屏幕后处理效果

Bloom

Bloom特效是游戏中常见的一种屏幕效果。这种特效可以模拟真实摄像机的一种图像效果，它让画面中较亮的区域“扩散”到周围的区域中，造成一种朦胧的效果。

大象之梦

一、Bloom的实现原理

1、首先根据一个阀值提取出图像中较亮的区域，把它们存储在一张渲染纹理中

像素亮度阀值为0.5
可以看见只有较亮的地方不是黑色，其余地方全是黑色（0.0.0）
2、然后对这个渲染纹理进行高斯模糊用来模拟扩散效果
高斯模糊
上面的模糊不是很明显，因为像素采样距离比较短
3、最后把这个渲染纹理和源图像进行混合，得到最终结果
原图像
Bloom

二、实现

using System;
using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class Bloom : PostEffectsBase
{
    public Shader bloomShader;
    private Material bloomMaterial = null;

    public Material material
    {
        get
        {
            bloomMaterial = CheckShaderAndCreateMaterial(bloomShader, bloomMaterial);
            return bloomMaterial;
        }
    }

    [Range(0, 4)] public int iterations = 3;
    [Range(0.2f,3.0f)] public float blurSpread = 0.6f;
    [Range(1, 8)] public int downSample = 2;
    [Range(0.0f, 1.0f)] public float luminanceThreshold = 0.6f;
    
    void OnRenderImage(RenderTexture src, RenderTexture dest)
    {
        if (material != null)
        {
            material.SetFloat("_LuminanceThreshold",luminanceThreshold);
            int rW = src.height / downSample;
            int rH = src.width / downSample;

            RenderTexture buffer0 = RenderTexture.GetTemporary(rW, rH, 0);
            buffer0.filterMode = FilterMode.Bilinear;
            //提取较亮的区域
            Graphics.Blit(src,buffer0,material,0);
            //Graphics.Blit(buffer0,dest);
            
            //进行高斯模糊
            for (int i = 0; i < iterations; i++)
            {
                material.SetFloat("_BlurSize",1.0f+i*blurSpread);
                RenderTexture buffer1 = RenderTexture.GetTemporary(rW,rH,0);
                Graphics.Blit(buffer0,buffer1,material,1);
                
                RenderTexture.ReleaseTemporary(buffer0);
                buffer0 = buffer1;
                buffer1 = RenderTexture.GetTemporary(rW, rH, 0);
                Graphics.Blit(buffer0,buffer1,material,2);
                buffer0 = buffer1;
            }
            //进行Bloom效果
            material.SetTexture("_Bloom",buffer0);
            Graphics.Blit(src,dest,material,3);
            
            RenderTexture.ReleaseTemporary(buffer0);
        }
        else
        {
            Graphics.Blit(src,dest);
        }
    }
}

Shader "Unlit/Bloom"
{
    Properties
    {
        _MainTex ("Texture", 2D) = "white" {}
        _Bloom("Bloom Texture",2D)="black"{}
        _LuminanceThreshold("Luminance Threshold",float)=0.5
        _BlurSize("BlurSize",float) = 1.0
    }
    SubShader
    {
        CGINCLUDE
        #include "UnityCG.cginc"

        sampler2D _MainTex;
        half4 _MainTex_TexelSize;
        sampler2D _Bloom;
        float _LuminanceThreshold;
        float _BlurSize;

        struct v2f
        {
            float4 pos:SV_POSITION;
            half2 uv:TEXCOORD0;
        };

        fixed luminance(fixed4 color)
        {
            return  0.2125 * color.r + 0.7154 * color.g + 0.0721 * color.b;
        }

        v2f vertExtraBright(appdata_img v)
        {
            v2f o;
            o.pos = UnityViewToClipPos(v.vertex);
            o.uv = v.texcoord;

            return o;
        }

        fixed4 fragExtraBright(v2f i):SV_TARGET
        {
            fixed4 texColor = tex2D(_MainTex,i.uv);
            fixed luminanceVal = luminance(texColor);
            fixed val = clamp(luminanceVal - _LuminanceThreshold,0,1);
            //return fixed4(val,val,val,1);
            return texColor * val;
        }

        v2f vertBloom(appdata_img v)
        {
            v2f o;
            o.pos = UnityObjectToClipPos(v.vertex);
            o.uv = v.texcoord;

            return o;
        }

        fixed4 fragBloom(v2f i):SV_TARGET
        {
            return tex2D(_MainTex,i.uv) + tex2D(_Bloom,i.uv);
        }

        ENDCG

        ZTest Always Cull Off ZWrite Off

        Pass
        {
            NAME "EXTRABRIGHT"
            CGPROGRAM
            #pragma vertex vertExtraBright
            #pragma fragment fragExtraBright
            ENDCG
        }

        UsePass "Unlit/GaussionBlur/GAUSSION_BLUR_VERTICAL"
        UsePass "Unlit/GaussionBlur/GAUSSION_BLUR_HORZONTAL"

        Pass 
        {
            NAME "BLOOM"
            CGPROGRAM
            #pragma vertex vertBloom
            #pragma fragment fragBloom
            ENDCG
        }
    }

    Fallback Off
}