From: joshuahab AT aol DOT com (Joshua Hab)
Newsgroups: comp.os.msdos.djgpp
Subject: Newbie seeks help with rotating triangle program
Date: 15 Oct 1997 04:11:16 GMT
Lines: 108
Message-ID: <19971015041101.AAA17302@ladder01.news.aol.com>
NNTP-Posting-Host: ladder01.news.aol.com
Organization: AOL http://www.aol.com
To: djgpp AT delorie DOT com
DJ-Gateway: from newsgroup comp.os.msdos.djgpp
Precedence: bulk

Hi everyone, I'm relatively new to programming and very new to DJGPP, and I was
 wondering if some kind soul(s) would help me with a program I'm writing that
 rotates a triangle (just an outline though) around the center of the screen
 (also the triangle's center)

The problem I'm having is that the function I wrote to determine the x and y
 values of the vertices is returning a value of 210 for x and 100 for y no
 matter what I put into them. The formula I'm using is as follows:
 (RADIUS_LENGTH is the radius of the circumscribed circle and CENTER_X is the
 center of it)

x=(RADIUS_LENGTH*(cos(theta/360*6.28)))+CENTER_X
                      ^^^^^^^^^^^^^^
                      this converts the degree based 'theta'
                      to radians

(this is derived from the polar equation x=r cos theta)

and likewise:

y=(RADIUS_LENGTH*(sin(theta/360*6.28)))+CENTER_Y

(derived from y=r sin theta)

So anyway, here is my source code. Any comments regarding programming style
 would be greatly appreciated, as I am very new and have only a vague
 definition of what is good and what is not good.
_________________________________
#include <math.h>
#include <stdlib.h>
#include <stdio.h>
#include <allegro.h>

#define CENTER_X 160
#define CENTER_Y 100
#define RADIUS_LENGTH 50

void main()
{


   int x1, x2, x3, y1, y2, y3;
   //x and y coordinates for the 3 points of a triangle

   int theta1=90, theta2=210, theta3=330; //rotation in degrees
  

   PALETTE palette;

   allegro_init();
   set_gfx_mode(GFX_VGA, 320, 200, 0, 0);
   install_keyboard();
   palette[20].g=63;     //I want the triangle to be green
   palette[20].r=0;
   palette[20].b=0;
   set_palette(palette);
   clear(screen);

     do
     {
        x1=calculate_x(theta1);      //calculate the x coordinate based
        x2=calculate_x(theta2);      //on the rotation or theta value
        x3=calculate_x(theta3);
        y1=calculate_y(theta1);      //likewise with y
        y2=calculate_y(theta2);
        y3=calculate_y(theta3);
        draw_triangle(x1, x2, x3, y1, y2, y3);  //draw the triangle outline
 with
                                                              //our newly
 discovered points
        theta1=theta1+5; //increment the theta values, rotating the figure
        theta2=theta2+5;
        theta3=theta3+5;
        }
        while ((readkey() & 0xFF) != 27);   //exit if "ESC" detected
        allegro_exit();
        return;
}

calculate_x(theta)
int theta;
{
    int x=0;
    x=(50*(cos(theta/360*6.28)))+160; //our equation for determining x
    return(x);
}

calculate_y(theta)
int theta;
{
    int y=0;
    y=100-(50*(sin(theta/360*6.28))); //our equation for determining y
    return(y);
}



draw_triangle(x1, x2, x3, y1, y2, y3)
int x1, x2, x3, y1, y2, y3;
{
     line(screen, x1, y1, x2, y2, 20); //draw 3 lines, connecting the 3
 vertices
     line(screen, x2, y2, x3, y3, 20); //in color 20, previously defined as 
     line(screen, x3, y3, x1, y1, 20); //green
     return;
}