colour_hdri.photometric_exposure_scale_factor_Lagarde2014#

colour_hdri.photometric_exposure_scale_factor_Lagarde2014(EV100: ArrayLike, T: ArrayLike = 9 / 10, f_v: ArrayLike = 98 / 100, theta: ArrayLike = 10) → NDArrayFloat[source]#

Convert the exposure value \(EV100\) to photometric exposure scale factor using Lagarde and de Rousiers (2014) formulation derived from the ISO 12232:2006 Saturation Based Sensitivity (SBS) recommendation.

The model implemented by this definition is appropriate to simulate a physical camera in an offline or realtime renderer.

Parameters:

  • T (ArrayLike) – Exposure value \(EV100\).

  • T – Transmission factor of the lens \(T\).

  • f_v (ArrayLike) – Vignetting factor \(f_v\).

  • theta (ArrayLike) – Angle of image point off axis \(\theta\).

  • EV100 (ArrayLike)

Returns:

Photometric exposure in lux-seconds (\(lx.s\)).

Return type:

numpy.ndarray

Notes

  • The saturation based speed, \(S_{sat}\), of an electronic still picture camera is defined as: \(S_{sat}=\cfrac{78}{H_{sat}}\) where \(H_{sat}\) is the minimum focal plane exposure, expressed in lux-seconds (\(lx.s\)), that produces the maximum valid (not clipped or bloomed) camera output signal. This provides \(1/2\) “stop” of headroom (41% additional headroom) for specular highlights above the signal level that would be obtained from a theoretical 100% reflectance object in the scene, so that a theoretical 141% reflectance object in the scene would produce a focal plane exposure of \(H_{sat}\).

  • Scene luminance \(L\) scaled with the photometric exposure value computed by this definition is almost equal to that given by the colour_hdri.saturation_based_speed_focal_plane_exposure() definition.

References

[ISO06], [LdeRousiers14]

Examples

>>> EV100 = exposure_value_100(8, 1 / 250, 400)
>>> H = photometric_exposure_scale_factor_Lagarde2014(EV100)
>>> print(H)  
0.0002088...
>>> H * 4000  
0.8353523...