Fixed shader for StackOverflow question.

vec2 rotate_uv(vec2 uv, float rotation) {
    return vec2(
        cos(rotation) * (uv.x) + sin(rotation) * (uv.y),
        cos(rotation) * (uv.y) - sin(rotation) * (uv.x)
    );
}

void mainImage(out vec4 fragColor, in vec2 fragCoord) {
    vec2 uv = fragCoord / iResolution.xy;
    vec2 fade_uv = uv;

    float time = abs(sin(iTime)) * 36.00 / 36.00;
    float angle = 70.00;
    float angle_1q = mod(angle, 90.0);
    float angle_rad = radians(angle);
    float angle_1q_rad = radians(angle_1q);

    float tan_1q_rad = tan(angle_1q_rad);
    float a = tan_1q_rad / (tan_1q_rad + 1.0);
    float scale = sqrt(2.0*a*a - 2.0*a + 1.0);

    fade_uv = fade_uv - 0.5;
    fade_uv = rotate_uv(fade_uv, angle_rad);
    fade_uv = fade_uv * scale;
    fade_uv = fade_uv + 0.5;

    float alpha = step(
        1.0 - time,
        1.0 - fade_uv.x
    );


    fragColor.xyz = mix(
        texture(iChannel0, uv).xyz,
        vec3(0.00),
        alpha
   );
}