1.利用的函数

当x>1时，让其变为0

2.代码

#define PI 3.1415926

float quadratic(float x) {
    if(x > 1.0){
        return 0.0;
    }
    return pow(1.0 - x,2.0);
}

void mainImage( out vec4 fragColor, in vec2 fragCoord ) {
 // 定义贴图坐标
    vec2 uv = fragCoord / iResolution.xy;

    float v = quadratic(iTime * 0.05);

    vec3 color = texture(iChannel0, uv).rgb;
    // 输出颜色
    vec3 c = vec3(0.0,0.0,0.0);
    //从1.0 变换到 0.0
    if(uv.y > v){
        c = color;
    }
    fragColor = vec4(vec3(c), 1.0);
}

效果: