meso-web

htrdr.md.in (16080B)

---

 # `htrdr`
 
 `htrdr` evaluates the intensity at any position (probe) of the scene, in
 any direction, in the presence of *surfaces* and an *absorbing and
 diffusing semi-transparent medium*, for both *internal* (longwave) or
 *external* (shortwave) *radiation sources*.
 The intensity is
 calculated using the *Monte-Carlo* method: a number of optical paths are
 simulated backward, from the probe position and into the medium.
 Various algorithms are used, depending on the specificities of the
 nature and shape of the radiation source.
 
 <video style="width:100%; text-align:center" controls poster="images/R8tr3.1.ARMCu.OUT.218.jpg">
   <source src="downloads/teapot_city_clouds.mp4" type="video/mp4">
 </video>
 
 > This film demonstrates the capacity of Monte-Carlo path-tracing
 > methods to handle large scale ratios from large cloud fields to cities
 > to buildings to trees and down to a teapot.
 > The 4D cloud field has been produced by the
 > Meso-NH
 > ([Lafore et al. 1998](https://www.ann-geophys.net/16/90/1998/angeo-16-90-1998.html),
 >  [Lac et al. 2018](https://www.geosci-model-dev.net/11/1929/2018/))
 > Large Eddy model.
 > Spectral materials are defined in particular from data from the
 > Spectral Library of impervious Urban Materials
 > ([Kotthaus et al. 2013](https://zenodo.org/record/4263842)).
 > Each frame was rendered with
 > [htrdr-atmosphere](man/man1/htrdr-atmosphere.1.html)
 > using 8192 samples per pixel component.
 > The resulting images are then
 > post-processed with the [htpp](man/man1/htpp.1.html) program.
 > Refer to the video for a complete description of how it was produced.
 
 Applications are theoretically possible to any configuration.
 However, it all eventually comes down to the possibility of using the
 physical data of interest, in their most common formats, in each
 scientific community.
 `htrdr` is currently suitable for three main application fields:
 
 1. [Atmospheric radiative transfer](man/man1/htrdr-atmosphere.1.html): a
    clear-sky atmosphere is vertically stratified, neglecting Earth
    sphericity, and described in terms of absorption coefficients as a
    function of height and spectral quadrature point as per a
    correlated-k model.
    Cloud physical properties are provided on a 3D rectangular grid.
    Surface geometrical and optical properties can be provided for an
    arbitrary number of geometries.
    Internal radiation and solar radiation are taken into account.
 
 2. [Combustion processes](man/man1/htrdr-combustion.1.html):
    thermodynamic data is provided at the nodes of an unstructured
    tetrahedral mesh, while surface properties can still be provided for
    various materials.
    The radiation source is only external: a monochromatic laser sheet
    illuminates the inside of the combustion chamber for diagnostic
    purposes.
 
 3. [Planetary science](man/man1/htrdr-planets.1.html): takes into
    account the geometry of a "ground" of arbitrary shape, described by a
    triangular mesh, with the possibility of using an arbitrary number of
    materials.
    The radiative properties of a gas mixture must be provided on a
    tetrahedral mesh, using the k-distribution spectral model.
    The radiative properties of an arbitrary number of aerosol and
    hydrometeores can also be provided on their individual tetrahedral
    mesh.
    Calculations can be made for both internal and external radiation
    sources.
    In the case of an external source, a sphere of arbitrary size and
    position is used.
    This sphere can radiate as a Planck source at a specified brightness
    temperature, or be associated with a high-resolution radiance
    spectrum.
 
 [![Titan](thumbs/titan_1280x960x4096.jpg)](images/titan_1280x960x4096.jpg)
 
 > Images of Titan rendered with
 > [htrdr-planets](man/man1/htrdr-planets.1.html).
 > Its 3D atmosphere is composed of a gas mixture and two aerosol modes
 > for haze and clouds.
 
 [![Titan transit](thumbs/titan_transit.jpg)](images/titan_transit.jpg)
 
 > Rendering of Titan in transit situation, i.e. the source is
 > positioned behind Titan to simulate a solar eclipse by Titan.
 
 Since any radiative transfer observable is expressed as an integral of
 the radiance, and since there is a strict equivalence between the
 integral to be solved and the underlying Monte-Carlo algorithm (each
 integral is associated with the sampling of a random variable), the
 algorithms that calculate the radiance are used for computing various
 quantities:
 
 - *Images* on a camera sensor, in a given field of view.
   For combustion applications, only monochromatic images are supported.
   In atmospheres and in planetary science, spectral integration is also
   possible, both for solar and thermal images: CIE colorimetry is used
   for solar images, while thermal images are in fact brightness
   temperature maps, obtained from the incoming radiative flux over a
   specified spectral interval.
 
 - *Flux density maps*, on a grid of sensors, integrated over an entire
   hemisphere.
   In the case of combustion chambers, flux density maps can be
   calculated, while spectrally integrated flux density maps are also
   possible for atmospheric application, both for solar and thermal
   radiation.
 
 [![Clouds](thumbs/CLEMENT.jpg)](images/CLEMENT.jpg)
 
 [![Clouds infrared](thumbs/CLEMENT_lw_9000_10000.jpg)](images/CLEMENT_lw_9000_10000.jpg)
 
 > Images rendered with
 > [htrdr-atmosphere](man/man1/htrdr-atmosphere.1.html)
 > of a 1000³ cloud field produced by the Meso-NH
 > ([Lafore et al. 1998](https://www.ann-geophys.net/16/90/1998/angeo-16-90-1998.html),
 >  [Lac et al. 2018](https://www.geosci-model-dev.net/11/1929/2018/))
 > Large Eddy Model.
 > The initial conditions and model set-up for the simulation are
 > described in
 > [Strauss et al. 2019](https://rmets.onlinelibrary.wiley.com/doi/full/10.1002/qj.3614).
 > The infrared rendering is calculated in [9, 10]&nbsp;µm spectral range;
 > the color map displays the brightness temperature in Kelvin.
 
 ## Related articles
 
 - [He et al. 2026](https://doi.org/10.1016/j.jqsrt.2025.109722),
   "Simultaneous estimation of radiance and its sensitivities to
   radiative properties in a spherical-heterogeneous atmospheric
   radiative transfer model by Monte Carlo method: Application to Titan",
   Journal of Quantitative Spectroscopy and Radiative Transfer
   ([open access](https://hal.science/hal-05370209v1/))
 
 - [El Hafi et al. 2025](https://doi.org/10.1016/j.jqsrt.2025.109661),
   "Application of null-collision backward Monte Carlo algorithm to
   digital image rendering of sooting flames in the visible range",
   Journal of Quantitative Spectroscopy and Radiative Transfer
   ([open access](https://www.sciencedirect.com/science/article/pii/S0022407325003231))
 
 - [Caliot et al. 2022](https://doi.org/10.1007/s10546-022-00750-5),
   "Model of Spectral and Directional Radiative Transfer in Complex Urban
   Canopies with Participating Atmospheres", Boundary-Layer Meteorology
   ([open access](https://hal.science/hal-03813906/))
 
 - [Villefranque et al. 2022](https://doi.org/10.1126/sciadv.abp8934),
   "The “teapot in a city”: A paradigm shift in urban climate modeling",
   Science Advances
   ([open access](https://arxiv.org/abs/2204.14227))
 
 - [Sans et al. 2021](https://doi.org/10.1016/j.jqsrt.2021.107725)
   "Null-collision meshless Monte Carlo - A new reverse Monte Carlo
   algorithm designed for laser-source emission in absorbing/scattering
   inhomogeneous media", Journal of Quantitative Spectroscopy and
   Radiative Transfer
   ([open access](https://imt-mines-albi.hal.science/hal-03224186v1))
 
 - [Villefranque et al. 2019](https://doi.org/10.1029/2018MS001602)
   "A Path-Tracing Monte Carlo Library for 3-D Radiative Transfer in
   Highly Resolved Cloudy Atmospheres", Journal of Advances in Modeling
   Earth Systems
   ([open access](https://arxiv.org/abs/1902.01137))
 
 
 <span id="rel_projects"/>
 
 ## Related projects
 
 `htrdr` has been used, developped and extended in the following
 research projects:
 
 - The development of `htrdr` began with the
   [High-Tune](https://anr.fr/Projet-ANR-16-CE01-0010) project.
   Originally, it simulated the radiative transfer of an *external source*
   (solar) in a scene composed of a triangulated ground and an
   *atmosphere*, neglecting Earth sphericity, in the presence of a *cloud
   field* provided over a structured grid.
   It was later extended in order to take into account a *non-gray
   surface*, and the possibility to perform radiative transfer
   computations for a *internal source* (ground and atmosphere).
 
 - In project
   [ModRadUrb](https://www.umr-cnrm.fr/spip.php?article1204) the
   emphasis was put on taking into account the representation of *complex
   geometries* (detailled city scenes) using *spectral properties of an
   arbitrary number of materials*.
   The solver was extended to solve upward and downward *hemispherical
   atmospheric fluxes* on a plane positioned anywhere in the scene, both
   in the visible and the infrared spectral ranges.
 
   Note that the
   [`htrdr` Urban](https://gitlab.com/edstar/htrdr/-/tree/main_urban) fork adds
   the calculation of the radiative flux density incident on or absorbed
   by a group of triangles to the geometry of the ground and humans.
 
 - In project
   [MCG-RaD](https://anr.fr/Projet-ANR-18-CE46-0012)
   the `htrdr` codebase was used to explore a whole new class of
   radiative transfer algorithms: instead of relying on the full
   atmospheric radiative properties data set (prerequisite for current
   algorithms), the so-called *line sampling* algorithms will *not*
   require *nor* compute the absorption coefficient of the atmosphere.
   Instead, it will sample energetic transitions and use a Line-by-Line
   parameters database (such as [HITRAN](https://hitran.org/)) in order
   to perform a *rigorous spectral integration*, both in the visible and
   the infrared spectral ranges.
 
 - In project
   [Astoria](https://anr.fr/Project-ANR-18-CE05-0015).
   `htrdr` was used to produce images in the visible, in the presence
   of *combustion chambers* where radiation scattering is performed by soot
   aggregates.
   One of the main difficulty resides in the fact that the chamber is
   *illuminated by a laser*: the classical solar radiative transfer
   algorithm then fails to converge because of the collimated radiation
   source, and a
   [new algorithm](https://doi.org/10.1016/j.jqsrt.2021.107725)
   was thus designed in order to ensure numerical convergence.
 
 - In project
   [Rad-Net](https://anr.fr/Projet-ANR-21-CE49-0020)
   `htrdr` was adapted for applications in *planetary science* and
   *astrophysics*.
   The application is now a scene composed of an *arbitrary number of
   solid surfaces* (a planet, satellites) represented by triangular
   meshes and materials which describe their *spectral
   reflectivity/emissivity* properties.
   The *3D atmopshere* is defined by a number of participating
   semi-transparent media (a gas mixture and an arbitrary number of
   aerosol modes) whose radiative properties are provided at the nodes of
   a *unstructured tetrahedral volumetric grid*, independant for ach
   medium.
 
 [![Gulder horizontal slides](thumbs/gulder_horizontal_slides.jpg)](images/gulder_horizontal_slides.jpg)
 
 > Renderings in the visible range calculated by
 > [htrdr-combustion](man/man1/htrdr-combustion.1.html) of a laminar
 > sooting flame seen from above and illuminated by a laser sheet.
 > The images display radiation that is emitted by the laser, scattered
 > and transmitted by the combustion medium.
 > The laser sheet is horizontal, and intersects the medium at various
 > heights.
 > One can see the difference in the scattered signal between the left
 > and right parts of the image; since the laser propagates from the left
 > to the right, it is progressively attenuated while traveling the
 > medium.
 > The intensity of radiation subject to scattering therefore decreases.
 > In addition to these trapping effect these images provide some insight
 > about the scattering cross-section of the medium as a function of
 > height.
 > Scattering properties of soot gradually increase from the injection
 > position to a height of approximately 35 mm.
 > A steep decrease follows:
 > the image for a height of 40 mm is very similar to the image obtained
 > at 10 mm.
 
 ## Installation
 
 No pre-compiled version of `htrdr` is provided;
 it must be compiled directly from its source tree. A simple way is to
 rely on [star-build](https://gitlab.com/meso-star/star-build/), which
 automates the build and installation of `htrdr` and its dependencies
 from source code.
 
 ### Prerequisites
 
 To build `htrdr` with `star-build`, first make sure your system has the
 following prerequisites:
 
 - POSIX shell
 - POSIX make
 - curl
 - git
 - mandoc
 - pkg-config
 - sha512sum
 - GNU Compiler Collection in version 8.3 or higher
 - netCDF4 library and headers
 - OpenMPI library and headers in version 2 or higher
 
 ### Build
 
 Assuming that the aforementioned prerequisites are available, the build
 procedure is summed up to:
 
     git clone https://gitlab.com/meso-star/star-build.git
     cd star-build
     make \
       PREFIX=~/htrdr_@VERSION@ \
       BUILD=src/rad-apps/htrdr_@VERSION@.sh
 
 With `PREFIX` defining the path where `htrdr` will be installed and
 `BUILD` defining the installation script to be run.
 
 By default, the whole `htrdr` project is built but you may prefer to
 deploy `htrdr` only for a specific application, i.e. only for
 atmospheric radiative transfer, combustion processes or planetary
 science.
 For example, to install only the atmospheric part of `htrdr`:
 
     make \
         PREFIX=~/htrdr_@VERSION@ \
         BUILD=src/rad-apps/htrdr_@VERSION@.sh \
         ATMOSPHERE=ENABLE \
         COMBUSTION=DISABLE \
         PLANETS=DISABLE
 
 ### Run
 
 Evaluate the installed `htrdr.profile` file in the current
 shell to register `htrdr` against it. You can then run
 `htrdr` and consult its manual pages:
 
     . ~/htrdr_@VERSION@/etc/profile
     htrdr -h
     man htrdr
 
 Refer to the Starter Packs
 ([atmosphere](htrdr-atmosphere-spk.html),
 [combustion](htrdr-combustion-spk.html) or
 [planets](htrdr-planets-spk.html))
 to quickly run a `htrdr` calculation; these archives provide input data
 and scripts that are good starting points to use `htrdr`.
 
 [![Downward ShortWave flux](thumbs/downward_flux_500x500x2048_sw_380_4000_879.349.jpg)](
 images/downward_flux_500x500x2048_sw_380_4000_879.349.jpg)
 
 > Shortwave downward flux density maps in W/m² computed by
 > [htrdr-atmosphere](man/man1/htrdr-atmosphere.1.html)
 > at 1 meter height with the [DZVAR](htrdr-atmosphere-spk.html) cloud
 > field.
 > The sun is located at the zenith.
 > The spectral integration range is [0.38, 4] µm .
 > Its spatially-avaraged flux is 879.349 W/m² .
 > One can observe the contrast between the shadows of the clouds and
 > fully illuminated areas.
 
 [![Downward LongWave flux](thumbs/downward_flux_500x500x2048_lw_4000_100000_425.156.jpg)](
 images/downward_flux_500x500x2048_lw_4000_100000_425.156.jpg)
 
 > Longwave downward flux density maps in W/m² computed on the same scene
 > of the previous image.
 > The spectral integration ranges is [4, 100] µm.
 > Its spatially-avaraged is 425.159 W/m².
 > Note the effect of clouds (higher values, due to the emission by the
 > base of the cloud at higher temperatures than for a clear-sky zone)
 > and also a "ripple" effect that is due to the spatial variations of
 > water vapor concentration, as provided by the LES simulation.
 
 ## History
 
 `htrdr` has been funded by the
 [ANR Rad-Net](https://anr.fr/Projet-ANR-21-CE49-0020) since 2021.
 |Méso|Star> is subcontractor of the project.
 
 `htrdr` was funded by the
 [ANR Astoria](https://anr.fr/Project-ANR-18-CE05-0015)
 from 2018 to 2022.
 |Méso|Star> was sub-contractor of the project.
 
 `htrdr` was funded by the
 [ADEME](https://www.ademe.fr/) (MODEVAL-URBA-2019) from 2019 to 2022.
 |Méso|Star> was partner of the project with
 [CNRM](https://www.umr-cnrm.fr/).
 
 `htrdr` was funded by the
 [ANR High-Tune](https://anr.fr/Project-ANR-16-CE01-0010) from
 2016 to 2019.
 |Méso|Star> was sub-contractor of the project.
 Visit the
 [High-Tune project web site](http://www.umr-cnrm.fr/high-tune/?lang=en)