ThRend

Infrared rendering for thermography simulation

How to cite

DOI: https://doi.org/10.26868/25222708.2021.30435

Aguerre, Jose; Fernandez, Eduardo. "ThRend: a ray tracing module for infrared rendering of urban scenes". Building Simulation Conference Proceedings, 2021.

Introduction

ThRend is a ray-tracing-based renderer of infrared radiation. Based on the Embree ray-tracing kernels, ThRend is designed to rapidly generate simulated thermograms based on few input data. This software can be considered as a post-processing tool that takes the output of other thermal simulation software and allows to simulate the behavior of long-wave radiation reaching an infrared sensor (thermal camera). ThRend gives the user the possibility of trying different emissivity and reflectivity configurations to render thermal images.

Input data

ThRend input data is handled through two configuration files: viewSettings and materials. viewSettings contains the configuration of the scene, camera and output images. materials describes the infrared properties of the materials to be used.

Example of viewSettings file:

#scene settings                                               
sceneFile bayonne14hs.inp
skyTempsFile tsky  

# camera and image settings
# location of the camera:
cameraCenter  -87.6051 9.4538 1.0

# direction of the camera:
cameraDirection 0.9623 -0.21 0.172
cameraUp 0 0 1

# field of view of camera in vertical direction (in degrees)
fovVertical 24

# resolution of the image 
imageWidth 180
imageHeight 250

# Primary rays per pixel (for antialiasing)
aa 16

# Number of reflected rays per pixel
reflSamples 100;

# Maximum bounces, greater than 0
MAX_BOUNCES 2

# Colormap settings
colormapFile colormap
tmin 10
tmax 40

tmin_reflected -10
tmax_reflected 30

In this example, the scene geometry and nodal temperatures are loaded from the AVS UCD file bayonne14hs.inp, as indicated by the tag sceneFile. This kind of file can be exported from most thermal software, and its specifications can be found in [1]. For example, in Cast3m, you can export your CHPOINT chp1 to this format with the following command:

SORT AVS geo1 chp1

Currently, ThRend supports geometries (geo1) composed of quad and tri surfaces, but it should be very easy to adapt the code to handle other element types.

The skyTempsFile tag specifies the file where to find the sky temperatures. This file has only one line with 10 values indicating the temperature of the sky in different zenith angles (specified in kelvin, from the zenith into the horizon with 10 degrees steps):

233.6 235.4 238.4 242.6 248 254.6 262.4 271.4 281.6 293.0

These temperatures are used for rays that do not hit any geometry. If you do not want to use this alternative, just input a fully closed mesh (eg. a room or a sky box).

The following tags set the camera properties, which are pretty self-explanatory. The tag aa indicates the number of primary rays per pixel for antialiasing. Please beware that the execution time of ThRend is linear with respect to this number, so try to keep it as small as possible. If you do not want antialiasing, just put aa 1.

The tag reflSamples indicates the number of reflected rays to cast per pixel. You want to have a number of rays that ensures sufficiently good sampling.

The tag MAX_BOUNCES contains the limit of radiation bounces. This number has to be greater than 0 to allow for infrared reflections. Larger number of bounces implies larger execution times.

The tag colormapFile indicates the file where to find the colormap specification of the output, using the same format as MATLAB colormaps (each line contains one RGB color). The tags tmin and tmax indicate the colormap temperature limits (in Celsius) used for the output (for example, minimum temperature 10C and maximum 40C). tmin_reflected and tmax_reflected are the equivalent for the reflected temperature output image, which in this example are lower because most of the reflected temperature correspond to the cold sky.

Example of materials file:

# material definition
name wood
UCD_id 2
normal_emissivity 0.95
diffuse_fraction 0.9
roughness 0.0995

name mortar
UCD_id 3
normal_emissivity 0.91
diffuse_fraction 0.85
roughness 0.0814

name glass
UCD_id 9
normal_emissivity 0.92
diffuse_fraction 0
roughness 0.0000

This file contains the definition of the infrared properties of the materials. Each material is associated with a different color in the UCD file. The idea here is to have the elements grouped by color (e.g. Cast3m colors), where each color has its own material properties. The tag name indicates the name of the created material, while the tag UCD_id indicates the associated id in the geometry file.

The tag normal_emissivity indicates the emissivity value at normal direction ε_n. This value is used to generate a directional emissivity curve following Schlick's approximation [2]:

ε (θ) = ε_n - ε_n(1 - cos(θ))⁵

The tag diffuse_fraction allows to generate an interpolation between the Schlick (Fresnel) emissivity curve and a diffuse curve of constant emissivity (using the value defined at normal_emissivity). This can be used to describe different levels of roughness in the materials, where for example glass is 0% diffuse, while wood is 90% diffuse. Note that this values define the emissivity behavior only; the reflectivity is handled with the next tag. Some examples of interpolated curves are:

Two types of reflections can be used in ThRend: glossy and diffuse reflections. Glossy reflections are handled through importance sampling of the microfacet BRDF with GGX [3]. Hammersley sampling [4] is used to choose the ray directions. The tag roughness indicates the roughness of the material, as defined by the following probability density function (where alpha_g is the roughness):

Here are some examples of sampling using different roughness:

See that smaller values of roughness imply more specular reflection behaviors, this is why, in the example above, the material glass has the lowest roughness value.

Diffuse reflections, on the other hand, are handled with Beckers-and-Beckers view factor sampling [5]. A purely diffuse reflection can be obtained by setting roughness to be -1:

name diffuseMaterialName
UCD_id 11
normal_emissivity 0.9
diffuse_fraction 0.7
roughness -1

One final note about material definition is that ThRend supports the definition of custom emissivity curves, such as in this example:

#custom material, defined with a curve of 91 emissivity values (from 90 to 0 degrees of viewing angle)
name customMat1
UCD_id 12
emissivity_curve 0.4650 0.5115 0.5528 0.5895 0.6223 0.6517 0.6779 0.7015 0.7228 0.7419 0.7591 0.7747 0.7889 0.8016 0.8132 0.8238 0.8333 0.8420 0.8500 0.8572 0.8637 0.8697 0.8752 0.8802 0.8847 0.8889 0.8927 0.8961 0.8993 0.9022 0.9048 0.9072 0.9095 0.9115 0.9133 0.9150 0.9165 0.9179 0.9192 0.9204 0.9215 0.9224 0.9233 0.9241 0.9249 0.9255 0.9261 0.9267 0.9272 0.9277 0.9281 0.9284 0.9288 0.9291 0.9294 0.9296 0.9298 0.9300 0.9302 0.9303 0.9305 0.9306 0.9307 0.9308 0.9309 0.9310 0.9310 0.9311 0.9312 0.9312 0.9312 0.9313 0.9313 0.9313 0.9313 0.9313 0.9314 0.9314 0.9314 0.9314 0.9314 0.9314 0.9314 0.9314 0.9314 0.9314 0.9314 0.9314 0.9314 0.9314 0.9314 
roughness 0.0814

The tag emissivity_curve tag replaces the normal_emissivity and diffuse_fraction tags. It allows defining a custom curve by entering 91 emissivity values: one value for each incidence angle (from 90 to 0 degrees).

Execution

If you want to try ThRend without compiling it, just download the folder "executable/". You will find all the configuration files and an example of scene. Go into "binary/", and execute "ThRend.exe". You should see a console similar to this one:

The results will be saved in the folder "executable/results". This executable is only compatible with Windows 8/10 x64. If you need a Linux executable, do not hesitate to contact me.

Output data

ThRend generates one main result and 4 auxiliary files. The main result is the file "apparent.png", which stores the rendered apparent surface temperature of the scene. The 4 auxiliary files are: "real.png", which shows the result as if every material was a blackbody, "emis.png", which stores a grayscale image showing the computed emissivity for each pixel, "refl.png" which shows the reflected temperature for each pixel (using the second colormap scale defined in viewSettings), and the file "temps", which stores the apparent temperatures as a matrix of numbers that can be loaded directly into Matlab.

This is the output of the default project that comes with the executable:

Source code and compilation

ThRend was developed in C++ with Visual Studio 2013. All the source code and VS projects are uploaded in this Github page. There are many dependencies but they are all portable, so you should be able to download and compile the project directly. The only prerequisite is Visual Studio 2013 or greater. If you need a Linux compilation project, do not hesitate to contact me.

The libraries used by ThRend are the following:

1. Intel Embree 3 for ray tracing operations (https://www.embree.org/)
2. GLM for vector and matrix operations (https://glm.g-truc.net/0.9.9/index.html).
3. FreeImage for saving images (http://freeimage.sourceforge.net/).

References

[1] AVS-UCD file format description. site: https://dav.lbl.gov/archive/NERSC/Software/express/help6.1/help/reference/dvmac/UCD_Form.htm. Accessed: May 8, 2020.

[2] Schlick, C. (1994, August). An inexpensive BRDF model for physically‐based rendering. In Computer graphics forum (Vol. 13, No. 3, pp. 233-246). Edinburgh, UK: Blackwell Science Ltd.

[3] Walter, B., Marschner, S. R., Li, H., & Torrance, K. E. (2007). Microfacet Models for Refraction through Rough Surfaces. Rendering techniques, 2007, 18th.

[4] Suffern, K. (2016). Ray Tracing from the Ground up. CRC Press.

[5] Beckers, B., and Beckers, P. (2016, September). Fast and accurate view factor generation. In FICUP, An International Conference on Urban Physics (Vol. 9).