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Some examples of the power of LPE are the ability to:

• Allow for additive compositing, preserving the physical nature of the image and avoiding artifacts through multiplication or division in post processing.
• Output a specific bounce of light (2nd, 3rd, etc.) or even a range of bounces (4th to 8th bounce of light). You can use this to isolate effects or even noisy bounces for subtraction.
• Output a specific object to an LPE. While you can use Cryptomatte to isolate objects with an ID, you can also output the object or collection of objects and their contribution separately.
• Isolate a particular noisy light path for subtraction in comp or an LPE without these light paths to composite.
• Create a non-physical effect like differently colored caustics from their source, change the color cast of an object, or scale lighting.
• Output per-light AOVs for manipulation later.Allow for additive compositing, preserving the physical nature of the image and avoiding artifacts through multiplication or division in post processing.
  
  

Note that trace sets can also be used to accomplish some of these effects but since trace sets are global in the render, LPE will obey these trace sets (meaning data that would have been collected otherwise is restricted by the trace set).

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You can specify a chain of events if you like, such as DDS, meaning Diffuse event to Diffuse event to a Specular in that order. The LPE would collect anything that happens in this specific order and store them. For simplicity we'll cover more common cases first.

Direct and Indirect Light

In ray tracing images, we mimic what happens as light travels through a real world scene.

• Direct Lighting is when light from a source strikes an object directly without anything between the light and the object and seen by the camera. Imagine a flashlight on the wall, the sun on a sidewalk, or the light from this screen onto your desk. That's direct lighting.
• Indirect Lighting is when light has already interacted with others objects before reaching the camera. Imagine a mirror, the light is illuminating everything that is seen indirectly in the mirror (you're lit by a light that then bounces to the mirror and then into the camera, you're seeing the effect of lit objects indirectly in the mirror). Or a whole room can be lit up by a window where the sun is only shining on the floor. These are all indirect lighting examples. Huge amounts of information in our daily lives is indirect. Right now you may be in a room where fixtures cover the lights, that means everything you see is indirectly lit.