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。

Shader "Shadertoy/Cloud" {

    Properties{

        iMouse ("Mouse Pos", Vector) = (, , , )

//        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(,map5);

            MARCH(,map4);

            MARCH(,map3);

            MARCH(,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,.,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

}