shader_type canvas_item;

uniform float ratio : hint_range(0.0, 1.0) = 0.0;
uniform float columns = 16.0;

float hash(float x) {
    return fract(sin(x * 12.9898) * 43758.5453);
}

void fragment() {
    float col = floor(UV.x * columns);
    float dir = hash(col) < 0.5 ? 0.0 : 1.0;

    // ★短冊ごとの開始タイミング（0〜1）
    float delay = hash(col + 10.0);

    // ratio が delay を超えたら動き始める
    float local_ratio = clamp((ratio - delay) * (1.0 / (1.0 - delay)), 0.0, 1.0);

    float show;
    if (dir == 0.0) {
        show = step(local_ratio, UV.y);
    } else {
        show = step(local_ratio, 1.0 - UV.y);
    }
    COLOR = vec4(0.0, 0.0, 0.0, show);
}