Hey all, I''ve recently been programming quite a few little games of varying styles in dos (mainly a 2d platformer and a Shooting Gallery), using DJGPP & the allegro library, but the one thing that keeps stumping me is how to do a 2d, top-down racing game... Most of it i''m comfortable with, except one key issue: the angle of the car, and therefore the direction it''s travelling in... I want to be able to rotate my car with the left and right keys (for example) and accelerate by pressing the up arrow, but I just can''t figure out how to calculate what angle the car should be facing... I don''t need to worry about gradual acceleration / decceleration (I can do that later), but I just want to get the simple car turning right and left and then moving in the correct direction... I assume I need to do something using er... well, degrees or something... erm... Forgive me if there''s no simple explanation (in which case i''m getting ahead of myself) but i''m sure I could just make an array with the necessary x and y velocities for each angle... or maybe not Cheers for any help, Nick - Head Designer, Llamasoft.net -- Visit our website... Llamasoft.net Games, goodies and ingenuity

well, you can give it an x,y, and a rotation variable. you set the rotation variable at the beginning of the game or level or whatever. anyways, each frame you rotate the sprite of the car by the rotation measurement. pressing left or right keys could incerment or decrement the rotation valuye. allegro comes with a function to rotate sprites so it should be pretty easy.

JoeMont001@aol.com www.polarisoft.n3.net

but remember, the allegro library''s rotation function takes a value from 0 to 255 as the rotation amount. So a rotation of 90 degrees would be a value of 64 and 180 degrees would be 128. Understand ?
So you need to do something like this

    int degreeAngle;int xPos;int yPos;void rotate(){  if (rightKeyPressed)  {    degreeAngle += turnRate;  }  else if (leftKeyPressed)  {    degreeAngle -= turnRate;  }  if (upKeyPressed)  {    int radians = degreeAngle * 0.0174444;  // convert degrees to radians    xPos += cos(radians);    yPos += sin(radians);   }}void drawPlayer(){  int rotateValue = degreeAngle * 0.711111; // convert to allegro rotate value   drawRotatedSprite(backBuffer, mySprite, xPos, Ypos, rotateValue);}    

now some of those function names might not be exact because it has been a long time since I used allegro but you can get the idea.

drawRotatedSprite(

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Yeah, but it''s not the drawing of the sprite i''m reffering to, specifically... I''m talking about, based on whatever angle the sprite is facing, what *direction* it should travel... I can''t just say ''if (we_want_to_go_forward) x ++;'' because that would only work if we were at an angle of 90 Degrees... So I need to work out what the x and y velocities should be, based on what angle we''re at (and the max speed of the car, for example). I was thinking of having something like...

    //at the top of the .cppfloat xv[360];float xy[360];//later, when we wan''t to accelerate or turnif (key[KEY_UP]){    x += xv[current_angle];    y += xy[current_angle];}if (key[KEY_LEFT]){   current_angle --;}    

etc. But, i need to fill those arrays with their correct values at all 360 angles... see? I hope that was slightly more clear


Nick - Head Designer, Llamasoft.net

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Ah, looks like ncsu121978 just beat me to it
Okidoke, i'll try something like that...
I assume, where you say:

xPos += cos(radians);
yPos += sin(radians);

I can multiply that by the speed I want to travel at? Maybe not, but bear with me, maths isn't my strong point

Nick - Head Designer, Llamasoft.net

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Edited by - Llamasoft.net on July 30, 2000 2:44:28 PM

Have a direction in degrees. Then do this:
x = sin(direction)
y = cos(direction)

That does a coordinate transform from polar to cartesian coordinates. Make sure you are using degrees sine not radians sine. Then you multiply x and y by the velocity of the car.

------------------------------
#pragma twice

xPos += (speed * cos(radians));
yPos += (speed * sin(radians));

and if I were you then I would use a lookup table for the sin and cos because they are SLOW functions and it seems as though you will be calling them alot

"Now go away or I shall taunt you a second time"
- Monty Python and the Holy Grail
themGames Productions

quote: Original post by ncsu121978

xPos += (speed * cos(radians));
yPos += (speed * sin(radians));

and if I were you then I would use a lookup table for the sin and cos because they are SLOW functions and it seems as though you will be calling them alot

That''s pretty much exactly what I thought... thankyou very, very much
I love this forum...
Cheers everyone!



Nick - Head Designer, Llamasoft.net

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First things first.
your doing a top-down 2D racer? STOPPPPPPPPPPP
thats my idea :-)
http://www.positech.co.uk