@Zongyan86
Hi!
Take a look at this post:
https://discourse.threejs.org/t/clouds-of-cubes/3218?u=prisoner849
and its jsfiddle.
Visibility of cubes is controlled in the vertex shader with simple scaling to zero:
float c = smoothNoise( ip * 20. ); // noise for scaling the clouds' cubes
vec3 n = position * step( 0.51, c ) + instancePosition; // scaled by noise