1.差集，即减法
下面是白色部分(即true的部分，在mask中的1部分)的差集
两个部分的白色相减后的图形。
1 - 0 = 1;
1 - 1 = 0;

void mainImage( out vec4 fragColor, in vec2 fragCoord )
{
    // Normalized pixel coordinates (from 0 to 1)
    vec2 uv = (fragCoord.xy - 0.5 * iResolution.xy) / iResolution.y;

    float c = smoothstep(-0.3,0.0,uv.x);
    float d = smoothstep(-0.1,0.2,uv.x);
    c =  c - d;

    fragColor = vec4(vec3(c),1.0);
}

效果:

2.并集，即加法
1 + 1 = 2 > 1(大于1视为1)
1 + 0 = 1

void mainImage( out vec4 fragColor, in vec2 fragCoord )
{
    // Normalized pixel coordinates (from 0 to 1)
    vec2 uv = (fragCoord.xy - 0.5 * iResolution.xy) / iResolution.y;

    float c = smoothstep(0.4,0.5,uv.x);
    float d = smoothstep(-0.3,-0.5,uv.x);
    c =  c + d;

    fragColor = vec4(vec3(c),1.0);
}

效果:

3.交集，即乘法
1 * 0 = 0;
1 * 1 = 1;

void mainImage( out vec4 fragColor, in vec2 fragCoord )
{
    // Normalized pixel coordinates (from 0 to 1)
    vec2 uv = (fragCoord.xy - 0.5 * iResolution.xy) / iResolution.y;

    float c = smoothstep(-0.2,0.1,uv.x);
    float d = smoothstep(0.3,-0.1,uv.x);
    c =  c * d;

    fragColor = vec4(vec3(c),1.0);
}

效果: