您当前的位置: 又是一个post-process后期效果，god ray 上帝之光，说起上帝之光就是咱们再看太阳时太阳周围一圈的针状光芒
先放组效果，本文的场景资源均来自浅墨大神，效果为本文shader效果
然后就来做下讲解，共有两个shader，一个负责制造ray，一个负责和原屏幕图像混合，于原屏幕图像混合很简单，就是单纯的把两个图像的颜色叠加，控制一下ray的权重，
接下来我们着重讲解一下，制造ray的shader
是一个fragement shader
共有4个外部变量
_ScreenLightPos屏幕上光线的位置，这个需要在c#脚本中计算并传出，稍后会讲解
_Density密度
_Decay衰减
_Exposure曝光，用来控制亮度，大家都知道，在相机中，曝光时间越长图像越亮
先看vertex shader
v2f vert(v2in v)
{
v2f o;
o.pos = mul(UNITY_MATRIX_MVP, v.vertex);
half2 texCoord = v.texcoord;
half2 deltaTexCoord = texCoord - _ScreenLightPos.xy;
deltaTexCoord *= 1.0f / 8 * _Density;
texCoord -= deltaTexCoord;
o.uv0 = texCoord;
texCoord -= deltaTexCoord;
o.uv1 = texCoord;
texCoord -= deltaTexCoord;
o.uv2 = texCoord;
texCoord -= deltaTexCoord;
o.uv3 = texCoord;
texCoord -= deltaTexCoord;
o.uv4 = texCoord;
texCoord -= deltaTexCoord;
o.uv5 = texCoord;
texCoord -= deltaTexCoord;
o.uv6 = texCoord;
texCoord -= deltaTexCoord;
o.uv7 = texCoord;
return o;
}v.texcoord为当前点的坐标
deltaTexCoord为当前点对光源点的反向向量，长度为两点间距离
密度越大deltaTexCoord越大，不超过8，deltaTexCoord始终是个分数
第一个采样点为此处本来位置
采样点渐渐接进光源处
_Density越大采样点间距越大
从0到7，点的位置从光源处越来越近，离此处点越来越远
看看我们的v2f结构体，存了多少坐标点
struct v2f {
float4 pos : POSITION;
float2 uv0 : TEXCOORD0;
float2 uv1 : TEXCOORD1;
float2 uv2 : TEXCOORD2;
float2 uv3 : TEXCOORD3;
float2 uv4 : TEXCOORD4;
float2 uv5 : TEXCOORD5;
float2 uv6 : TEXCOORD6;
float2 uv7 : TEXCOORD7;
};传入值的结构体v2in
struct v2in {
float4 vertex : POSITION;
float2 texcoord : TEXCOORD0;
};我们就得到了当前点到光源点的一条直线中的八个点的坐标，为fragement shader取色混色用
当然本步骤也可在fragement shader中完成，但效率没有vertex shader好，因为不用每个像素都取样，只是每个顶点取样就好
再看fragement shader
half4 frag(v2f i) : COLOR
{
half illuminationDecay = 1.0f;
half4 color = tex2D(_MainTex, i.uv0)*illuminationDecay;
illuminationDecay *= _Decay;
color += tex2D(_MainTex, i.uv1)*illuminationDecay;
illuminationDecay *= _Decay;
color += tex2D(_MainTex, i.uv2)*illuminationDecay;
illuminationDecay *= _Decay;
color += tex2D(_MainTex, i.uv3)*illuminationDecay;
illuminationDecay *= _Decay;
color += tex2D(_MainTex, i.uv4)*illuminationDecay;
illuminationDecay *= _Decay;
color += tex2D(_MainTex, i.uv5)*illuminationDecay;
illuminationDecay *= _Decay;
color += tex2D(_MainTex, i.uv6)*illuminationDecay;
illuminationDecay *= _Decay;
color += tex2D(_MainTex, i.uv7)*illuminationDecay;
color /= 8;
return half4(color.xyz * _Exposure, 1);
}illuminationDecay光照衰减，_Decay是我们外部可控衰减
_Exposure增加亮度
然后就是混色，基本上的原理就是从光源处打出无数条射线，嗯，可以这么理解。
Ray我们就制造好了，接下来我们需要把光线ray与原屏幕图像混合，这一步就比较简单了，只给出源代码，各位自己意会。
Shader "Custom/god ray 2 blend" {
Properties{
_MainTex("Base (RGB)", 2D) = "" {}
_GodRayTex ("God (RGB)", 2D) = ""{}
_Alpha("_Alpha", Float) = 0.5
}
// Shader code pasted into all further CGPROGRAM blocks
CGINCLUDE
#include "UnityCG.cginc"
struct v2in {
float4 vertex : POSITION;
float2 texcoord : TEXCOORD0;
};
struct v2f {
float4 pos : POSITION;
float2 uv : TEXCOORD0;
};
sampler2D _MainTex;
sampler2D _GodRayTex;
uniform float _Alpha;
v2f vert(v2in v)
{
v2f o;
o.pos = mul(UNITY_MATRIX_MVP, v.vertex);
o.uv = v.texcoord;
return o;
}
half4 frag(v2f i) : COLOR
{
half4 color = tex2D(_MainTex, i.uv) + tex2D(_GodRayTex, i.uv)*_Alpha;
//half4 color = tex2D(_MainTex, i.uv);
return color;
}
ENDCG
Subshader{
Tags{ "Queue" = "Transparent" }
Pass{
ZWrite Off
BindChannels
{
Bind "Vertex", vertex
Bind "texcoord", texcoord0
Bind "texcoord1", texcoord1
}
Fog{ Mode off }
CGPROGRAM
#pragma fragmentoption ARB_precision_hint_fastest
#pragma vertex vert
#pragma fragment frag
ENDCG
}
}
Fallback off
} // shader然后就是最后一步，也是十分重要的一步就是通过脚本把它弄到屏幕上，
此处的要点就是要求出光源在屏幕中的位置，
Camera类中有这么一个函数可以把世界坐标转换为屏幕坐标
Camera.WorldToScreenPoint(position)
官网介绍如下
Transforms position from world space into screen space.
把position从世界坐标转换为屏幕坐标
Screenspace is defined in pixels. The bottom-left of the screen is (0,0); the right-top is (pixelWidth,pixelHeight). The z position is in world units from the camera.
左下角是屏幕坐标系的原点，右上角是屏幕的最大范围，超出这个范围的光源我们都不进行god ray渲染了，以此作为判断，否则就会进行错误渲染，屏幕超出光照范围了仍在闪烁。
我们把光源的transport传入脚本，然后检验光源的position
另外还有重要一点就是判断光源在相机前面还是在后面，如果只判断是否在屏幕内的话，相机转到光源后面也会被渲染god ray，解决方法在此，WorldToScreenPoint返回的z值为世界空间内光源与相机的距离，为矢量，所以我们就能用z值正负来判断前后了，为正则光源在相机前可渲染god ray，为负则光源在相机后不可渲染god ray
if (lightScreenPos.z > 0 && lightScreenPos.x > 0 && lightScreenPos.x < camera.pixelWidth && lightScreenPos.y >0 && lightScreenPos.y < camera.pixelHeight)
其实就这么渲染也可以，但是效果并不好，god ray变成了“god point”，原因刚才分析的，shader的原理是取点到光源的八个点，那渲染的结果也就是出现了好多点，层次很分明，就是因为之混乱和了那8次，解决方式就是多次渲染，点多了，就变成线了
我们要想使效果更好一点就要多次渲染
建立两个renderTexure tempRtA和tempRtB用来互相传值
Graphics.Blit(sourceTexture, tempRtA, material);
第一次过滤结果存在tempRtA
传到下一次渲染做_MainTex
Graphics.Blit(tempRtA, tempRtB, material);
再传出tempRtB到第三次渲染，再传出tempRtA。。。
Graphics.Blit(tempRtB, tempRtA, material);
Graphics.Blit(tempRtA, tempRtB, material);
Graphics.Blit(tempRtB, tempRtA, material);
最后做混合，把ray texture传到blend shader作为GodRayTex。然后得到最终结果
materialBlend.SetTexture("_GodRayTex", tempRtA);
Graphics.Blit(sourceTexture, destTexture, materialBlend, 0);
代码如下：
using UnityEngine;
using System.Collections;
[ExecuteInEditMode]
public class godRay2 : MonoBehaviour
{
public Transform lightpos;
public Shader curShader;
public Shader curShaderblend;
private Material curMaterial;
private Material curMateriaBlend;
public Vector4 ScreenLightPos = new Vector4(0, 0, 0, 0);
public float Density = 0.01f;
public float Decay = 0.5f;
public float Exposure = 0.5f;
public float Alpha = 1;
public RenderTexture tempRtA = null;
public RenderTexture tempRtB = null;
private Vector3 lightScreenPos;
#region Properties
Material material
{
get
{
if (curMaterial == null)
{
curMaterial = new Material(curShader);
curMaterial.hideFlags = HideFlags.HideAndDontSave;
}
return curMaterial;
}
}
Material materialBlend
{
get
{
if (curMateriaBlend == null)
{
curMateriaBlend = new Material(curShaderblend);
curMateriaBlend.hideFlags = HideFlags.HideAndDontSave;
}
return curMateriaBlend;
}
}
#endregion
void Start()
{
if (!SystemInfo.supportsImageEffects)
{
enabled = false;
return;
}
if (!curShader && !curShader.isSupported)
{
enabled = false;
}
}
void OnRenderImage(RenderTexture sourceTexture, RenderTexture destTexture)
{
if (curShader != null)
{
lightScreenPos = Camera.main.WorldToScreenPoint(lightpos.position);
if (lightScreenPos.z > 0 && lightScreenPos.x > 0 && lightScreenPos.x < camera.pixelWidth && lightScreenPos.y > 0 && lightScreenPos.y < camera.pixelHeight)
{
material.SetVector("ScreenLightPos", new Vector4(lightScreenPos.x / camera.pixelWidth, lightScreenPos.y / camera.pixelHeight, 0, 0));
// material.SetVector("ScreenLightPos", ScreenLightPos);
material.SetFloat("Density", Density);
material.SetFloat("Decay", Decay);
material.SetFloat("Exposure", Exposure);
materialBlend.SetFloat("Alpha", Alpha);
CreateBuffers();
Graphics.Blit(sourceTexture, tempRtA, material);
Graphics.Blit(tempRtA, tempRtB, material);
Graphics.Blit(tempRtB, tempRtA, material);
Graphics.Blit(tempRtA, tempRtB, material);
Graphics.Blit(tempRtB, tempRtA, material);
materialBlend.SetTexture("_GodRayTex", tempRtA);
Graphics.Blit(sourceTexture, destTexture, materialBlend, 0);
// Graphics.Blit(tempRtA, destTexture, material, 0);
}
else
{
Graphics.Blit(sourceTexture, destTexture);
}
}
else
{
Graphics.Blit(sourceTexture, destTexture);
}
}
void CreateBuffers()
{
if (!tempRtA)
{
tempRtA = new RenderTexture(Screen.width / 4, Screen.height / 4, 0);
tempRtA.hideFlags = HideFlags.DontSave;
}
if (!tempRtB)
{
tempRtB = new RenderTexture(Screen.width / 4, Screen.height / 4, 0);
tempRtB.hideFlags = HideFlags.DontSave;
}
}
void OnDisable()
{
if (curMaterial)
{
DestroyImmediate(curMaterial);
}
}
}
本shader有几个缺点，在比较暗的场景不要使用，因为光源处不亮，所以效果不好，Ray的质量不高，从例子就可以看出来，Ray很不清晰，此处可以和Unity ImageEffect的Sun shafts作比较
最后放上两组效果
林中闪耀的光芒
------ by wolf96 http://blog.csdn.net/wolf96