Question

Can someone explain this?

double distance( int x1, int y1, int x2, int y2 )
{
    //Return the distance between the two points
    return sqrt( pow( x2 - x1, 2 ) + pow( y2 - y1, 2 ) );
}
bool check_collision( Circle &A, Circle &B )
{
    //If the distance between the centers of the circles is less than the sum of their radii
    if( distance( A.x, A.y, B.x, B.y ) < ( A.r + B.r ) )
    {
        //The circles have collided
        return true;
    }

    //If not
    return false;
}

I dont get how this bit of code

//Return the distance between the two points
return sqrt( pow( x2 - x1, 2 ) + pow( y2 - y1, 2 ) );

returns the distance between two circles.. Code is from http://lazyfoo.net/SDL_tutorials/lesson19/index.php

Answer 1

This

sqrt( pow( x2 - x1, 2 ) + pow( y2 - y1, 2 ) )

returns the euclidean distance between the circle centres. As a formula this distance is simply

sqrt((a1-b1)^2 + (a2-b2)^2)

where (a1,a2) and (b1,b2) are the centres of the 2 circles.