这个水波效果并非真实的让顶点去运动而产生的的效果,而是通过法线扰动而产生的模拟效果,产生效果主要分为三个步骤,第一个步骤是计算折射,第二个步骤是计算反射,最后一个步骤是混合反射和折射的值。
首先是计算折射值,这个也是这三个中最复杂的一个部分,首先折射所采样的纹理是通过grabpass来实现的,grabpass可以在渲染完毕所有的不透明物体之后把渲染完毕的图形存储到一个纹理中,然后在下一个pass中就可以调用这个纹理了,而这个例子中的grabpass得到的纹理就是上图去掉水流的纹理。
然后我们需要获取屏幕坐标来对grabpass得到的_RefractionTex进行采样,想弄明白这个采样过程还是得回去好好看看渲染流水线。
首先使用o.scrPos = ComputeGrabScreenPos(o.pos);来获取屏幕坐标,不过得注意,这个坐标是齐次坐标,是没有除过w的,而且顶点着色器里面也不能去除w,因为顶点着色器得到的结果将会差值到片元着色器中,除以w之后会破坏它的线性,导致差值的结果变的不正确,因此,我们还需要在片元着色器中除以w来得到真正的屏幕坐标。
我们来观察一下ComputeGrabScreenPos的源码:
inline float4 ComputeGrabScreenPos (float4 pos) {
#if UNITY_UV_STARTS_AT_TOP
float scale = -1.0;
#else
float scale = 1.0;
#endif
float4 o = pos * 0.5f;
o.xy = float2(o.x, o.y*scale) + o.w;
#ifdef UNITY_SINGLE_PASS_STEREO
o.xy = TransformStereoScreenSpaceTex(o.xy, pos.w);
#endif
o.zw = pos.zw;
return o;
}
去掉那些对平台进行特殊处理的代码,这代码翻译过来的意思就是,
o.x = pos.x/2+pos.w/2,o.y = pos.y/2+pos.w/2,o.zw = pos.zw;
稍微变一下
o.x = (pos.x/pos.w+1)*pos.w/2(把(-1,1)范围的NDC映射到(0,1)的视口坐标)
把它最后再除一个pos.w就是最终的视口坐标了,也就是说把NDC转换为了除以w之前的视口坐标
接下来就是计算偏移的代码
float2 offset = bump.xy * _Distortion * _RefractionTex_TexelSize.xy;
i.scrPos.xy = offset * i.scrPos.z + i.scrPos.xy;
要是问我为什么要这样计算偏移,其实我也不知道,乍一看都是没有根据的,大概是一些业内的经验所得的公式吧。
用偏移后的屏幕坐标对折射贴图进行采样就能得到折射的值。
相对来说,反射的值计算要容易的多,直接对cubemap进行采样即可,最后我们再利用freshnel公式来获得反射和折射的比例,根据这个比例混合反射和折射的值即可。
完整源码:
Shader "Unity Shaders Book/Chapter 15/Water Wave" {
Properties {
_Color ("Main Color", Color) = (0, 0.15, 0.115, 1)
_MainTex ("Base (RGB)", 2D) = "white" {}
_WaveMap ("Wave Map", 2D) = "bump" {}
_Cubemap ("Environment Cubemap", Cube) = "_Skybox" {}
_WaveXSpeed ("Wave Horizontal Speed", Range(-0.1, 0.1)) = 0.01
_WaveYSpeed ("Wave Vertical Speed", Range(-0.1, 0.1)) = 0.01
_Distortion ("Distortion", Range(0, 100)) = 10
}
SubShader {
// We must be transparent, so other objects are drawn before this one.
Tags { "Queue"="Transparent" "RenderType"="Opaque" }
// This pass grabs the screen behind the object into a texture.
// We can access the result in the next pass as _RefractionTex
GrabPass { "_RefractionTex" }
Pass {
Tags { "LightMode"="ForwardBase" }
CGPROGRAM
#include "UnityCG.cginc"
#include "Lighting.cginc"
#pragma multi_compile_fwdbase
#pragma vertex vert
#pragma fragment frag
fixed4 _Color;
sampler2D _MainTex;
float4 _MainTex_ST;
sampler2D _WaveMap;
float4 _WaveMap_ST;
samplerCUBE _Cubemap;
fixed _WaveXSpeed;
fixed _WaveYSpeed;
float _Distortion;
sampler2D _RefractionTex;
float4 _RefractionTex_TexelSize;
struct a2v {
float4 vertex : POSITION;
float3 normal : NORMAL;
float4 tangent : TANGENT;
float4 texcoord : TEXCOORD0;
};
struct v2f {
float4 pos : SV_POSITION;
float4 scrPos : TEXCOORD0;
float4 uv : TEXCOORD1;
float4 TtoW0 : TEXCOORD2;
float4 TtoW1 : TEXCOORD3;
float4 TtoW2 : TEXCOORD4;
};
v2f vert(a2v v) {
v2f o;
o.pos = mul(UNITY_MATRIX_MVP, v.vertex);
o.scrPos = ComputeGrabScreenPos(o.pos);
o.uv.xy = TRANSFORM_TEX(v.texcoord, _MainTex);
o.uv.zw = TRANSFORM_TEX(v.texcoord, _WaveMap);
float3 worldPos = mul(unity_ObjectToWorld, v.vertex).xyz;
fixed3 worldNormal = UnityObjectToWorldNormal(v.normal);
fixed3 worldTangent = UnityObjectToWorldDir(v.tangent.xyz);
fixed3 worldBinormal = cross(worldNormal, worldTangent) * v.tangent.w;
o.TtoW0 = float4(worldTangent.x, worldBinormal.x, worldNormal.x, worldPos.x);
o.TtoW1 = float4(worldTangent.y, worldBinormal.y, worldNormal.y, worldPos.y);
o.TtoW2 = float4(worldTangent.z, worldBinormal.z, worldNormal.z, worldPos.z);
return o;
}
fixed4 frag(v2f i) : SV_Target {
float3 worldPos = float3(i.TtoW0.w, i.TtoW1.w, i.TtoW2.w);
fixed3 viewDir = normalize(UnityWorldSpaceViewDir(worldPos));
float2 speed = _Time.y * float2(_WaveXSpeed, _WaveYSpeed);
// Get the normal in tangent space
fixed3 bump1 = UnpackNormal(tex2D(_WaveMap, i.uv.zw + speed)).rgb;
fixed3 bump2 = UnpackNormal(tex2D(_WaveMap, i.uv.zw - speed)).rgb;
fixed3 bump = normalize(bump1 + bump2);
// Compute the offset in tangent space
float2 offset = bump.xy * _Distortion * _RefractionTex_TexelSize.xy;
i.scrPos.xy = offset * i.scrPos.z + i.scrPos.xy;
fixed3 refrCol = tex2D( _RefractionTex, i.scrPos.xy/i.scrPos.w).rgb;
// Convert the normal to world space
bump = normalize(half3(dot(i.TtoW0.xyz, bump), dot(i.TtoW1.xyz, bump), dot(i.TtoW2.xyz, bump)));
fixed4 texColor = tex2D(_MainTex, i.uv.xy + speed);
fixed3 reflDir = reflect(-viewDir, bump);
fixed3 reflCol = texCUBE(_Cubemap, reflDir).rgb * texColor.rgb * _Color.rgb;
fixed fresnel = pow(1 - saturate(dot(viewDir, bump)), 4);
fixed3 finalColor = reflCol * fresnel + refrCol * (1 - fresnel);
return fixed4(finalColor, 1);
}
ENDCG
}
}
// Do not cast shadow
FallBack Off
}