多角形

n 角形を作る。
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precision highp float;
varying vec2 uv;
uniform sampler2D t;
uniform vec2 resolution;
#define PI acos(-1.)
#define NGON 5.
float polygon(vec2 p,float n,vec2 sv){
    float n1=2.*PI/n;//中心から見て次の頂点までの角度
    float theta=acos(dot(normalize(p),normalize(sv)));
    theta=mod(theta,n1);
    
    float n2=(n-2.)*PI/n;//n角形の1内角
    float a=theta+n2/2.;
    float d=length(sv)*(cos(theta)-cos(a)*sin(theta)/sin(a));
    
    return 1.-smoothstep(d,d+.01,length(p));
}
float polygonLine(vec2 p,float n,vec2 sv){
    return polygon(p,n,sv)-polygon(p,n,length(sv)*.9*normalize(sv));
}
 
 
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