移动端下雪系统的实现

 

做完下雨做下雪，（下雨文章地址：http://gad.qq.com/article/detail/288769）下雪其实和下雨有很多类似的地方。类似的部分我就不再仔细说明，主要还是看不同的部分。个人认为下雪比下雨更加困难。

 

首先还是从雪粒子开始，雪粒子无法像雨一样根据深度来直接判断阻挡，因为雨是垂直下落的，但雪不是，雪是会有各种方向的，所以这里需要做实实在在的碰撞来处理遮挡。

      

 private void OnTriggerEnter(Collider other)
｛
        SnowManager.inst.reclaim(this);
｝

然后要开始处理积雪。第一步还是要实现一个雪的材质，起码要看上去像雪。

基本原理就是雪是白色的，然后加上一个法线，模拟雪表面的纹理。

                           

VertexOutputBaseSimple1 vertForwardBase(VertexInput1 v)
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         UNITY_SETUP_INSTANCE_ID(v);
         VertexOutputBaseSimple1 o;
         UNITY_INITIALIZE_OUTPUT(VertexOutputBaseSimple1, o);
         float4 posWorld = mul(unity_ObjectToWorld, v.vertex);
         o.pos = UnityObjectToClipPos(v.vertex);
         o.pack0.xy = TRANSFORM_TEX(v.texcoord, _MainTex);
         o.pack0.zw = TRANSFORM_TEX(v.texcoord, _BumpMap);
         float3 worldPos = mul(unity_ObjectToWorld, v.vertex).xyz;
         float3 worldNormal = UnityObjectToWorldNormal(v.normal);
         float3 worldTangent = UnityObjectToWorldDir(v.tangent.xyz);
         float tangentSign = v.tangent.w * unity_WorldTransformParams.w;
         float3 worldBinormal = cross(worldNormal, worldTangent) * tangentSign;
         o.tSpace0 = float4(worldTangent.x, worldBinormal.x, worldNormal.x, worldPos.x);
         o.tSpace1 = float4(worldTangent.y, worldBinormal.y, worldNormal.y, worldPos.y);
         o.tSpace2 = float4(worldTangent.z, worldBinormal.z, worldNormal.z, worldPos.z);
         o.vlight = ShadeSH9(float4(worldNormal, 1));
         o.depthuv = mul(depthMat, float4(worldPos, 1));
         TRANSFER_SHADOW(o);
         UNITY_TRANSFER_FOG(o, o.pos);
         return o;
｝ 
float3 H = normalize(_WorldSpaceLightPos0.xyz + viewDir);
float NdotH = max(0, dot(worldNormal, H));
float nl = dot(float3(i.tSpace0.z, i.tSpace1.z, i.tSpace1.z), _WorldSpaceLightPos0.xyz);
float NdotL = dot(worldNormal, _WorldSpaceLightPos0.xyz);
float NdotV = dot(worldNormal, viewDir);
float3 shadow = atten * _LightColor0.rgb;
float y = NdotL * shadow;
float2 uv_ramp = float2(_Strength * NdotV, y);
float3 ramp = tex2D(_SnowTex, uv_ramp.xy);
half ssatten = 1.0;
half3 specular = saturate(pow(NdotH, _Shininess * 128.0) * _Specular);
float shadow1 = SHADOW_ATTENUATION(i);
c.rgb = lerp(albedo.rgb * _Color * shadow * nl * _LightColor0.rgb + albedo.rgb * _Color * 0.5, i.vlight, coverage) + coverage * (ramp + albedo * (specular) * shadow);
c.rgb *= shadow1;

这样就有了基本的感觉。
然后是处理雪的逐渐覆盖，首先想到的就是利用柏林噪声生成fbm，来逐步覆盖雪地。

                                 

float f;
float3 q = worldPos;
f = 0.5000*noise(q);
q = q * 2.02;
f += 0.2500*noise(q);
q = q * 2.03;
f += 0.1250*noise(q);
q = q * 2.01;
f += 0.0625*noise(q);
float NdotD = saturate(dot(normal, _WorldSpaceLightPos0.xyz));
float coverage = NdotD - lerp(1, -1, _cover);
coverage = saturate(coverage);
coverage = f * _noise - lerp(1, -1, coverage) + max(-0.1,i.tSpace1.z * _cover);

                                

  
float4 grassuv1 = mul(MoveMatrix, float4(worldPos, 1));
float2 grassuv2 = grassuv1.xy / grassuv1.w * 0.5 + 0.5;
#if UNITY_UV_STARTS_AT_TOP
       grassuv2.y = 1 - grassuv2.y;
#endif
float4 n = tex2D(_MoveTex, grassuv2);
n.xz = (n.xz - 0.5) * 2;
n.y *= _cover;
UNITY_LIGHT_ATTENUATION(atten, i, worldPos)
float4 c = 0;
float3 worldNormal;
worldNormal.x = dot(i.tSpace0.xyz, normal);
worldNormal.y = dot(i.tSpace1.xyz, normal);
worldNormal.z = dot(i.tSpace2.xyz, normal);
worldNormal = normalize(worldNormal);
c.rgb += albedo.rgb * i.vlight;
float3 H = normalize(_WorldSpaceLightPos0.xyz + viewDir);
float NdotH = max(0, dot(worldNormal, H));
float nl = dot(float3(i.tSpace0.z, i.tSpace1.z, i.tSpace1.z), _WorldSpaceLightPos0.xyz);
float NdotL = dot(worldNormal, _WorldSpaceLightPos0.xyz);
float NdotV = dot(worldNormal, viewDir);
float3 shadow = atten * _LightColor0.rgb;
float y = NdotL * shadow;
float2 uv_ramp = float2(_Strength * NdotV, y);
float3 ramp = tex2D(_SnowTex, uv_ramp.xy);
half ssatten = 1.0;
half3 specular = saturate(pow(NdotH, _Shininess * 128.0) * _Specular);
float shadow1 = SHADOW_ATTENUATION(i);
c.rgb = lerp(albedo.rgb * _Color * shadow * nl * _LightColor0.rgb + albedo.rgb * _Color * 0.5, i.vlight, coverage) + min(1.03, coverage * (coverage * 0.05 + 1)) * (ramp + albedo * (specular) * shadow);
c.rgb *= shadow1;
if (needLine > 0.5 && coverage > 0.7 && n.y > 0.1)
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         c.rgb *= (1 - pow(1 - n.y * f, 5) * 0.3  * pow(coverage, 5));
｝

顶点片段着色器并不复杂，而且可以适当减少噪声计算来获得好的帧率。问题大的反而是为了处理雪的碰撞，以及雪的速度较慢，会积累大量的粒子在屏幕里。

所以改良措施就是直接用雪粒子特效代替，因为粒子特效是多线程优化过的，也支持碰撞，性能会好不少。

其他应该和下雨类似，需要自己整合到项目中。

最后是插件地址：

https://assetstore.unity.com/packages/vfx/shaders/mobilesnowsystem-135279