shader 2 : use shaderToy in unity

shadertoy 原型，https://www.shadertoy.com/view/XslGRr

先说几个概念

　　Shader language目前有3种主流语言：基于OpenGL的GLSL（OpenGL Shading Language，也称为GLslang），基于Direct3D的HLSL（High Level Shading Language），还有NVIDIA公司的Cg （C for Graphic）语言。

其中unity使用的是GLSL，unity使用的是Cg，并且进一步封装，称为shaderLab。

转unity代码如下：

Shader "Shadertoy/Cloud" { 
    Properties{
        iMouse ("Mouse Pos", Vector) = (100, 100, 0, 0)
//        iChannel0("iChannel0", 2D) = "white" {}  
//        iChannelResolution0 ("iChannelResolution0", Vector) = (10, 10, 0, 0)
    }

    CGINCLUDE    
        #include "UnityCG.cginc"   
        #pragma target 3.0      

        #define vec2 float2
        #define vec3 float3
        #define vec4 float4
        #define mat2 float2x2
        #define mat3 float3x3
        #define mat4 float4x4
        #define iTime _Time.y
        // fmod用于求余数，比如fmod(1.5, 1.0) 返回0.5；
        #define mod fmod
        // 插值运算，lerp(a,b,w) return a + w*(a-b);
        #define mix lerp
        // fract(x): 取小数部分 
        #define fract frac
        #define texture2D tex2D
        // 屏幕的尺寸
        #define iResolution _ScreenParams
        // 屏幕中的坐标，以pixel为单位.(_iParam.scrPos.xy/_iParam.scrPos.w)在屏幕中归一化后的屏幕位置,即返回分量范围在(0, 1)的屏幕横纵坐标值。屏幕的左下角值为(0, 0)，右上角值为(1, 1)。
        #define gl_FragCoord ((_iParam.scrPos.xy/_iParam.scrPos.w) * _ScreenParams.xy)

        #define PI2 6.28318530718
        #define pi 3.14159265358979
        #define halfpi (pi * 0.5)
        #define oneoverpi (1.0 / pi)

        fixed4 iMouse;
//        sampler2D iChannel0;
//        fixed4 iChannelResolution0;

        struct v2f {    
            float4 pos : SV_POSITION;    
            float4 scrPos : TEXCOORD0;   
        };              

        //ComputeScreenPos是在UnityCG.cginc中定义的函数，它就作用如名字一样，计算该顶点转换到屏幕上的位置
        v2f vert(appdata_base v) {  
            v2f o;
            o.pos = mul (UNITY_MATRIX_MVP, v.vertex);
            o.scrPos = ComputeScreenPos(o.pos);
            return o;
        }  

        vec4 main(vec2 fragCoord);

        fixed4 frag(v2f _iParam) : COLOR0 { 
            vec2 fragCoord = gl_FragCoord;
//            vec2 pos =fragCoord.xy / iResolution.xy * 2. - 1.;
            return main(fragCoord);
        } 
         
        ////////////////////////////////////////////////////////////////////
        //  https://www.shadertoy.com/view/4sfGzS
        mat3 setCamera( in vec3 ro, in vec3 ta, float cr )
        {
            vec3 cw = normalize(ta-ro);
            vec3 cp = vec3(sin(cr), cos(cr),0.0);
            vec3 cu = normalize( cross(cw,cp) );
            vec3 cv = normalize( cross(cu,cw) );
            return mat3( cu, cv, cw );
        }

        vec3 sundir = normalize( vec3(-1.0,0.0,-1.0) );

        #define MARCH(STEPS,MAPLOD) for(int i=0; i<STEPS; i++) { vec3  pos = ro + t*rd; if( pos.y<-3.0 || pos.y>2.0 || sum.a > 0.99 ) break; float den = MAPLOD( pos ); if( den>0.01 ) { float dif =  clamp((den - MAPLOD(pos+0.3*sundir))/0.6, 0.0, 1.0 ); sum = integrate( sum, dif, den, bgcol, t ); } t += max(0.05,0.02*t); }

        float iqhash( float n )
        {
            return fract(sin(n)*43758.5453);
        }

        float noise( vec3 x )
        {
        // The noise function returns a value in the range -1.0f -> 1.0f
        vec3 p = floor(x);
        vec3 f = fract(x);

        f       = f*f*(3.0-2.0*f);
        float n = p.x + p.y*57.0 + 113.0*p.z;
        return mix(mix(mix( iqhash(n+0.0  ), iqhash(n+1.0  ),f.x),
                       mix( iqhash(n+57.0 ), iqhash(n+58.0 ),f.x),f.y),
                   mix(mix( iqhash(n+113.0), iqhash(n+114.0),f.x),
                       mix( iqhash(n+170.0), iqhash(n+171.0),f.x),f.y),f.z);
        }

        float map5( in vec3 p )
        {
            vec3 q = p - vec3(0.0,0.1,1.0)*iTime;

            float f;

            f  = 0.50000*noise( q ); q = q*2.02;
            f += 0.25000*noise( q ); q = q*2.03;
            f += 0.12500*noise( q ); q = q*2.01;
            f += 0.06250*noise( q ); q = q*2.02;
            f += 0.03125*noise( q );
            return clamp( 1.5 - p.y - 2.0 + 1.75*f, 0.0, 1.0 );
        }
        float map4( in vec3 p )
        {
            vec3 q = p - vec3(0.0,0.1,1.0)*iTime;
            float f;
            f  = 0.50000*noise( q ); q = q*2.02;
            f += 0.25000*noise( q ); q = q*2.03;
            f += 0.12500*noise( q ); q = q*2.01;
            f += 0.06250*noise( q );
            return clamp( 1.5 - p.y - 2.0 + 1.75*f, 0.0, 1.0 );
        }
        float map3( in vec3 p )
        {
            vec3 q = p - vec3(0.0,0.1,1.0)*iTime;
            float f;
            f  = 0.50000*noise( q ); q = q*2.02;
            f += 0.25000*noise( q ); q = q*2.03;
            f += 0.12500*noise( q );
            return clamp( 1.5 - p.y - 2.0 + 1.75*f, 0.0, 1.0 );
        }
        float map2( in vec3 p )
        {
            vec3 q = p - vec3(0.0,0.1,1.0)*iTime;
            float f;
            f  = 0.50000*noise( q ); q = q*2.02;
            f += 0.25000*noise( q );;
            return clamp( 1.5 - p.y - 2.0 + 1.75*f, 0.0, 1.0 );
        }

        vec4 integrate( in vec4 sum, in float dif, in float den, in vec3 bgcol, in float t )
        {
            // lighting
            vec3 lin = vec3(0.65,0.7,0.75)*1.4 + vec3(1.0, 0.6, 0.3)*dif;        
            vec4 col = vec4( mix( vec3(1.0,0.95,0.8), vec3(0.25,0.3,0.35), den ), den );
            col.xyz *= lin;
            col.xyz = mix( col.xyz, bgcol, 1.0-exp(-0.003*t*t) );
            // front to back blending    
            col.a *= 0.4;
            col.rgb *= col.a;
            return sum + col*(1.0-sum.a);
        }

        vec4 raymarch( in vec3 ro, in vec3 rd, in vec3 bgcol, in int2 px )
        {
            vec4 sum = vec4(0.0,0.0,0.0,0.0);

            float t = 0.0;//0.05*texelFetch( iChannel0, px&255, 0 ).x;

            MARCH(30,map5);
            MARCH(30,map4);
            MARCH(30,map3);
            MARCH(30,map2);
            //clamp(x,min,max),min(max(x, min), max),
            //if  min < x < max , return x;
            //if x < min , return min;
            //if x > max , return max;
            return clamp( sum, 0.0, 1.0 );
        }

        //得到像素点的颜色值
        vec4 render( in vec3 ro, in vec3 rd, in int2 px )
        {
            // background sky 
            // dot 点积，各分量分别相乘 后 相加    
            float sun = clamp( dot(sundir,rd), 0.0, 1.0 );
            vec3 col = vec3(0.6,0.71,0.75) - rd.y*0.2*vec3(1.0,0.5,1.0) + 0.15*0.5;
            col += 0.2*vec3(1.0,.6,0.1)*pow( sun, 8.0 );

            // clouds    
            vec4 res = raymarch( ro, rd, col, px );
            col = col*(1.0-res.w) + res.xyz;
            
            // sun glare 
            // pow(x, y): x的y次方；   
            col += 0.2*vec3(1.0,0.4,0.2)*pow( sun, 3.0 );

            return vec4( col, 1.0 );
        }

        vec4 main(vec2 fragCoord) {
            //iResolution:screen size
            //fragCoord:在屏幕上的位置，单位pixel
             vec2 p = (-iResolution.xy + 2.0*fragCoord.xy)/ iResolution.y;

            vec2 m = iMouse.xy/iResolution.xy;
        
            //camera
            vec3 ro = 4.0*normalize(vec3(sin(3.0*m.x), 0.4*m.y, cos(3.0*m.x)));
            vec3 ta = vec3(0.0, -1.0, 0.0);
            mat3 ca = setCamera( ro, ta, 0.0 );
            //ray
            vec3 rd = mul(ca , normalize( vec3(p.xy,1.5)));
            //fragCoord屏幕像素位置.
            vec2 pos = fragCoord.xy / iResolution.xy;
            return render( ro, rd, int2(fragCoord-0.5) ); 
        }
    ////////////////////////////////////////////////////////////////////

    ENDCG    

    SubShader {    
        Pass {    
            CGPROGRAM    

            #pragma vertex vert    
            #pragma fragment frag    
            #pragma fragmentoption ARB_precision_hint_fastest     

            ENDCG    
        }    
    }     
    FallBack Off    
}

参考了乐乐（candycat）的博客，想深入学习shader的可以多看看她的博客和书，很多干货，然后就是https://msdn.microsoft.com/en-us/library/windows/apps/dn166865.aspx，GLSL转HLSL参考文档。

还有一个人一定要说，shaderToy网站上的iq，创造了好多shader，效果非常非常赞。

最后有两个东西没解决，textureLod，和texelFetch这两个函数怎么映射到unity？