Filament Basic Physical Lighting

light units (统一度量衡)

• luminous intensity 和 illuminance 的转换关系

direct Lighting

directional light

• diffuse almost correct, but specular has artifacts

E⊥ is the illuminance of the light source for a surface perpendicular to said light source

punctual light

point light

球面积分的式子应该少了一项sin(theta)项

spot light

但是为了方便artist调节参数，需要把outer angle和perceived illuminance解耦，近似成 但是也仅限于a matte, diffuse mask that absorbs light的材质才近似物理正确。 最终，spot light 的 render equation有如下形式：

attenuation function

但其实point light和spot light并不遵循inverse square law attenuation.

Photometric lights

如何给artist更大的自由去控制the distribution of light within the space. Photometric lights use a photometric profile to describe their luminous intensity distribution.

• IES (Illuminating Engineering Society)
• EULUMDAT (European Lumen Data format)

比punctual light多一项各个方向采样的intensity IES profile生成pass使用的a photometric profile as a texture的两种方式：

- Photometric profile as a mask
- Photometric profile

float multiplier;
// Photometric profile used as a mask
if (photometricLight.isMasked()) {
    // The desired intensity is set by the artist
    // The integrated intensity comes from a Monte-Carlo
    // integration over the unit sphere around the luminaire
    multiplier = photometricLight.getDesiredIntensity() /
            photometricLight.getIntegratedIntensity();
} else {
    // Multiplier provided for convenience, set to 1.0 by default
    multiplier = photometricLight.getMultiplier();
}

// The max intensity in cd comes from the IES profile
float lightIntensity = photometricLight.getMaxIntensity() * multiplier;