Light Map in "Seal Guardian"

Light mapping is a common technique used in games for storing lighting data. "Seal Guardian" used light map in order to support large variety of hardware from iOS, Mac to PC because of its low run time cost. There are many methods to bake the light map such as photon mapping and radiosity. Our baking method is similar to radiosity hemicube[1], but we render a full cube map for each light map texel to store incoming lighting data instead.

Scene with light map
Scene without light map

Light Map Atlas
In each level, light map is built for all static meshes with a second unique UV set. We gather all those static meshes and pack them into a large light map atlas by using this method[2], others method can be chosen, we just pick a simple one.

Packing a single large light map atlas for all static mesh in the scene

Compute Light Map Texel Position
Then we render all the meshes into a RGBA32Float world position render target using the light map atlas layout created before (by a vertex shader which transform the mesh 3D world position vertex to its unique 2D light map UV). Then we query back the render target to store all the written texels which correspond to the world position of each light map texel. Those position will be used for rendering cube maps for radiosity.

Each square represent a single light map texel,
we query back those texel world space position to render cube map for radiosity

Radiosity Baking
As talked before, we use a method similar to hemicube, but rendering a full cube map instead, so we will render a cube map at each light map texel with all the post processing effect/tone mapping off and just storing the lighting data. Because our light map is intended to store the incoming indirect static lighting for each texel, we convert the incoming lighting data cube map rendered at each texel to 2nd order spherical harmonics coefficients(i.e. 4 coefficients for each RGB channels), the conversion method can be found in "Stupid Spherical Harmonics (SH) Tricks"[3]. So we will need 1 RGBA32Float(or RGBA16Float) cube map and 3 temporal RGBA32Float(or RGBA16Float) textures for each radiosity iteration.

No light map, direct lighting and emissive materials only 
Lighting without albedo texture

Radiosity pass 1
In the first pass, we render all the meshes without analytical light source (e.g. directional light) into the cube map. Only the emissive material such as sky and static light placed in the scene get rendered to inject the initial lighting into the radiosity iterations. We support sphere and box shape static light which get rendered into the cube just like an emissive mesh during rendering the cube map. Once the cube map render is completed, we convert the cube map to SH coefficients and store the values. After all the texels are rendered, we will have an incoming lighting light map from emissive mesh and static light source ready for the next pass.
Light map baked with the emissive sky material
Lighting with light map using the emissive sky

Radiosity pass 2
In the second pass, we render all the meshes only with analytical light source and the SH light map from previous pass into a new cube map to calculate the first bound incoming lighting. Then convert the cube map to SH coefficients. After all the texels are rendered and converted to an SH light map, we need to sum this SH light map to the previous pass SH light map to get the accumulated lighting for pass 1 and 2 into another 3 accumulated SH light maps for our final storage (this accumulated light map is not used in the radiosity iteration, just for final radiosity output.).
Light map baked with direct lighting and emissive material
Lighting with light map using direct lighting and emissive sky

Radiosity pass >= 3
For the sub-sequence passes, we can use the SH light map from previous iteration to render the cube map and repeat the conversion to SH and accumulate SH lighting for all passes steps to get the incoming indirect lighting for each light map texels.

Final baked result, showing both direct and indrect lighting
Lighting using light map, without albedo texture 

Storage Format
To store the light map data for runtime and reduce memory usage (3 SH light maps in float format, i.e. 12 values for each texels, is too much data to store...), we decompose the the incoming lighting color data to luma and chroma. We only store the luma data in SH format and compute an average chroma value by integrating the SH RGB incoming lighting data with a SH cosine transfer function along the static mesh normal direction, this will get a reflected Lambertian lighting and we use this value to compute the chroma value. By doing this, we can preserve the directional variation of indirect lighting, keeping an average color of incoming lighting and reduce the light map storage to 6 values per texels. To further reduce the memory usage, we clamp the incoming SH luma value to a predefined range so that we can store it in 8-bit texture. However, using compression like DXT will result in artifacts, so we just store the light map data in 2 RGBA8 textures.

Final light map used in run-time, storing SH luma and average chroma

In this post, we have briefly outlined how the light maps are created in "Seal Guardian". It is based on a modified version of radiosity hemicube and using SH as an intermediate representation for baking and reduce the storage size (by splitting the lighting data to luma and chroma.). We skipped some of the baking details like padding the lighting data for each UV shell in each radiosity iteration to avoid light leaking from empty light map texels. Also "Seal Guardian" is rendered using PBR, that means we have metallic material which doesn't work well with radiosity. Instead of converting the metallic to a diffuse material, we pre-filter all the environment probe in each radiosity pass to get the lighting for metallic material. Also, we would like to improve the light map baking in the future, like improving the baking time, fixing the compression problem, we may try BC6H (but need to find another method for iOS compression...), or using a smaller texture size for chroma light map than the luma SH light map texture...

Lastly, if you are interested in "Seal Guardian", feel free to check it out and its Steam store page is live now. It will be released on 8th Dec, 2017 on iOS/Mac/PC. Thank you.

The yellow light bounce on the floor is done by the yellow metallic wall with pre-filtering the environment map in each radiosity pass
Showing only the indirect lighting



1 則留言:

  1. 非常高兴读到这篇文章.我现在正在做lightmap这一块.我有一个问题想请教,根据什么来定义每个对象的atlas在lightmap中所占的密度是多少?