htrdr.md.in (14115B)
1 # `htrdr` 2 3 `htrdr` evaluates the intensity at any position (probe) of the scene, in 4 any direction, in the presence of *surfaces* and an *absorbing and 5 diffusing semi-transparent medium*, for both *internal* (longwave) or 6 *external* (shortwave) *radiation sources*. 7 The intensity is 8 calculated using the *Monte-Carlo* method: a number of optical paths are 9 simulated backward, from the probe position and into the medium. 10 Various algorithms are used, depending on the specificities of the 11 nature and shape of the radiation source. 12 13 <video style="width:100%; text-align:center" controls poster="images/R8tr3.1.ARMCu.OUT.218.jpg"> 14 <source src="downloads/teapot_city_clouds.mp4" type="video/mp4"> 15 </video> 16 17 > This film demonstrates the capacity of Monte-Carlo path-tracing 18 > methods to handle large scale ratios from large cloud fields to cities 19 > to buildings to trees and down to a teapot. 20 > The 4D cloud field has been produced by the 21 > Meso-NH 22 > ([Lafore et al. 1998](https://www.ann-geophys.net/16/90/1998/angeo-16-90-1998.html), 23 > [Lac et al. 2018](https://www.geosci-model-dev.net/11/1929/2018/)) 24 > Large Eddy model. 25 > Spectral materials are defined in particular from data from the 26 > Spectral Library of impervious Urban Materials 27 > ([Kotthaus et al. 2013](https://zenodo.org/record/4263842)). 28 > Each frame was rendered with 29 > [htrdr-atmosphere](man/man1/htrdr-atmosphere.1.html) 30 > using 8192 samples per pixel component. 31 > The resulting images are then 32 > post-processed with the [htpp](man/man1/htpp.1.html) program. 33 > Refer to the video for a complete description of how it was produced. 34 35 Applications are theoretically possible to any configuration. 36 However, it all eventually comes down to the possibility of using the 37 physical data of interest, in their most common formats, in each 38 scientific community. 39 `htrdr` is currently suitable for three main application fields: 40 41 1. [Atmospheric radiative transfer](man/man1/htrdr-atmosphere.1.html): a 42 clear-sky atmosphere is vertically stratified, neglecting Earth 43 sphericity, and described in terms of absorption coefficients as a 44 function of height and spectral quadrature point as per a 45 correlated-k model. 46 Cloud physical properties are provided on a 3D rectangular grid. 47 Surface geometrical and optical properties can be provided for an 48 arbitrary number of geometries. 49 Internal radiation and solar radiation are taken into account. 50 51 2. [Combustion processes](man/man1/htrdr-combustion.1.html): 52 thermodynamic data is provided at the nodes of an unstructured 53 tetrahedral mesh, while surface properties can still be provided for 54 various materials. 55 The radiation source is only external: a monochromatic laser sheet 56 illuminates the inside of the combustion chamber for diagnostic 57 purposes. 58 59 3. [Planetary science](man/man1/htrdr-planets.1.html): takes into 60 account the geometry of a "ground" of arbitrary shape, described by a 61 triangular mesh, with the possibility of using an arbitrary number of 62 materials. 63 The radiative properties of a gas mixture must be provided on a 64 tetrahedral mesh, using the k-distribution spectral model. 65 The radiative properties of an arbitrary number of aerosol and 66 hydrometeores can also be provided on their individual tetrahedral 67 mesh. 68 Calculations can be made for both internal and external radiation 69 sources. 70 In the case of an external source, a sphere of arbitrary size and 71 position is used. 72 This sphere can radiate as a Planck source at a specified brightness 73 temperature, or be associated with a high-resolution radiance 74 spectrum. 75 76 [](images/titan_1280x960x4096.jpg) 77 78 > Images of Titan rendered with 79 > [htrdr-planets](man/man1/htrdr-planets.1.html). 80 > Its 3D atmosphere is composed of a gas mixture and two aerosol modes 81 > for haze and clouds. 82 83 [](images/titan_transit.jpg) 84 85 > Rendering of Titan in transit situation, i.e. the source is 86 > positioned behind Titan to simulate a solar eclipse by Titan. 87 88 Since any radiative transfer observable is expressed as an integral of 89 the radiance, and since there is a strict equivalence between the 90 integral to be solved and the underlying Monte-Carlo algorithm (each 91 integral is associated with the sampling of a random variable), the 92 algorithms that calculate the radiance are used for computing various 93 quantities: 94 95 - *Images* on a camera sensor, in a given field of view. 96 For combustion applications, only monochromatic images are supported. 97 In atmospheres and in planetary science, spectral integration is also 98 possible, both for solar and thermal images: CIE colorimetry is used 99 for solar images, while thermal images are in fact brightness 100 temperature maps, obtained from the incoming radiative flux over a 101 specified spectral interval. 102 103 - *Flux density maps*, on a grid of sensors, integrated over an entire 104 hemisphere. 105 In the case of combustion chambers, flux density maps can be 106 calculated, while spectrally integrated flux density maps are also 107 possible for atmospheric application, both for solar and thermal 108 radiation. 109 110 [](images/CLEMENT.jpg) 111 112 [](images/CLEMENT_lw_9000_10000.jpg) 113 114 > Images rendered with 115 > [htrdr-atmosphere](man/man1/htrdr-atmosphere.1.html) 116 > of a 1000³ cloud field produced by the Meso-NH 117 > ([Lafore et al. 1998](https://www.ann-geophys.net/16/90/1998/angeo-16-90-1998.html), 118 > [Lac et al. 2018](https://www.geosci-model-dev.net/11/1929/2018/)) 119 > Large Eddy Model. 120 > The initial conditions and model set-up for the simulation are 121 > described in 122 > [Strauss et al. 2019](https://rmets.onlinelibrary.wiley.com/doi/full/10.1002/qj.3614). 123 > The infrared rendering is calculated in [9, 10] µm spectral range; 124 > the color map displays the brightness temperature in Kelvin. 125 126 ## Related articles 127 128 - [He et al. 2026](https://doi.org/10.1016/j.jqsrt.2025.109722), 129 "Simultaneous estimation of radiance and its sensitivities to 130 radiative properties in a spherical-heterogeneous atmospheric 131 radiative transfer model by Monte Carlo method: Application to Titan", 132 Journal of Quantitative Spectroscopy and Radiative Transfer 133 ([open access](https://hal.science/hal-05370209v1/)) 134 135 - [El Hafi et al. 2025](https://doi.org/10.1016/j.jqsrt.2025.109661), 136 "Application of null-collision backward Monte Carlo algorithm to 137 digital image rendering of sooting flames in the visible range", 138 Journal of Quantitative Spectroscopy and Radiative Transfer 139 ([open access](https://www.sciencedirect.com/science/article/pii/S0022407325003231)) 140 141 - [Caliot et al. 2022](https://doi.org/10.1007/s10546-022-00750-5), 142 "Model of Spectral and Directional Radiative Transfer in Complex Urban 143 Canopies with Participating Atmospheres", Boundary-Layer Meteorology 144 ([open access](https://hal.science/hal-03813906/)) 145 146 - [Villefranque et al. 2022](https://doi.org/10.1126/sciadv.abp8934), 147 "The “teapot in a city”: A paradigm shift in urban climate modeling", 148 Science Advances 149 ([open access](https://arxiv.org/abs/2204.14227)) 150 151 - [Sans et al. 2021](https://doi.org/10.1016/j.jqsrt.2021.107725) 152 "Null-collision meshless Monte Carlo - A new reverse Monte Carlo 153 algorithm designed for laser-source emission in absorbing/scattering 154 inhomogeneous media", Journal of Quantitative Spectroscopy and 155 Radiative Transfer 156 ([open access](https://imt-mines-albi.hal.science/hal-03224186v1)) 157 158 - [Villefranque et al. 2019](https://doi.org/10.1029/2018MS001602) 159 "A Path-Tracing Monte Carlo Library for 3-D Radiative Transfer in 160 Highly Resolved Cloudy Atmospheres", Journal of Advances in Modeling 161 Earth Systems 162 ([open access](https://arxiv.org/abs/1902.01137)) 163 164 165 [](images/gulder_horizontal_slides.jpg) 166 167 > Renderings in the visible range calculated by 168 > [htrdr-combustion](man/man1/htrdr-combustion.1.html) of a laminar 169 > sooting flame seen from above and illuminated by a laser sheet. 170 > The images display radiation that is emitted by the laser, scattered 171 > and transmitted by the combustion medium. 172 > The laser sheet is horizontal, and intersects the medium at various 173 > heights. 174 > One can see the difference in the scattered signal between the left 175 > and right parts of the image; since the laser propagates from the left 176 > to the right, it is progressively attenuated while traveling the 177 > medium. 178 > The intensity of radiation subject to scattering therefore decreases. 179 > In addition to these trapping effect these images provide some insight 180 > about the scattering cross-section of the medium as a function of 181 > height. 182 > Scattering properties of soot gradually increase from the injection 183 > position to a height of approximately 35 mm. 184 > A steep decrease follows: 185 > the image for a height of 40 mm is very similar to the image obtained 186 > at 10 mm. 187 188 <span id="rel_projects"/> 189 190 ## Related projects 191 192 `htrdr` has been used, developped and extended in the following 193 research projects: 194 195 - The development of `htrdr` began with the 196 [High-Tune](https://anr.fr/Projet-ANR-16-CE01-0010) project. 197 Originally, it simulated the radiative transfer of an *external source* 198 (solar) in a scene composed of a triangulated ground and an 199 *atmosphere*, neglecting Earth sphericity, in the presence of a *cloud 200 field* provided over a structured grid. 201 It was later extended in order to take into account a *non-gray 202 surface*, and the possibility to perform radiative transfer 203 computations for a *internal source* (ground and atmosphere). 204 205 - In project 206 [ModRadUrb](https://www.umr-cnrm.fr/spip.php?article1204) the 207 emphasis was put on taking into account the representation of *complex 208 geometries* (detailled city scenes) using *spectral properties of an 209 arbitrary number of materials*. 210 The solver was extended to solve upward and downward *hemispherical 211 atmospheric fluxes* on a plane positioned anywhere in the scene, both 212 in the visible and the infrared spectral ranges. 213 214 Note that the 215 [`htrdr` Urban](https://gitlab.com/edstar/htrdr/-/tree/main_urban) fork adds 216 the calculation of the radiative flux density incident on or absorbed 217 by a group of triangles to the geometry of the ground and humans. 218 219 - In project 220 [MCG-RaD](https://anr.fr/Projet-ANR-18-CE46-0012) 221 the `htrdr` codebase was used to explore a whole new class of 222 radiative transfer algorithms: instead of relying on the full 223 atmospheric radiative properties data set (prerequisite for current 224 algorithms), the so-called *line sampling* algorithms will *not* 225 require *nor* compute the absorption coefficient of the atmosphere. 226 Instead, it will sample energetic transitions and use a Line-by-Line 227 parameters database (such as [HITRAN](https://hitran.org/)) in order 228 to perform a *rigorous spectral integration*, both in the visible and 229 the infrared spectral ranges. 230 231 - In project 232 [Astoria](https://anr.fr/Project-ANR-18-CE05-0015). 233 `htrdr` was used to produce images in the visible, in the presence 234 of *combustion chambers* where radiation scattering is performed by soot 235 aggregates. 236 One of the main difficulty resides in the fact that the chamber is 237 *illuminated by a laser*: the classical solar radiative transfer 238 algorithm then fails to converge because of the collimated radiation 239 source, and a 240 [new algorithm](https://doi.org/10.1016/j.jqsrt.2021.107725) 241 was thus designed in order to ensure numerical convergence. 242 243 - In project 244 [Rad-Net](https://anr.fr/Projet-ANR-21-CE49-0020) 245 `htrdr` was adapted for applications in *planetary science* and 246 *astrophysics*. 247 The application is now a scene composed of an *arbitrary number of 248 solid surfaces* (a planet, satellites) represented by triangular 249 meshes and materials which describe their *spectral 250 reflectivity/emissivity* properties. 251 The *3D atmopshere* is defined by a number of participating 252 semi-transparent media (a gas mixture and an arbitrary number of 253 aerosol modes) whose radiative properties are provided at the nodes of 254 a *unstructured tetrahedral volumetric grid*, independant for ach 255 medium. 256 257 []( 258 images/downward_flux_500x500x2048_sw_380_4000_879.349.jpg) 259 260 > Shortwave downward flux density maps in W/m² computed by 261 > [htrdr-atmosphere](man/man1/htrdr-atmosphere.1.html) 262 > at 1 meter height with the [DZVAR](htrdr-atmosphere-spk.html) cloud 263 > field. 264 > The sun is located at the zenith. 265 > The spectral integration range is [0.38, 4] µm . 266 > Its spatially-avaraged flux is 879.349 W/m² . 267 > One can observe the contrast between the shadows of the clouds and 268 > fully illuminated areas. 269 270 []( 271 images/downward_flux_500x500x2048_lw_4000_100000_425.156.jpg) 272 273 > Longwave downward flux density maps in W/m² computed on the same scene 274 > of the previous image. 275 > The spectral integration ranges is [4, 100] µm. 276 > Its spatially-avaraged is 425.159 W/m². 277 > Note the effect of clouds (higher values, due to the emission by the 278 > base of the cloud at higher temperatures than for a clear-sky zone) 279 > and also a "ripple" effect that is due to the spatial variations of 280 > water vapor concentration, as provided by the LES simulation. 281 282 ## History 283 284 `htrdr` has been funded by the 285 [ANR Rad-Net](https://anr.fr/Projet-ANR-21-CE49-0020) since 2021. 286 |Méso|Star> is subcontractor of the project. 287 288 `htrdr` was funded by the 289 [ANR Astoria](https://anr.fr/Project-ANR-18-CE05-0015) 290 from 2018 to 2022. 291 |Méso|Star> was sub-contractor of the project. 292 293 `htrdr` was funded by the 294 [ADEME](https://www.ademe.fr/) (MODEVAL-URBA-2019) from 2019 to 2022. 295 |Méso|Star> was partner of the project with 296 [CNRM](https://www.umr-cnrm.fr/). 297 298 `htrdr` was funded by the 299 [ANR High-Tune](https://anr.fr/Project-ANR-16-CE01-0010) from 300 2016 to 2019. 301 |Méso|Star> was sub-contractor of the project. 302 Visit the 303 [High-Tune project web site](http://www.umr-cnrm.fr/high-tune/?lang=en)