Water drop 2b – Dynamic rain and its effects

Version : 1.1 – Living blog – First version was 3 january 2013

This is the second post of a series about simulating rain and its effect on the world in game. As it is a pretty big post, I split it in two parts a and b:

Water drop 1 – Observe rainy world
Water drop 2a – Dynamic rain and its effects
Water drop 2b – Dynamic rain and its effects
Water drop 3a – Physically based wet surfaces
Water drop 3b – Physically based wet surfaces
Water drop 4a – Reflecting wet world
Water drop 4b – Reflecting wet world

Directly following the part a let’s continue with other rain effects:

Water influence on material, water accumulation and puddles

In the observation post we see that a wet surface is darker and brighter depends on its material properties. The influence of water on a material is complex and will be discuss in detail in the next post : Water drop 3 – Physically based wet surfaces. For now we will follow the guideline define by Nakamae et al in “A Lighting Model Aiming at Drive Simulators” [1]. When it rains water accumulate in ground crack, gap and deformation. With sufficient precipitation puddles can appear and stay a long time even after it stop to rain (as highlight by the observation post). To define the different states of the ground surfaces [1] introduce the following classification:

Type 1: a dry region
Type 2: a wet region; i.e., the region where the road surface is wet but no water gathers
Type 3: a drenched region; i.e., the region where water remains to some extent but no puddles are formed, or the region of the margin of a puddle
Type 4: a puddle region

When a surface is wet (type 2), the paper suggest to apply a reflection coefficient of 5 to 10 on the specular and 0.1 to 0.3 on the diffuse. In the pseudo code of this section, we will represent this water influence by a function DoWetProcess. This function take a percentage of wetting strength under the form of a shader variable we will call wet level. When wet level is 0, the surface is dry, when 1 it is wet. This value is different from the raindrop intensity of the previous sections. Wet level variable is increase when the rain starts and takes some time to decrease it after it stops. Allowing simulating some drying. Here is a simple pseudo code:

void DoWetProcess(inout float3 Diffuse, inout float Gloss, float WetLevel)
{
   // Water influence on material BRDF
   Diffuse    *= lerp(1.0, 0.3, WetLevel);
   // Not the same boost factor than the paper
   Gloss       = min(Gloss * lerp(1.0, 2.5, WetLevel), 1.0);
}

// Water influence on material BRDF
DoWetProcess(Diffuse, Gloss, WetLevel);

Note that’s there is no change apply on the normal, when the surface is wet, we simply use the original normal. Here is shot with an environment map apply:

WaterInfluenceWetDry

For puddles (type 4), the paper suggest a two layers reflection model as the photos of real rainy world show us. For now we keep it simple and just use the water BRDF parameters coupled with the diffuse attenuation of a wet surface. For the margin region of puddles (type 3) a simple weighting function between the two previous models is proposed. Here we lerp between the current material BRDF parameters (wet or not) and result of type 4 BRDF parameters to simulate the presence of accumulate water. Puddles placement need to be control by the artists and we use the alpha channel of vertex colors of a mesh for this purpose.  We provide a tool to our artists to paint vertex color in the editor directly on the mesh instance (more exactly Unreal Engine 3 provides tools). The blend weight of our lerping method is defined by the value of the vertex’s color alpha channel: 0 mean puddle and 255 mean no puddle (the default value for vertex color is often white opaque). Pseudo code:

AccumulatedWater = VertexColor.a;

// Type 2 : Wet region
DoWetProcess(Diffuse, Gloss, WetLevel);

// Apply accumulated water effect
// When AccumulatedWater is 1.0 we are in Type 4
// so full water properties, in between we are in Type 3
// Water is smooth
Gloss    = lerp(Gloss, 1.0, AccumulatedWater);
// Water F0 specular is 0.02 (based on IOR of 1.33)
Specular = lerp(Specular, 0.02, AccumulatedWater);
N        = lerp(N, float3(0, 0, 1), AccumulatedWater);

View in editor: no puddle, puddle paint with vertex color’s tools, puddle

PuddleEditing

Puddle paint can be seen in action at the begin of the puddles, heightmap and ripples youtube video.
Read more of this post