1.代码

float Box(vec2 uv,vec2 bottomLeft,vec2 topRight,vec2 blur)
{
    vec2 c = smoothstep(bottomLeft,bottomLeft + blur,uv);
    vec2 co2 = smoothstep(topRight,topRight - blur ,uv);
    c *= co2;
    return c.x * c.y;
}


mat2 rotate2d(float angle)
{
    mat2 m = mat2(cos(angle),-sin(angle),sin(angle),cos(angle));
    return m;
}

void mainImage( out vec4 fragColor, in vec2 fragCoord )
{
    // Normalized pixel coordinates (from 0 to 1)
    vec2 uv = fragCoord/iResolution.xy;
    uv.x *= iResolution.x / iResolution.y;
    uv *= 4.0;
    uv = fract(uv);
    //先移动到原点
    vec2 uv2 = uv - vec2(0.5,0.5);
    vec2 uvr = uv2 * rotate2d(iTime * 2.0);
    //再恢复回去
    vec2 uv3 = uvr + vec2(0.5,0.5);
    float c1 = Box(uv3,vec2(0.1,0.1),vec2(0.9,0.9),vec2(0.01,0.01));

    fragColor = vec4(c1 * vec3(1.0,1.0,0.0),1.0);
}

效果:

参考:
https://www.acwing.com/blog/content/58672/