From: Dan <dyoon AT home DOT com>
Newsgroups: comp.os.msdos.djgpp
Subject: Allegro and Asteroids
Date: Sun, 01 Mar 1998 15:52:17 -0800
Organization: @Home Network
Lines: 53
Message-ID: <34F9F4B1.6CE0@home.com>
Reply-To: dyoon AT home DOT com
NNTP-Posting-Host: cc104221-a.bnapk1.occa.home.com
Mime-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Transfer-Encoding: 7bit
To: djgpp AT delorie DOT com
DJ-Gateway: from newsgroup comp.os.msdos.djgpp
Precedence: bulk

Ok, I'm having some problems trying to make my little asteroids clone
game do the following...turn the ship in a certain direction, and if i 
press the up arrow key, have the ship move in the direction it's front
end is pointing.

I have some code written but it doesn't work properly.  If anyone could
tell me what I'm doing wrong I would really appreciate it.

[begin loop]

      if(key[KEY_UP])
      {
         float radian_angle = (angle*180)/PI;

         scale_x = cos(radian_angle);
         scale_y = sin(radian_angle);

         acc_x = scale_x*1;
         acc_y = scale_y*1;
      }
      vel_x += acc_x;
      vel_y += acc_y;

         if(vel_x > SHIP_MAX_SPEED)
            vel_x = SHIP_MAX_SPEED;
         if(vel_y > SHIP_MAX_SPEED)
            vel_y = SHIP_MAX_SPEED;
         if(vel_x < -SHIP_MAX_SPEED)
            vel_x = -SHIP_MAX_SPEED;
         if(vel_y < -SHIP_MAX_SPEED)
            vel_y = -SHIP_MAX_SPEED;

      pos_x += vel_x;
      pos_y += vel_y;

[end loop]

the ship starts off with the front of the ship pointing north, so the
ship's "angle" variable starts off at 90.  pressing the right arrow
key will decrement the angle, and the left arrow key will increment it.

i know that my code is a little bit off because on the monitor, going up
the screen will actually mean decrementing the Y axis...but my main 
problem is that the angle isn't even right.  for example, i start the
ship off at 90 degrees...so if i just press the up arrow key from the
start of the game, the ship should go up the screen?  instead it goes at
some weird angle.

help will be much appreciated.  Oh, and i'm sorry if this is the wrong
place to ask this question...but i was thinking that maybe there are 
some djgpp specific math routines???

Daniel