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# Simplify reference system and vector movement

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5 replies to this topic

### #1nimrodson  Members

Posted 01 February 2013 - 05:55 PM

Greetings,

Well, I spend a whole afternoon trying to implement a simple navigation system using vectors. Finally it worked, here's a fragment of what I did (from a Spaceship class):

def moveForward(self):
self.center -= self.step * Vector2D(1,0).rotate(self.angle).invX()
self.rect.center = self.center.toPoint()


Basically, I use right and left arrow keys to rotate the spaceship to right or left direction, respectly. And, I have the previous fragmente code to move forward. Due the fact that (0,0) grows from left to right (x-coordinate) and from top to bottom, I have to invert the x-coordinate (invX() method) from the rotated vector in order to "correct" the spaceship navigation. Is there another way to make this simpler, for example, changing the origin or somewhat like that?

Thank you

### #2Álvaro  Members

Posted 01 February 2013 - 08:49 PM

That looks a bit awkward. Instead of keeping an angle to indicate where the spaceship is pointing, use a vector of length 1. The code would then look something like this (ignoring the issue of the flipped x coordinate, which I think should be dealt with in the rendering code, not the scene update code):
def moveForward(self):
self.rect.center = self.center.toPoint() # I don't know what this line does

What is self.rect.center? And why do you need to call toPoint' on self.center? What is self.center if not a point?

EDIT: Oh, the tricky part with using heading as a length-one vector is how to rotate it when the user presses a key. Instead of adding something to the angle, rotate the vector by some amount (you seem to already have code for that). You may have to normalize the vector every so often (like every frame, for simplicity) so that it doesn't end up being much longer or shorter by accumulated floating-point errors.

Edited by Álvaro, 01 February 2013 - 08:51 PM.

### #3nimrodson  Members

Posted 02 February 2013 - 09:03 AM

What is self.rect.center? And why do you need to call toPoint' on self.center? What is self.center if not a point?

self.rect.center is a python tuple. I need to set it to render the spaceship later, and it doesn't accept a "vector". toPoint() method converts a vector in a tuple. I know, it seems ugly but i couldn't figure out how deal with it.

EDIT: I hadn't understood your full answer but now it's clear to me. Thank you very much!

Edited by nimrodson, 02 February 2013 - 09:14 AM.

### #4nimrodson  Members

Posted 02 February 2013 - 09:28 AM

You may have to normalize the vector every so often (like every frame, for simplicity) so that it doesn't end up being much longer or shorter by accumulated floating-point errors.

Have you got any additional information (websites, books) of what you wrote it? Thanks

### #5Álvaro  Members

Posted 02 February 2013 - 08:38 PM

I don't, but what I describe in that particular sentence you quoted (you need to normalize your vector every so often) is the standard thing to do when controlling a 3D attitude using a quaternion (which has to be kept with length 1 as well). The situation here is analogous.

If you want to know something else about how to do 2D geometry without using angles, I love to evangelize about it.

### #6nimrodson  Members

Posted 03 February 2013 - 03:33 PM

Hahaha, OK. Thanks!

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