Material

In 3D data visualization, material is the new color! By assigning a material to the 3D geometry, we can not only achieve more photorealistic visual quality but also encode data in more channels than just RGB. Hakowan leverages Mitsuba's material support to provide a number of material-based visual channels.

All materials support the following parameters:

Parameter	Type	Description
two_sided	bool	Wheter the material is two-sided. (default: false)

A material can be specified via the Layer.channel() method.

# Assume `m` is a `Material` object
l = hkw.layer().channel(material=m)

Hakowan also provides the shortcut method Layer.material() to specify material directly.

l = hkw.layer().material("Diffuse", "orange", two_sided=True)
l = hkw.layer().material("Principled", "ivory", roughness=0.2, metallic=0.8)

Diffuse material

Diffuse material provides a matte shading to the shape. It is good for visualizing scalar field data without any interference from specular highlight.

Channel	Type	Description
reflectance	TextureLike	Base color of the material (default: 0.5)

Here is a simple example of creating a Diffuse material.

m = hkw.material.Diffuse("ivory")

Here, we assign a uniform color, "blue", to the reflectance channel. To encode actual data, we need to assign a texture to the reflectance channel.

m = hkw.material.Diffuse(hkw.texture.ScalarField(data = "attr_name"))

Check the Mitsuba doc for more details.

Conductor material

Conductor material gives the geometry a metallic look and feel. It takes a material parameter as input, but it cannot be used to encode any data.

Parameter	Type	Description
material	str	Material type (see supported material types)

Here is a snippet to create a Conductor material of type Al.

m = hkw.material.Conductor(material="Al")

Check the Mitsuba doc for more details.

Rough conductor material

RoughConductor material gives the geometry a matte metallic look and feel. It takes a material parameter just like Conductor. In addition, it also takes a distribution parameter and a alpha channel.

Channel	Type	Description
alpha	Texture or float	Roughness value from 0 (smooth) to 1 (rough) (default: 0.1)

Parameter	Type	Description
material	str	Material type (see supported material types)
distribution	str	Microfacet normal distribution: ggx or beckmann (default)

Here is a simple example of using RoughConductor.

m = hkw.material.RoughConductor(material = "Cu")

Here is a more complex example where we map an attribute to the alpha channel:

m = hkw.material.RoughConductor(
    material="Cu", alpha=hkw.texture.ScalarField(data="attr_name")
)

Note that alpha=0.1 is relatively rough, and alpha from 0.3 to 0.7 is very rough. Beyond 0.7, the result does not look very realistic.

Check out the Mitsuba doc for more details.

Plastic material

Plastic material provides a smooth plastic look and feel. Hakowan exposes two visual channels in this material:

Channel	Type	Description
diffuse_reflectance	TextureLike	Base color of the material (default: 0.5)
specular_reflectance	Texture or float	Specular reflectance component (default: 1.0)

Here is a snippet to create a Plastic material:

m = hkw.material.Plastic(
    diffuse_reflecctance=hkw.texture.ScalarField(data="attr_name"),
    specular_reflectance=hkw.texture.ScalarField(data="attr2_name")
)

Check out the Mitsuba doc for more details.

Rough plastic material

Similar to RoughConductor, the distribution parameter controls the microfacet normal distribution, and its valid values are ggx and beckmann (default). The alpha parameter control the roughness of the material.

Channel	Type	Description
diffuse_reflectance	TextureLike	Base color of the material (default: 0.5)
specular_reflectance	Texture or float	Specular reflectance component (default: 1.0)

Addition to the above visual channels, this material also expose the following parameters.

Parameter	Type	Description
distribution	str	Microfacet normal distribution: ggx or beckmann (default)
alpha	float	Roughness value from 0 (smooth) to 1 (rough) (default: 0.1)

Here is a snippet to create a RoughPlastic material:

m = hkw.material.RoughPlastic(
    diffuse_reflecctance=hkw.texture.ScalarField(data="attr_name"),
    specular_reflectance=hkw.texture.ScalarField(data="attr2_name"),
    alpha=0.1
)

Note that alpha=0.1 is relatively rough, and alpha from 0.3 to 0.7 is very rough. Beyond 0.7, the result does not look very realistic.

Check out the Mitsuba doc for more details.

Principled material

Principled material is the most versatile material. It is based on paper "Physically Based Shading" and its extension. It can be used to approximate almost all other materials.

Channel	Type	Description
color	TextureLike	Base color of the material (default: 0.5)
roughness	Texture or float	Roughness value from 0 (smooth) to 1 (rough) (default: 0.5)
metallic	Texture or float	Metallic value from 0 (not metallic) to 1 (very metallic) (default: 0.0)

As a rule of thumb, roughness and metallic are effective at encoding binary categorical data, but not very effective for quantitative data.

Here is a snippet to create a Principled material:

m = hkw.material.Principled(
    color=hkw.texture.ScalarField(data="attr_name"),
    roughness=hkw.texture.ScalarField(data="attr_name2"),
    metallic=hkw.texture.ScalarField(data="attr_name3"),
)

Here is another example where the color, roughness and metallic channels are uniform across the entire shape.

m = hkw.material.Principled(color="#252525", roughness=0.2, metallic=0.8)

Check out the Mitsuba doc for more details.

Dielectric material

Dielectric material provides a smooth glossy look and feel. This material exposes no visual channels and have the following parameters.

Parameter	Type	Description
int_ior	str or float	Interior index of refraction (default: bk7, see supported ior list)
ext_ior	str or float	Exterior index of refraction (default: air, see supported ior list)
medium	Medium	Medium of the enclosed material (default: None)

Note that by default, Dielectric material does not expose a color channel. Color is specified indirectly through the medium parameter. Medium describes the material/medium enclosed by the shape. It can provide fancy effects such as sub-surface scattering. However, medium setting requires the volpath integrator.

Here is a snippet for creating a Dielectric material.

m = hkw.material.Dieletric(int_ior="water")

Check out the Mitsuba doc for more details.

Rough dielectric material

RoughDielectric material provides a matte glass look and feel. Similar to the RoughConductor material, This material exposes an alpha visual channel.

Channel	Type	Description
alpha	Texture or float	Roughness value from 0 (smooth) to 1 (rough) (default: 0.1)

Although alpha channel is available to encode data, based on our experiments, its effect is somewhat limited. Here are the parameters of this material.

Parameter	Type	Description
int_ior	str or float	Interior index of refraction (default: bk7, see supported ior list)
ext_ior	str or float	Exterior index of refraction (default: air, see supported ior list)
medium	Medium	Medium of the enclosed material (default: None)
distribution	str	Microfacet normal distribution: ggx or beckmann (default)

Here is a snippet for creating a RoughDielectric material.

m = hkw.material.RoughDielectric(int_ior="water")

Thin dielectric material

ThinDielectric material provides a thin glossy shell look and feel for a given shape. It does not expose any visual channels.

Parameter	Type	Description
int_ior	str or float	Interior index of refraction (default: bk7, see supported ior list)
ext_ior	str or float	Exterior index of refraction (default: air, see supported ior list)
medium	Medium	Medium of the enclosed material (default: None)

Here is a snippet for creating a ThinDielectric material.

m = hkw.material.ThinDielectric(int_ior="water")

Hair material

Hair material from Mitsuba provides a hair-like look and feel. This material is designed to specifically work with curve mark. It does not provide any channels for encoding data.

Parameter	Type	Description
eumelanin	float	The amount of dark/black/brown pigments (default: 1.3)
pheomelanin	float	The amount of red/yellow pigments (default: 0.2)

Here is a snippet for creating a Hair material.

m = hkw.material.Hair(eumelanin=0.3, pheomelanin=0.1)

Check out the Mitsuba doc for more details.

Two-sided material

Sometimes, the mesh we have may not be correctly oriented. Inverted facets will appear to be black. To avoid such artifact, all material classes support an optional two_sided parameter to enabled two-sided normals.

m = hkw.material.Diffuse("salmon", two_sided=True)

Bump mapped material

A bump map can be applied to any material by setting the bump_map texture and bump_scale parameter. The former parameters defines the raw bump map texture where the latter defines the multiplicative factor to scale the bump magnitude.

m = hkw.material.Diffuse(
    "salmon",
    bump_map=hkw.texture.Image("bump_map.png"),
    bump_scale=0.5,
)

See the Moon example for an application of bump map.