<p>In the context of global illumination and the scattering equation, do we need to understand "light sources" as surfaces with a "purely emissive" material on which no scattering is present?</p>

<p>In contrast, any surface in the scene geometry could be equipped with a self-emissive material and hence being somehow a "light source" (it's kind of a philosophical question, I guess).</p>

<p>However, since we often talk about light-carrying paths of "length" <span class="math-container">$k$</span>, where one endpoint is located on the camera and the other on a light source, I guess that's how we need to see it. Am I correct or is there something I'm missing?</p>

CC BY-SA 4.0
2019-12-26T07:20:52.663Z
It can have a brdf, there's no "no scattering is present" requirement.
lightxbulb
CC BY-SA 4.0
2019-12-26T07:35:42.930Z
@lightxbulb But isn't it the case that we usually stop tracing after we hit a light source (when we start a random walk on the camera)?
0xbadf00d
CC BY-SA 4.0
2019-12-26T10:36:44.143Z
No, we usually do not, unless the brdf is perfectly absorbing. If you did this for light sources with a brdf that is not perfectly absorbing you will get a wrong solution of the rendering equation. Whether you decide to model all your light sources as perfectly absorbing or not in practice is a separate question that does not affect the theory.
lightxbulb
CC BY-SA 4.0
2019-12-27T13:59:03.060Z
@lightxbulb Please take note of my comment below [httpdigest's answer](https://computergraphics.stackexchange.com/a/9445/9254).
0xbadf00d
<raytracing><pathtracing><global-illumination><material>
What is a "light source" in global illumination?
0xbadf00d
# What is a "light source" in global illumination?

(Asked by: username_0 on 2019-12-26)

In the context of global illumination and the scattering equation, do we need to understand "light sources" as surfaces with a "purely emissive" material on which no scattering is present?

In contrast, any surface in the scene geometry could be equipped with a self-emissive material and hence being somehow a "light source" (it's kind of a philosophical question, I guess).

However, since we often talk about light-carrying paths of "length" $k$, where one endpoint is located on the camera and the other on a light source, I guess that's how we need to see it. Am I correct or is there something I'm missing?

**Tags:** raytracing, pathtracing, global-illumination, material

> **Comments:**
> **username_1**: It can have a brdf, there's no "no scattering is present" requirement.  
> **username_0**: @username_1 But isn't it the case that we usually stop tracing after we hit a light source (when we start a random walk on the camera)?  
> **username_1**: No, we usually do not, unless the brdf is perfectly absorbing. If you did this for light sources with a brdf that is not perfectly absorbing you will get a wrong solution of the rendering equation. Whether you decide to model all your light sources as perfectly absorbing or not in practice is a separate question that does not affect the theory.  
> **username_0**: @username_1 Please take note of my comment below [httpdigest's answer](https://computergraphics.stackexchange.com/a/9445/9254).  

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