Workflows

Sky and Horizon View Factors

Sky and horizon view factor diagram

This guide explains how the current voxel shading implementation computes sky view factor (SVF), horizon view factor (HVF), and dome sky-bin fractions for diffuse radiation.

Source Implementation

  • EnergyAtlasCore/VoxelShading/SkyViewFactor.cs
  • EnergyAtlasCore/VoxelShading/HorizonViewFactor.cs
  • EnergyAtlasCore/VoxelShading/ShadingCalculation.cs
  • EnergyAtlasLib/Preprocessing/TiledSensorShading.cs

Sky View Factor

SkyViewFactor.ComputeSingleSensorSVF starts from a ShadingSensor with a location, unit normal, and represented area.

  1. Generate SkyDomeSamples cosine-weighted Hammersley hemisphere directions in a local +Z frame.
  2. Build an orthonormal basis from the sensor normal and rotate every sample into world space.
  3. Skip directions whose world Z component is less than or equal to zero.
  4. Bin each remaining ray into a 6 x 24 EnergyPlus-style sky matrix:
  5. altitude bands: [0,15), [15,30), [30,45), [45,60), [60,75), [75,90]
  6. azimuth sectors: 24 sectors at 15 degree spacing
  7. Ray-march from the sensor location through the voxel grid up to CalculationRadius.
  8. Count rays with no voxel hit as visible.
  9. Return visibleCount / SkyDomeSamples.

For a group, ComputeGroupSVF averages all sensor SVFs and averages the per-bin DomeSFs matrices.

Horizon View Factor

HorizonViewFactor.ComputeSingleHVF uses horizontal directions instead of dome samples.

  1. Generate HorizonSamples evenly spaced directions around the horizontal plane.
  2. If the sensor normal is almost vertical upward, return 0.0; the roof does not see horizon diffuse in this model.
  3. Keep only directions in front of the sensor plane, where direction dot normal > 0.
  4. Ray-march each kept horizontal direction.
  5. Weight visibility by direction dot normal.
  6. Return visibleSum / sum.

For a group, ComputeGroupHVF averages all sensor HVFs.

Parameters

Parameter Source field Effect
SkyDomeSamples VoxelShadingCalcParams.SkyDomeSamples Number of dome rays per sensor. Default 512.
HorizonSamples VoxelShadingCalcParams.HorizonSamples Number of horizontal rays per sensor. Default 24.
CalculationRadius VoxelShadingCalcParams.CalculationRadius Ray-marching max distance. Default 500.
CalculateForDiffuse VoxelShadingCalcParams.CalculateForDiffuse Enables SVF/HVF/DomeSF calculation. Default true.

Outputs

Output Meaning Consumer
SkyViewFactor Group mean sky visibility fraction. PerezSky.Diffuse dome visibility.
HorizonViewFactor Group mean horizon visibility fraction. PerezSky.Diffuse horizon visibility.
DomeSFs 6 x 24 sky-bin lit fractions. Stored on ShadingGroupResult; not yet used in the simplified incident-radiation path.

Notes

  • SVF and HVF use the same voxel-grid ray-marching backends as direct sunlit fraction.
  • DomeSFs bins with no sampled rays default to 0.5.
  • If CalculateForDiffuse is false, ShadingGroupResult keeps default SVF/HVF values from the result model rather than computing them.