Rotation & movement

Rotation & movement

Demons

Member #8,807

July 2007

I'm able to rotate my sprite with:

    if(key[KEY_Q])
      _angle--;
    if(key[KEY_E])
      _angle++;

rotate_sprite(buffer, _sprite, _position.X, _position.Y, itofix(_angle));

I have a vector for the position of the sprite. Using the above to rotate, how can I get the direction vector so when I move forward it moves the direction that the sprite is rotated? I'd sure it's fairly easy and I just can't find anything.

Or, based on the angle, how can I get the direction vector?

Vanneto

Member #8,643

May 2007

You should have a velocity on the x and y axis for the sprite, and then do this:

vx = cos(angle) * speed;
vy = sin(angle) * speed;

Where speed is the speed by which the sprite should be moving. Then you update the x and y values by the vx,vy values:

_position.X += vx;
_position.Y += vy;

And youre done!

In capitalist America bank robs you.

Demons

Member #8,807

July 2007

Oh ok. I do have a velocity, and I also have a direction vector, but it seems since I have the angle variable I don't need the direction vector. Thanks.

Also, that would call cos() and sin() each cycle. I imagine I could call those and store the value in a variable only when I rotate right?

What variables do I need these declared as? I'm getting errors about fixed and such and when I make the angle 'fix' and the vectors float or anything else I get strange results.

I have my angle init to 0 and when I use that to move forward it moves right. ie I have it so when you press the W key it sets the speed value to > 0 to start the movement.

Vanneto

Member #8,643

May 2007

1. Yes you could calculate the new position only when the ship is rotated. That is a good idea.

2. Well sin() and cos() take floats. So you could make _angle a float and then use ftofix() when calling rotate_sprite().

3. Hmm... Dont know about this one. Try starting with 90 degrees.

In capitalist America bank robs you.

Demons

Member #8,807

July 2007

OK, I must be missing something. Here is my player code, would be great if someone could take a look and let me know what I'm doing wrong.

The sprite does rotate, but the direction of moving doesn't work correctly.

1class Vector2D
2{
3private:
4public:
float X, Y;
6};
7 
8 
9class Player
10{
11private:
BITMAP *_sprite;
Vector2D _position;
Vector2D _velocity;
Vector2D _direction;
float _angle;
float _speed;
bool _moveForward;
19public:
Player()
{
  _moveForward = false;
  _speed = 0;
  _angle = 90;
  _position.X = SCREEN_W/2;
  _position.Y = SCREEN_H/2;
  _direction.X = cos(_angle);
  _direction.Y = sin(_angle);
  _sprite = load_bitmap("C:\\player.bmp",0);
}
void draw(BITMAP *buffer)
{
  rotate_sprite(buffer, _sprite, _position.X, _position.Y, ftofix(_angle));
}
void move()
{
  //_velocity.X = _direction.X * _speed;
  //_velocity.Y = _direction.Y * _speed;
39 
  _velocity.X = cos(_angle) * _speed;
  _velocity.Y = sin(_angle) * _speed;
42 
  _position.X += _velocity.X;
  _position.Y += _velocity.Y;
}
void input()
{
  if(key[KEY_Q])
  {
    _angle--;
    _direction.X = cos(_angle);
  }
  if(key[KEY_E])
  {
    _angle++;
    _direction.Y = sin(_angle);
  }
58 
  if(key[KEY_W] && !_moveForward)
  {
    _speed = .5;
    _moveForward = true;
  }
  else if(!key[KEY_W] && _moveForward)
  {
    _speed = 0;
    _moveForward = false;
  }
}
70};

TeamTerradactyl

Member #7,733

September 2006

For your move() function doesn't seem to be truly using your _velocity. I would suggest something similar to:

  void move()
  {
    _velocity.X = cos(_angle) * _speed;
    _velocity.Y = sin(_angle) * _speed;

    _position.X += _velocity.X;
    _position.Y += _velocity.Y;
  }

Could you also explain

Demons said:

the direction of moving doesn't work correctly

and say WHAT isn't working?

Demons

Member #8,807

July 2007

I must be missing something. That looks the same except you have ) after speed which shouldn't compile since there is no matching (.

Quote:

and say WHAT isn't working?

I don't know what isn't working or what it's doing. I just know it's not moving it in the right direction. When my guy is facing up, and I try to move (I don't rotate yet) then he goes right.

[EDIT]

OK, something that seems odd. When the angle is 0, cos(angle) is returning a 1, and that gets multiplies by the speed which is .5 making the _velocity.X = to .5, which isn't right I'm thinking, because that makes my image go right, when the image I have visually is pointing up.

TeamTerradactyl

Member #7,733

September 2006

What direction is your BITMAP *_sprite stored? You may need to rotate the sprite's image by 90 degrees. The "unturned" direction should be facing right when you have _angle=0.0, it should be facing up when _angle=PI/2 (or in the case of Allegro rotations, 128/2, or 64.0).

And the ")" artifact was something that happened when copying/pasting; sorry. I'll edit my post and fix that typo.

EDIT: Sorry; I guess I had not correctly read your code post, so I had re-keyed what you already had.

So please disregard that first bit; I thought your math was wrong, but it turns out, you were correct all along. The relevant thing is that I think your sprite is simply turned 90 degrees. What direction does it go if you use your Q/E keys and accelerate? Does it go "straight" and even rotate correctly, but simply have the bitmap facing the wrong direction?

Demons

Member #8,807

July 2007

My bitmap is stored normally I guess. I created a top down bitmap where you are looking directly down on a person. So you see hit head, shoulders, adn a gun that is pointing "up" or forward.

When angle is 0 the image is facing up. In my code I init angle to 0, and it uses that in rotate_sprite. So if angle is 0 it seems rotate_sprite just draws it normally (up), but moving makes it go right. So how do I rotate my bitmap yet not the angle? Seems I need the bitmap to be facing right at first.

When I move and rotate, sometimes it seems to go the right away and sometimes it doesn't. Seems odd.

[EDIT]

Ok, I rotated the image in the paint editor so now it starts out looking right. So now if I move forward and don't rotate it works by moving right and looks visually right. BUT, if I press Q once then move again the character starts moving up/left, when visually he should move up/right.

Hmm maybe it's because the sprite is rotating around the top/left corner instead of the center of the sprite? I assume rotate_sprite() is doing the rotation around the center of the sprite, and my cos()/sin() is doing it around the top/left of teh sprite.

IonBlade

Member #3,521

May 2003

[EDIT] Nevermind, I read wrong

------

"I've got to choose my words carefully so that I'm not misunderstood, but I think there are fewer developers around today who think about making a good game. As a creator, I find that sad..." - Hideo Kojima

TeamTerradactyl

Member #7,733

September 2006

1Player::Player()
2{
3  _moveForward = false;
4  _speed = 0.0;
5  _angle = 0.0;
6  _position.X = SCREEN_W/2;
7  _position.Y = SCREEN_H/2;
8  _direction.X = cos(_angle);
9  _direction.Y = sin(_angle);
10  _sprite = load_bitmap("player.bmp",0);
11}
12 
13void Player::draw(BITMAP *buffer)
14{
15  rotate_sprite(buffer, _sprite, _position.X, _position.Y, ftofix(_angle * 128.0));
16}
17 
18 
19void Player::move()
20{
21  _velocity.X = cos(_angle * M_PI) * _speed;
22  _velocity.Y = sin(_angle * M_PI) * _speed;
23 
24  _position.X += _velocity.X;
25  _position.Y += _velocity.Y;
26}
27 
28void Player::input()
29{
30  if(key[KEY_Q])
31  {
32    _angle-=0.05; // If this is too large, it rotates much too quickly
33    _direction.X = cos(_angle);
34  }
35  if(key[KEY_E])
36  {
37    _angle+=0.05; // Same as rotation for KEY_Q
38    _direction.Y = sin(_angle);
39  }
40  if(key[KEY_W] && !_moveForward)
41  {
42    _speed = .5;
43    _moveForward = true;
44  }
45  else if(!key[KEY_W] && _moveForward)
46  {
47    _speed = 0;
48    _moveForward = false;
49  }
50}