1.距离与符号
length(uv) 表示到圆心的距离

sign 函数:
如果输入值大于或等于0，则返回1。
如果输入值小于0，则返回-1

sign(Circle(uv,0.7)) 半径大于0.7返回1,小于0.7返回-1

-1的话就是黑色，1- 那就是白色了.
c = 1.0 - sign(Circle(uv,0.7)) * c;

#define PI 3.1415926

vec2 fixUV(in vec2 uv){
    return (2.0 * uv - iResolution.xy) / 
           min(iResolution.x,iResolution.y);
}

float Circle(vec2 uv,float r){
    return length(uv) - r;
}

void mainImage(out vec4 fragColor,in vec2 fragCoord){
    vec2 uv = fixUV(fragCoord);
    vec3 c = vec3(0.4,0.5,0.6);
    c = 1.0 - sign(Circle(uv,0.7)) * c;
    fragColor = vec4(c,1.0);
}

效果:

2.远近明暗

#define PI 3.1415926

vec2 fixUV(in vec2 uv){
    return (2.0 * uv - iResolution.xy) / 
           min(iResolution.x,iResolution.y);
}

float Circle(vec2 uv,float r){
    return length(uv) - r;
}

void mainImage(out vec4 fragColor,in vec2 fragCoord){
    vec2 uv = fixUV(fragCoord);
    vec3 c = vec3(0.4,0.5,0.6);
    float d = Circle(uv,0.7);
    c = 1.0 - sign(d) * c;
    //离圆心越近越亮
    c *= 1.0 - exp(-3.0 * abs(d));
    fragColor = vec4(c,1.0);
}

效果: