Dear all,
When simulating ground-level ambient smoke in a fire, incorporating the exponential volumetric fog component provided by Unreal Engine is quite handy.
However, it seems it has limitations, particularly when attempting to simulate light attenuation in an atmosphere dominated by soot particles (BC), which absorb much of the intensity. Indeed, it looks like applying a scattering to absorption ratio equal to zero (in UE ‘fog albedo’=0,0,0) to the volumetric fog results in light extinction following a sigmoidal curve, rather than the expected exponential Bouguer-Beer-Lambert pattern.
Conversely, for isotropic light scattering in air dominated by water droplets (in UE ‘fog albedo’=255,255,255), everything seems to return to normal. In intermediate situations, the sigmoidal curve smooths out but still deviates from the expected behavior.
This limitation complicates the accurate numerical simulation of the optical effect of aerosols presence (particularly, the soot volume fraction) in the air in Unreal Engine, although the visual results are still quite convincing.
- Can you please confirm that this is correct?
- Do you know the maths/Physics behind light attenuation when fog albedo=black?
- Can you please suggest an alternative workflow to obtain realistic results (i.e., given an aerosol particles concentration, provide the parameterization of volumetric fog to mimic the real effect of light attenuation)?
The experiment for measuring this features targets with alternating black and white regions. The targets are spaced 10 meters apart along the depth axis (Y). The camera is positioned 10 meters in front of the first target. Two types of materials are applied to the white part: a set with reflective material, and other set with self-emissive (unlit) material. Each target is sampled in grayscale (0-255) in the resulting image, and the values for the white and black parts are compared to derive the contrast. As we move away from the camera, the resulting color increasingly resembles the fog ‘scattering color’, decreasing light intensity and contrast.
Sampling of the white part of the targets indicates a sigmoidal behavior of light extinction in an environment dominated by soot (‘fog albedo’ = 0,0,0). The measurements have been repeated for increasing values of smoke density. The reference value of light intensity (Io) is sampled for white targets in the situation where the smoke density is 0. This behavior notably deviates from the curve drawn by the Bouguer-Beer-Lambert law (exponential curve in light blue in the graph).
Here you have an example of applying exponential volumetric fog in Unreal Engine for the visual simulation of ground-level ambient smoke in the VR experience I developed for the WUICOM-BCN project dealing with forest fires in WUI microscale (2023).
Any help on this matter will be more than welcome.
Many thanks
David - Gnomusy