• Vertex Shader
• Overview of the pixel shader
• Light Radiance Calculation
• Local Light Attenuation
• Lambert Diffuse Lighting
• GGX Specular Lighting

Vertex Shader

The engine renders a sphere geometry with the same radius as the point light source.

Then, trigger the pixel rendering for the pixels covered by this sphere.

Overview of the pixel shader

The main process of the pixel shader can be treated as follows:

• Obtain the light data and depth of the current pixel.
• Obtain the shadow attenuation. In our case, the cube is not covered by any shadow. However, if it were, the data would be loaded from here.
• Call the GetDynamicLighting function.

The final output buffer looks like this:

The core calculation is performed inside the GetDynamicLighting function. Let's take a look.

Light Radiance Calculation

This part is already discussed in an old unreal engine 4 siggraph presentation : https://cdn2.unrealengine.com/Resources/files/2013SiggraphPresentationsNotes-26915738.pdf

I highly recommend reading the PDF to understand the theory behind the calculation. In this chapter, I will only explain how it works, rather than talking too much about why.

This flow chart shows how the 4 important terms: diffuse, specular, shadow and distance attenuation effect the final radiance of this light source.

Local Light Attenuation

The local light attenuation step follows the inverse squared rule.

The inverse squared light attenuation is a physical phenomenon that describes how the intensity of light decreases as the distance from its source increases. This phenomenon follows the inverse square law, which states that the intensity of light is inversely proportional to the square of the distance from the light source.

In other words, if you double the distance from a light source, the intensity of the light will decrease by a factor of four. This is because the light is spreading out over a larger surface area as it moves away from the source, which means that the same amount of light is being distributed over a larger area.

You can check the function GetLocalLightAttenuation to see how the inverse square falloff is calculated.

Lambert Diffuse Lighting

The diffuse term is actually really simple:

Then:

GGX Specular Lighting

This part is also perfectly matching the paper, so I don’t have too much to discuss here: