Thursday, 10 October 2013

Finding the relative position of a point to a players orientation...

So for many things in a 3D game we need to convert the Relative Offset position of one object to another based on how one of the objects would see the other one!!!


   Player 1 and Player 2 are near to each other.  Player 1 is at 0,0 and Player 2 is at 100,5.
Player 1 is facing towards 0 degrees or North.  So from Player 1`s point of view then Player 2 is to his right by 100 and in front by 5 units.
But IF Player 1 turns 90 degrees to his left then Player 2s new relative position to Player 1 becomes 5,-100.

You can see that the X and Y relative coordinates have changed based on which direction Player 1 is facing.

The best way to think of this is light a Radar on a vehicle (in our case a Submarine).  The dots on the radar are Always Relative to the Orientation of the vehicle.

Why do we care about this?

Well it turns out that (apart from having a Radar) it is a lot easier to check for collision between 2 objects by using simple Point within a Box Area check, than for checking if a single point is Anywhere Within a Box Area that could be pointing in any 3 Dimensional direction!

We can rotate the Point to relative to the Player, which basically means the Player is Always pointing at 0 degrees ie North. And only the Relative point moves around the player.

Here is the actual code function from our game that does this:-

Vector3 Find_Relative_Position(Matrix R_Orientation, Vector3 R_Position)
Vector3 Xaxis = R_Orientation.Right;

Vector3 Yaxis = R_Orientation.Up;

Vector3 Zaxis = R_Orientation.Forward;

float Rx = (Xaxis.X * R_Position.X) + (Xaxis.Y * R_Position.Y) + (Xaxis.Z * R_Position.Z);

float Ry = (Yaxis.X * R_Position.X) + (Yaxis.Y * R_Position.Y) + (Yaxis.Z * R_Position.Z);

float Rz = (Zaxis.X * R_Position.X) + (Zaxis.Y * R_Position.Y) + (Zaxis.Z * R_Position.Z);

return new Vector3(Rx, Ry, -Rz);


Just send it the Orientation Matrix of the Viewer (ie Player that you want to check collision with) and also the Point relative to the viewers position (ie Player Position - Point Position)

The above function will then return the Relative Point vector in respect to the Players Orientation View.

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