Shadow in "Seal Guardian"

"Seal Guardian" uses a mix of static and dynamic shadow systems to support long range shadow to cover the whole level. "Seal Guardian" only use a single directional for the whole level, so part of the shadow information can be pre-computed. It mainly consists of 3 parts: baked static shadow on static meshes stored along with the light map, baked static shadow for dynamic objects stored along with the irradiance volume and dynamic shadow with optional ESM soft shadow.

Static shadow for static objects
During the baking process of the light map, we also compute static shadow information. We first render a shadow map for the whole level in a big render target (e.g. 8192x8192), then for each texel of light map, we can compare against its world position to the shadow map to check whether that texel is in shadow. But we are using a 1024x1024 light map for the whole scene, storing the shadow term directly will not have enough resolution. So we use distance field representation[1] to reduce storage size similar to the UDK[2]. To bake the distance field representing of the shadow term, instead of comparing a single depth value at texel world position as before, we compare several values within a 0.5m x 0.5m grid, oriented along the normal at position similar to the figure below:
Blue dots indicate the positions for sampling shadow map
to compute distance field value for the texel at red dot position.
(The gird is perpendicular to the red vertex normal of the texel.)

By doing this, we can get the shadow information around the baking texel to compute the distance field. We choose this method instead of computing the distance field from a large baked shadow texture because we want to have the shadow distance filed consistently computed in world space no matter how the mesh UV is and this can also avoid UV seam too. But this method may cause potential problem for concave mesh, but so far, for all levels in "Seal Guardian", it is not a big problem.
Static shadow only

Static shadow for dynamic objects
For dynamic objects to receive baked shadow, we baked shadow information and store it along with the irradiance volume. For each irradiance probe location, we compare it to the whole scene shadow map and get a binary shadow value. During runtime, we interpolate this binary shadow value by using the position of dynamic object and the probe location to get a smooth transition of shadow value, just like interpolating the SH coefficients of irradiance volume.

Circled objects does not have light map UV, so they are treated the same as dynamic objects and shadowed with the shadow value stored along with irradiance volume
Each small sphere is a sampling location for storing the SH coefficients and shadow value of the irradiance for dynamic objects.

Dynamic Shadow
We use standard shadow mapping algorithm with exponential shadow map(ESM)[3] to support dynamic shadow in "Seal Guardian".  However due to we need to support a variety of hardware(from iOS, Mac to PC) and minimise code complexity, we choose not to use any cascade shadow map. Instead we use a single shadow map to support dynamic shadow for a very short distance (e.g. 30m-60m) and rely on baked shadow to cover the remaining part of the scene.
Dynamic shadow mixed with static shadow
Dynamic shadow only

Shadow Quality Settings
With the above systems, we can make a few shadow quality settings:
  1. mix of static shadow with dynamic ESM shadow
  2. mix of static shadow with dynamic hard shadow
  3. static shadow only
On iOS platform, we choose the shadow quality depends on the device capability. Besides, as we are using a forward renderer, when we are drawing objects that outside the dynamic shadow distance, those objects can use the static shadow only shader to save a bit of performance.
Soft Shadow
Hard Shadow
No Shadow

We have briefly describe the shadow system in "Seal Guardian", which uses distance field shadow map for static mesh shadow, interpolated static shadow value for dynamic objects and ESM dynamic shadow for a short distance. Also a few shadow quality settings can be generated with very few coding effort.

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.


2 則留言:

  1. The distance field shadow is get min distance to the occluder of the 5x5 grid, and save it to light map shadow channel?The grid plane seems that it's parallel to the red vertex normal of the texel, the plane is vertical, the normal is vertical too.

    1. Sorry for my poor drawing skill, I intended to draw a grid plane which is perpendicular to the vertex normal. And yes, we compute the distance field shadow by getting min distance to the occluding samples of the 0.5m x 0.5m grid (5x5 grid is for illustration purpose and in practice, we use 1cm interval between samples, which result in 50x50 grid. Also the post is mistakenly written 5m x5m grid before, the correct value is 0.5m x 0.5m) and save the result to a light map shadow channel.