Trying to repaint all image pixels recursively

Question

I'm trying to repaint all the image pixels recursively but it fails because the stack overflows. But why?

void go( short x, short y ) {

    if ( x < 0 || y < 0 || x >= _w || y >= _h ) return ;

    _image[ x ][ y ] = someColor;

    go( x + 1, y );
    go( x - 1, y );
    go( x, y - 1 );
    go( x, y + 1 ); 

}

Answer 1

This function call will never stop.

void go( short x, short y ) {    

    if ( x < 0 || y < 0 || x >= _w || y >= _h ) return ;

    _image[ x ][ y ] = someColor;

    go( x + 1, y );
    go( x - 1, y ); 
    go( x, y - 1 );
    go( x, y + 1 ); 

}

If you call your function with x=0,y=0 (go( 0, 0 );) it will do this

    _image[ 0 ][ 0 ] = someColor;    
    go( 1, 0 );
    go( -1, 0 ); 
    go( 0, -1 );
    go( 0, 1 ); 

OK now let's check the function call to go( 1, 0 );:

    _image[ 1 ][ 0 ] = someColor;    
    go( 2, 0 );
    go( 0, 0 );  //ohoh
    go( 1, -1 );
    go( 1, 1 ); 

So you can see that go( 0, 0 ) calls go( 1, 0 ) callsgo( 0, 0 ) calls go( 1, 0 ) callsgo( 0, 0 ) calls go( 1, 0 ) callsgo( 0, 0 ) calls go( 1, 0 ) callsgo( 0, 0 ) calls go( 1, 0 ) calls ....

That's why you have a stack overflow. You won't ever get out.

---

Why don't you just create 2 loops and iterate over these? As your example always iterates x from 0 to _w and yfrom 0 to _hI would just get rid of these parameters at all.

You can just use this:

void go( short x, short y ) {    
    for(short x = 0; x < _w; x++) {
        for(short y = 0; y < _h; y++) {
            _image[ x ][ y ] = someColor;
        }
    }
}

Answer 2

I think you're missing one base case of initial value of _image array, as to stop re-filling same values over and over

if ( _image[x][y] != previousColor ) 
        return;