1.双向垂直的扫描效果
加法表示并集

#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 * 0.1);

    //up要从1变换到0.5
    float up = v;
    if(up < 0.5){
        up = 0.5;
    }
    //down 从0到0.5
    float down = 1.0 - v;
    if(down > 0.5){
        down = 0.5;
    }

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

上面代码等价于下面，||或也表示并集

#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 * 0.1);

    //up要从1变换到0.5
    float up = v;
    if(up < 0.5){
        up = 0.5;
    }
    //down 从0到0.5
    float down = 1.0 - v;
    if(down > 0.5){
        down = 0.5;
    }

    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 > up || uv.y < down){
        c = color;
    }
    fragColor = vec4(vec3(c), 1.0);
}

效果如下:

2.从左到右扫描

#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 * 0.1);

    //从0到1
    v = 1.0 - v;
    //从0到0.25
    v /= 4.0;
    vec3 a = texture(iChannel0, uv).rgb;
    vec3 c = vec3(0.0,0.0,0.0);
    if(uv.x > 0.25 && uv.x < 0.25 + v){
        c += a;
    }
    if(uv.x > 0.0 && uv.x < v){
        c += a;
    }
    if(uv.x > 0.5 && uv.x < 0.5 + v){
        c += a;
    }
    if(uv.x > 0.75 && uv.x < 0.75 + v){
        c += a;
    }
    fragColor = vec4(vec3(c), 1.0);
}

效果如下: