してください。
ここでは、サポートされているすべての表面拡散モデルと、そのパラメータの概要を説明します。
表面散乱モデルの概略図
Mitsuba で最も重要な表面散乱モデルの概略図 (Weidlich と Wilkie のスタイルで表示 ［Arbitrarily Layered Micro-Facet Surfaces］)
矢印は、各モデルを適用したサーフェスの相互作用として起こりうる結果を示しています。
 Smooth diff use material (diffuse) |
 Smooth dielectric material (dielectric) |
 Smooth conducting material (conductor) |
 Smooth plastic material (plastic) |
 Rough diff use material (roughdiffuse) |
 Rough dielectric material (roughdielectric) |
 Rough conducting material (roughconductor) |
 Rough plastic material (roughplastic) |
 Smooth diff use transmitter (difftrans) |
 Bump map modi er (bump) |
 Single-scattering layer (hk) |
 Smooth dielectric coating (coating) |
凡例
 Incident illumination |
 Smooth surface |
 Exterior (normal-facing side) |
 Tinted layer |
 Scattered illumination (primary component) |
 Rough/bumpy surface |
 Interior-facing side |
 Scattering layer |
 Scattered illumination (secondary component) |
 Diff use scattering |
 Clear coating |
 Arbitrary BSDF |
 Scattered illumination (tertiary component) |
|||
 Lobe shape/presence is up to the nested model |
BSDFs
現実的な結果を達成するために、Mitsuba には、汎用の表面散乱モデル (滑らかまたは粗いガラス、金属、プラスチックなど) と特殊な素材専門 (織布、マスクなど) の両方のライブラリーが付属しています。
いくつかのモデルプラグインはどちらのカテゴリも当てはまらず、一つ以上の拡散モデル上に適用されたモディファイアとして最適に表現されます。
ドキュメントの中やシーン記述言語の中では、BSDF が「表面散乱モデル」と同義に用いられています。
This is an abbreviation for Bidirectional Scattering Distribution Function, a more precise technical term.
In Mitsuba, BSDFs are assigned to shapes, which describe the visible surfaces in the scene. In the scene description language, this assignment can either be performed by nesting BSDFs within shapes, or they can be named and then later referenced by their name.The following fragment shows an example of both kinds of usages:
<scene version="0.5.0">
<!-- Creating a named BSDF for later use -->
<bsdf type=".. BSDF type .." id="myNamedMaterial">
<!-- BSDF parameters go here -->
</bsdf>
<shape type="sphere">
<!-- Example of referencing a named material -->
<ref id="myNamedMaterial"/>
</shape>
<shape type="sphere">
<!-- Example of instantiating an unnamed material -->
<bsdf type=".. BSDF type ..">
<!-- BSDF parameters go here -->
</bsdf>
</shape>
</scene>
It is generally more economical to use named BSDFs when they are used in several places, since this reduces Mitsuba’s internal memory usage.
Correctness considerations
A vital consideration when modeling a scene in a physically-based rendering system is that the used materials do not violate physical properties, and that their arrangement is meaningful. For instance,
 Slice through a glass lled with water |
 Description using surfaces in Mitsuba (normals in gray) |
| Surface | Interior IOR | Exterior IOR |
| ■■■ | 1.50 | 1.00 |
| ■■■ | 1.50 | 1.33 |
| ■■■ | 1.33 | 1.00 |
Figure 7: Some of the scatteringmodels inMitsuba need to know the indices of refraction on the exterior and interior-facing side of a surface. It is therefore important to decompose the mesh into meaningful separate surfaces corresponding to each index of refraction change.The example here shows such a decomposition for a water-filled Glass.
imagine having designed an architectural interior scene that looks good except for a white desk that seems a bit too dark. A closer inspection reveals that it uses a Lambertian material with a diffuse reflectance of 0.9.
In many rendering systems, it would be feasible to increase the reflectance value above 1.0 in such a situation. But in Mitsuba, even a small surface that reflects a little more light than it receives will likely break the available rendering algorithms, or cause them to produce otherwise unpredictable results. In fact, the right solution in this case would be to switch to a different the lighting setup that causes more illumination to be received by the desk and then reduce the material’s reflectance—after all, it is quite unlikely that one could find a real-world desk that reflects 90% of all incident light.
As another example of the necessity for ameaningfulmaterial description, consider the glassmodel illustrated in Figure 7. Here, careful thinking is needed to decompose the object into boundaries that mark index of refraction-changes. If this is done incorrectly and a beam of light can potentially pass through a sequence of incompatible index of refraction changes (e.g. 1.00 → 1.33 followed by 1.50 → 1.33), the output is undefined and will quite likely even contain inaccuracies in parts of the scene that are far away from the glass.
- 最終更新:2014-05-22 14:42:51
