ray marching shader

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shadertoy上的第一篇关于ray marching的教程，实现了在屏幕上绘制一个3D球体

float minDist=0.0;float maxDist=100.0;float getDist(vec3 cameraOrign , vec3 dir ){       float depth=minDist;       for(int i =0;i<255;i+=1){     vec3 rayPosition=cameraOrign+depth*dir;          float  sphereSDF=1.0;           float dist=length(rayPosition)-1.0;          if(dist<0.01){      return depth;              }            depth+=dist;      if(depth>=maxDist){       return maxDist;          }          }    return maxDist;    }void mainImage( out vec4 fragColor, in vec2 fragCoord ){vec2 size=iResolution.xy;           vec2 rayMarchXY=fragCoord-size/2.;        float fieldOfView=45.0;    float z = size.y / tan(radians(fieldOfView) / 2.0);        vec3 rayDir=normalize( vec3(rayMarchXY,-z) );        vec3 cameraOrign=vec3( 0,0 , 5);        float dist=getDist(cameraOrign,rayDir);       if(dist>maxDist-0.001){    fragColor=vec4(0.0,0.0,0.0,1.0);    return;    }        vec3 p = cameraOrign + dist * rayDir;    vec3 K_a = vec3(0.2, 0.2, 0.2);    vec3 K_d = vec3(0.7, 0.2, 0.2);    vec3 K_s = vec3(1.0, 1.0, 1.0);    float shininess = 10.0;                vec3 color = vec3 (0.25,0.25,0.25);        vec3 light1Pos = vec3(4.0 * sin(iGlobalTime),                          2.0,                          4.0 * cos(iGlobalTime));        float EPSILON=0.001;    vec3 N = normalize(vec3(        length(vec3(p.x + EPSILON, p.y, p.z)) - length(vec3(p.x - EPSILON, p.y, p.z)),        length(vec3(p.x, p.y + EPSILON, p.z)) - length(vec3(p.x, p.y - EPSILON, p.z)),        length(vec3(p.x, p.y, p.z  + EPSILON)) - length(vec3(p.x, p.y, p.z - EPSILON))    ));    vec3 L = normalize(light1Pos - p);        float dotLN = dot(L, N);        if (dotLN < 0.0) {        color+= vec3(0.0, 0.0, 0.0);    }            color+= vec3( dotLN,dotLN,dotLN);         fragColor = vec4(color, 1.0);}

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