Drawing/animating circle arc in glsl

circle-slice.glsl

const float PI = 3.141592653589793;

vec2 rotateUV(vec2 uv, float rotation)
{
    float mid = 0.5;
    return vec2(
        cos(rotation) * (uv.x - mid) + sin(rotation) * (uv.y - mid) + mid,
        cos(rotation) * (uv.y - mid) - sin(rotation) * (uv.x - mid) + mid
    );
}

float halfSlice(vec2 uv, float angle) {
    float circle = 1.0 - length(uv - 0.5);
    circle = step(0.5, circle);
    circle -= step(uv.x, 0.5);
    circle -= step(rotateUV(uv, PI - angle).x, 0.5);
    circle = clamp(circle, 0.0, 1.0);

    return circle;
}

float slice(vec2 uv, float angle) {
    if (angle <= PI) return halfSlice(uv, angle);

    float remainingAngle = angle - PI;
    vec2 ruv = rotateUV(uv, PI);
    return halfSlice(ruv, remainingAngle) + halfSlice(uv, PI);
}

void main() {
    float angle = (0.5 + 0.5 * sin(time)) * PI * 2.0;
    float circle = slice(vUv, angle);
    
    gl_FragColor = vec4(circle);
}