• Tile-based Lighting
• Shadow
• Calculation of Visible Area Shadow
• Off-screen shadow calculation
• Global Signed Distance Field
• Lighting Combination
• Direct Lighting
• Combine Indirect Lighting

Next is the Direct Lighting injection phase, which can be seen as a deferred lighting process in Card space, which also includes shadow calculation.

Tile-based Lighting

Before performing lighting calculations, it is necessary to organize the relationship between the light source and the page, that is, to determine the list of light sources that actually affect each area. This is the process of Tiled-based Lighting in many engines.

Relationships between LightSources, Tiles, CardPages, and Cards:

The size of a tile is an important consideration in the design of a display or screen. The standard size of a tile is 8 x 8 pixels. The total pixel number is obviously 64.

Tiles are often used in combination with lights, which have a many-to-many relationship with tiles. This combination is typically calculated in a program or system called LightTiles.

One key feature of tiles is their ability to store light sources. In fact, each tile can store up to 8 light sources.

Additionally, there is a function that is used to determine whether a light source affects the tile. This function is called DoesLightAffectCardPageUVRange, and it plays a crucial role in ensuring that the display is accurate and visually appealing.

Shadow

Compared to shadow calculation under the standard Camera view, Card space needs to deal with a special issue:

• VSM and other shadow mapping systems are designed to cull based on the Camera view. Therefore, areas outside the view may not contain shadow information.
• However, Lumen Card still needs this information to produce correct shadows during light calculation, otherwise it will result in light leaking.

This issue leads to the direct lighting calculation stage of Lumen Card being divided into three parts:

• Shadow calculation using shadow maps
• Before 5.2 official release, this was true, but it is not true now. Please check below.
• Offscreen Shadow calculation using distance field tracing.
• Finally, based on the shadow information, material information captured by SurfaceCache, and information from the light source itself, the Tile is illuminated by the light source.

Calculation of Visible Area Shadow

After this, the visible area does not actually sample the shadow map, but rather marks the affected area.

ShadowMaskRay.ShadowFactor = 1.0f;
ShadowMaskRay.bShadowFactorComplete = false;

Afterwards, the task of shadow calculation was left to the off-screen shadow calculation.

Off-screen shadow calculation

The basic steps of this step are consistent with the previous content:

• Each thread obtains the information it needs to perform shadow Trace, reconstruct the spatial information of the Surface Cache, etc.

Off-screen shadow calculation is performed by tracing the Global Signed Distance Field.

Global Signed Distance Field

GSDF is divided and stored by page in a way similar to Virtual Texture. It is important to note that this is a Texture3D rather than a 2D texture.

Lighting Combination

The final result will merge direct lighting, emissive, and indirect lighting.

In fact, this is divided into 2 steps:

Direct Lighting

Once again, pull the information related to the light tiles and combine it with the previously calculated shadow information. If the current area is not within the shadow, calculate the light intensity using the GetIrradianceForLight function. Output the following buffer:

Combine Indirect Lighting

FinalLighting = (DirectLighting + IndirectLighting) * Diffuse_Lambert(Albedo) + Emissive;

The final buffer: