This is a proposal for an extension to the Khronos Group glTF 2.0 specification. The goal of this extension is to provide a physically accurate description of materials under different wavelengths.
OpenMaterial_material_parameters addresses the physics of wave-matter interaction. It uses physical properties of materials to describe their behavior for various environmental conditions. This physical material model works for different sensors such as camera, lidar and radar. The extension allows the material model to be used by multiple sensors in parallel.
Various physical concepts are considered, such as geometrical optics for the visible spectrum (camera sensors) and uniform theory of diffraction and physical optics for millimeter and sub-millimeter range (radar sensors). These concepts are used in their validity region to cover the full-wave spectrum. The proposed physical concepts and their implementation are independent of geometry.
In order to use this extension, it has to be listed in the extensionsUsed section of the corresponding asset:
"extensionsUsed" : [
"OpenMaterial_material_parameters"
]
The material properties used within the extension are defined based on the material model. This list of parameters not only includes physical properties of the material but also allows differentiation of surface, volume and subsurface interactions. Physical properties of a material are stored in the same file, regardless of the type of sensor. A particular class (i.e. WavelengthRange) defines if the material is detectable from the specific type of sensor, which is used to render the scene, through a window of valid wavelenghth values.
The index of refraction (IOR), Poynting Vector, Snell and Fresnel laws (geometric options) and coating (interference) are the material properties describing the surface interactions. In contrast, the mean free path, particle density, absorbtion and particle cross section are parameters describing volume interactions. For radar sensors, besides geometrical optics calculations, diffraction models and possibly numerical simulations can be included in the wave-matter interaction. Surface roughness is defined by the surface height root-mean-square (rms) and correlation length both in the unit of micrometer. A definition of all above mentioned parameters enables usage of a material for camera, lidar and radar simulations.
Within the following enlistment of properties specified by the proposed extension, items labeled as required are mandatory and must be present. Properties without required label are optional and may be omitted:
-
user_preferences[required] Material properties that could change when material is applied to geometry.geometrical_optics[boolean][required] If true, geometrical optics will be used. This is valid assumption when the size of the geometry structure of the object is much larger than the incident wavelength. [-]include_diffraction[boolean][required] If true, diffraction models will be included in the interaction calculations. Examples of methods are Unified Theory of Diffraction (UTD) - Geometrical Theory of Diffraction (GTD). [-]include_numerical_simulation[boolean][required] If true, numerical methods will be used to compute the electromagnetic field. Examples of methods are BEM (Boundary Elements Method), FEM (Finite Element Method), FDTD (Finite Difference Time Domain), and FMM (Fast Multiple Method). [-]material_scheme[string][required] Valid options aresurface,subsurfaceandvolume. [-]material_classification[string][required] Property specifying the hierarchical classification of the material. The first hierarchy level is eithersolid,liquidorgas. Lower hierarchy levels are user-defined. Hierarchy levels are separated by the–sign, e.g.solid-metal-aluminium. [-]material_type[required]isotropic[boolean][required] If true, it will be assumed that the material's physical properties are independent of the incident direction. This is true for most metals and glasses. If the property is set to false, anisotropic material behavior (including orthotropic) will be considered. Then the input parameters should be provided as a function of the incident direction. [-]homogeneous[boolean][required] If false, non-homogeneous material (a variation of physical properties varies inside the material will be considered. Physical properties have to be provided accordingly. [-]magnetic[boolean][required] If true, it is assumed that the material has magnetic properties. If false, the material is assumed to be non-magnetic. [-]
temperature[number] Material base-temperature [K] (T(°C) = T(K) - 273.15). Some assets have varying temperatures e.g. pedestrians, vehicle parts, ice, snow, rain.lambert_emission[number][required] A Lambertian emitter has the same brightness (i.e. current per area per solid angle) when observed from all angles. Lambertian emission follows the Lambert's cosine law. Value "0" means that the property is not used, while "1" enables it. [-]subsurface[required]subsurface[boolean][required] If true, subsurface interactions will be considered on top of surface interactions. [-]subsurface_thickness[number][required] Thickness of the volume to be considered as subsurface. The default value is the penetration depth calculated by the material model. [μm]
surface_displacement_uri[string] URI of external file with displacement data. Surface displacement is the macro-surface pattern. Assets with multiple displacements should be linked to their appropriate "displacement group". [-]surface_roughness[required] Surface roughness is defined by the surface root-mean-square and the correlation length.surface_height_rms[number][required] Surface height root-mean-square. [μm]surface_correlation_length[number][required] Surface correlation length. [μm]
coating_materials[array] Coating refers to a layer of transparent or semi-transparent material on top of another material, e.g. a layer of oil or water on top of asphalt. Each array element has the following properties:material_ref[string][required] Reference to an external material. [-]layer_thickness[number][required] Thickness of the coating layer. [μm]
ingredients[array] Ingredients are considered as impurities on top of the main material, e.g. oxidization is consiedered an ingredient of metal. Each array element has the following properties:material_ref[string][required] Reference to an external material. [-]distribution_pattern_uri[string] Reference to an external map of material distribution which describes the distribution of material ingredients over geometry. [-]
-
physical_properties[required] Intrinsic physical material properties that do not change when material is applied to geometry.refractive_index_uri[string][required] URI of an external file with tabular refractive index data. The refractive index is a complex function N (n + ik) that depends on the temperature and the wavelength. [-]mean_free_path[number][required] Mean free path for volumetric materials [μm]. The mean free path is the average distance travelled by light until it scatters at a scatterer. Value 0 indicates the property is not used.particle_density[number][required] Density of scatterers in a volume [μm^3]. Value 0 indicates the property is not used.particle_cross_section[number][required] Effective cross section of scatterers in a volume [μm^2]. Value 0 indicates the property is not used.emissive_coefficient_uri[number][required] URI of an external file with emissivity coefficient values [-]. An ideal black body has the emissivity coefficient of 1.0.detection_wavelength_ranges[array][required] Array of wavelength ranges in which the material can be detected by sensors. Each array element has the following properties:min[number][required] Minimum wavelength [m].max[number][required] Maximum wavelength [m].typical_sensor[string][required] Sensor type corresponding to the wavelength range. Valid options are:camera,lidar,radarandultrasound. [-]
effective_particle_area[number][required] Effective particle area of a material. This value is required for radar simulation. [μm]relative_permittivity_uri[string][required] Ratio of the capacitance of a capacitor using that material as a dielectric, compared with a similar capacitor that has vacuum as its dielectric. Relative permittivity is also commonly known as the dielectric constant and is required for radar simulation. This strings represents the URI of an external file with tabular relative permeability data. The data is structured by incident angle, temperature and wavelength. [-]relative_permeability_uri[string][required] Denoted by the symbol μ_r, relative permeability is the ratio of the permeability of a specific medium to the permeability of free space μ0. In SI units, μ_r is measured in henries per meter (H/m), or equivalently in newtons per ampere squared (N⋅A−2). This value is required for radar simulation. This strings represents the URI of an external file with tabular relative permeability data. The data is structured by incident angle, temperature and wavelength. [-]conductivity_uri[string][required] Conductivity quantifies how a material conducts electric current. The SI unit of electrical conductivity is Siemens per meter (S/m). This value is required for radar simulation. This strings represents the URI of an external file with tabular conductivity data. [-]acoustic_impedance[number][required] Acoustic impedance describes how much resistance an ultrasound beam encounters as it passes through a tissue [kg/(m^2s)]. This value is required for ultrasound simulation.shear_velocity[number][required] Shear velocity is used to describe shear-related motion in moving fluids. This value is required for ultrasound simulation. [m/s]
Within this project, wave-matter interaction is demonstrated based on the proposed material model for the case of perfectly smooth aluminium and gold. This demonstration considers camera sensors and implements Fresnel equations for surface interactions in visible light.
Visible light (light) is the electromagnetic radiation in the spectrum of 390 to 700 nanometers. Light interaction with material depends on the physical properties of the material. These physical properties can be measured and are provided in physics and chemistry literature. In the case of specular reflection, the Fresnel equations are used to describe how much of the incident ray is reflected and refracted. The Fresnel equations satisfy the energy conservation, so the total amount of the reflected and refracted light is equal to the incoming light. The angle of the refracted ray is given by Snell's law. Both the Fresnel equations and Snell's law depend on the index of refraction (IOR) which is a function of temperature and wavelength.
The index of refraction is a complex function which has a real and an imaginary part. The real part accounts for refraction, while the imaginary part describes the light attenuation. Instead of experimental data, the index of refraction can also be described by theoretical models such as the Lorentz oscillator model. This model provides a good description for many materials like metals, water, polystyrene, mercury, or silicone at wavelengths typically bigger than one micrometer.
The surface of geometry files can be manipulated by applying distortion of the normals and vertexes position of the meshes. This effect can be controlled by mapping displacement textures to the geometry. The former are gray-scale/ coloured pictures associating the distortion of the displacement as a function of the local coordinate u and v. In this context, the displacement direction is the local surface normal. Brighter textures will lead to higher displacement.
In the following image, the material model structure is presented:
Material composition and assignment of physical properties is defined using the following two building blocks:
AssetMaterial:
1) userPreferences: (MaterialUserPreferences)
-SurfaceRoughness
...
-Subsurface
...
-MaterialType
...
- Ingredients
...
-CoatingModels
...
...
2) physicalProperties: (MaterialPhysicalProperties)
-AssetMaterialIor
-IorData
...
-WavelenghtRange
...
...
Within its material section, the file describing the geometry of a 3D model has to provide a reference to a file based on the
OpenMaterial_material_parameters extension and therefore providing physical material parameters and the description of
the material:
"materials" : [
{
"name" : "aluminium",
"extensions": {
"OpenMaterial_reference_link": {
"id": "a3a62d4531e64ae5a620030b1deaff7c",
"title": "aluminium",
"uri": "../materials/aluminium.gltf" <======
}
}
}
],
This way, the same material can be used by different geometries (i.e. 3D models, meshes, etc.):
"meshes": [
{
"primitives": [
"indices": 0,
"mode": 4,
"material": 0 <======
}
],
"name": "body_115"
},
{
"primitives": [
"indices": 3,
"mode": 4,
"material": 0 <======
}
],
"name": "body_72"
},
The physical parameters provided in the referenced material file (e.g. aluminium.gltf, iron.gltf or gold.gltf) are used to
compute reflection of rays at the geometry (see above sections "Properties" and "Code structure"):
"materials": [
{
"name": "aluminium",
"extensions": {
"OpenMaterial_material_parameters": {
"user_preferences": {
"geometrical_optics": true,
"include_diffraction": false,
"include_numerical_simulation": false,
"material_scheme": "surface",
"material_classification": "solid-metal",
"material_type": {
"isotropic": true,
"homogeneous": true,
"magnetic": false
},
"temperature": 300.0,
"surface_displacement_uri": "",
"surface_roughness": {
"surface_height": 0.0,
"surface_correlation_length": 0.0
},
"coating_materials": [],
"lambert_emission": 0.0,
"subsurface": {
"subsurface": false,
"subsurface_thickness": 0.0
"ingredients": []
},
},
"physical_properties": {
"refractive_index_uri": "data/aluminium_ior.gltf",
"mean_free_path": 0.0,
"particle_density": 0.0,
"particle_cross_section": 0.0,
"emissive_coefficient_uri": "",
"detection_wavelength_ranges": [
{
"min": 0,
"max": 1E06,
"typical_sensor": "camera"
}
"effective_particle_area": 0.0,
"relative_permittivity_uri": "",
"relative_permeability_uri" : "",
"conductivity_uri": "",
"acoustic_impedance": 0.0,
"shear_velocity": 0.0
],
}
}
}
}
],
In the given example, wavelength and temperature specific refractive index values (i.e. aluminium_ior.gltf) are specified
in a separate file using the OpenMaterial_ior_data extension.
To represent emissivity, the OpenMaterial_emissivity_data extension can be used.
Relative permeability and relative permittivity are defined by extensions OpenMaterial_permeability_data and
OpenMaterial_permittivity_data.
N/A
OpenMaterial_material_parameters.schema.json
N/A