Fall of black pixels in a Quadtree

Question

Hello I am trying to create a function Gravity that make all the black pixels of a Quadtree to "fall" if they can (if there is a white pixel under). for instance Gravity(Qt1) gives Qt2 :

The Quadtree class:

typedef struct Qtree{
    bool allblack;
    struct Qtree * son[4];
}Qtree;

I have made some auxiliariy functions: create a black pixel

Qtree* create_Black(){
    Qtree* I -> malloc(sizeof(Qtree));
    I->son[0] = I->son[1] = I->son[2] = I->son[3] = NULL
    return I;

create a white pixel

Qtree* create_White(){ 
    return NULL;

create a Qtree based on 4 sons

Qtree* create_Comp(Qtree* i0, Qtree* i1, Qtree* i2, Qtree i3){         
    Qtree* I = malloc(sizeof(Qtree));         
    I->son[0] = i0;         
    ...         
    return I;
}

I already have done some code that exchanges black pixel but I can obtain the result wanted :( If some guys could help me, I can't find any ressources on internet