I have object A, with a speed. Speed is specified as 3D vector a = (x, y, z)
. Position is 3D point A [X, Y, Z]
. I need to find out, if the current speed leads this object to another object B on position B [X, Y, Z]
.
I've sucessfully implemented this in 2 dimensions, ignoring the third one:
/*A is projectile, B is static object*/
//entity is object A
// - .v[3] is the speed vector
//position[3] is array of coordinates of object B
double vector[3]; //This is the vector c = A-B
this->entityVector(-1, entity.id, vector); //Fills the correct data
double distance = vector_size(vector); //This is distance |AB|
double speed = vector_size(entity.v); //This is size of speed vector a
float dist_angle = (float)atan2(vector[2],vector[0])*(180.0/M_PI); //Get angle of vector c as seen from Y axis - using X, Z
float speed_angle = (float)atan2((double)entity.v[2],entity.v[0])*(180.0/M_PI); //Get angle of vector a seen from Y axis - using X, Z
dist_angle = deg180to360(dist_angle); //Converts value to 0-360
speed_angle = deg180to360(speed_angle); //Converts value to 0-360
int diff = abs((int)compare_degrees(dist_angle, speed_angle)); //Returns the difference of vectors direction
I need to create the very same comparison to make it work in 3D - right now, the Y positions and Y vector coordinates are ignored.
What calculation should I do to get the second angle?
Edit based on answer:
I am using spherical coordinates and comparing their angles to check if two vectors are pointing in the same direction. With one vector being the A-B and another A's speed, I'me checking id A is heading to B.
I'm assuming the "second angle" you're looking for is φ. That is to say, you're using spherical coordinates:
(x,y,z) => (r,θ,φ)
r = sqrt(x^2 + y^2 + z^2)
θ = cos^-1(z/r)
φ = tan^-1(y/x)
However, if all you want to do is find if A is moving with velocity a towards B, you can use a dot product for a basic answer.
1st vector: B - A (vector pointing from A to B)
2nd vector: a (velocity)
dot product: a * (B-A)
If the dot product is 0, it means that you're not getting any closer - you're moving around a sphere of constant radius ||B-A|| with B at the center. If the dot product > 0, you're moving towards the point, and if the dot product < 0, you're moving away from it.