Rotation

Hi I'm trying to rotate a circle, well it's a circle made out of circles that I'm creating programatically. For some reasone when I rotate my circle it gets all distorted(streached flattened) but I can't figure out why?? What I want is for the circle to rotate without loosing its shape. I Wrote this program using Allegro... here's the source sorry if it's confusing I'm just hacking on this program to try to learn stuff. I put a //NOTE comment at the part I have a question about so you can find it quickly I just thought I'd include the entire code incase someone wanted to compile it keep in mind you'll need allegro if you want to compile it.

#include <stdio.h>
#include <math.h>
#include "allegro.h"

volatile int frame_count, fps;

void fps_proc(void)
{
   fps = frame_count;
   frame_count = 0;
}

END_OF_FUNCTION(fps_proc);

int main(void)
{
	#define PI 3.14159265358979
    BITMAP *dblbuffer;
    int x;
	int i = 0;
	int theta = 0;
	int plotx, ploty;
	int rotate = 0;

	char buffer[20];
	char score_buf[80];
	frame_count = fps = 0;

    allegro_init();

    install_keyboard();
    install_timer();        /* needed for `rest' function */

    if (set_gfx_mode(GFX_AUTODETECT, 1280, 1024, 0, 0) < 0) {
	//printf("%s\n", allegro_error);

	exit(1);
    }
    clear(screen);

	/* set up the interrupt routines... */

	LOCK_VARIABLE(frame_count);
	LOCK_VARIABLE(fps);
	LOCK_FUNCTION(fps_proc);




   install_int(fps_proc, 1000);

    dblbuffer = create_bitmap(SCREEN_W, SCREEN_H);
    clear(dblbuffer);

    vsync();
    blit(dblbuffer, screen, 0, 0, 0, 0, SCREEN_W, SCREEN_H);


	do{
	
	poll_keyboard();
	if(key[KEY_ESC])
		break;


		for (x = 0; x < 360; x++) {
		//rest(2);		/* slow it down so we can see it */

		//circlefill(dblbuffer, x - 1, 100, 50, 0);	/* erase from last place */

		//circlefill(dblbuffer, x, 450+100*( sin((x+y*90)*PI/180)) , 10, x%30 + 32 ); //sin

		//circlefill(dblbuffer, x, 450+50*( tan((x+y*90)*PI/180)) , 10, x%30 + 32 );//tan

		//putpixel(dblbuffer,x,450+100*( sin((x+y*90)*PI/180)),x%30 + 32);



			//calculate x, y for circle elements

			plotx = 300* cos(x*PI/180);
			ploty = 300* sin(x*PI/180);

			//rotate x, y 


			if(rotate > 0)
			{
//NOTE This is the part I'm having trouble with!!		

				
				plotx = (plotx * cos(theta*PI/180) - ploty * sin(theta*PI/180));
				ploty = (ploty *cos(theta*PI/180) + plotx*sin(theta*PI/180));
			}

			//shift

			plotx = plotx + SCREEN_W / 2;
			ploty = ploty + SCREEN_H / 2;

			circlefill(dblbuffer, plotx, ploty , 10, x%30 + 32 ); //sin

			poll_keyboard();
			if(key[KEY_R])
			rotate = 1;		



		
		//textout(dblbuffer, font, "Double buffering (mode 13h)", 0, 0, 15);

		
		}


		if (fps)
		sprintf(score_buf, "(%ld fps)",(long)fps);
	else
		sprintf(score_buf, "0");

	sprintf(buffer,"theta: %d degrees", theta);

	textout(dblbuffer, font, score_buf, 0, 0, 7);
	textout(dblbuffer, font, "Press R to begin Rotation", 1015, 0, 7);
	textout(dblbuffer,font,buffer,0,20,7);
	theta++;
	if(theta > 360)
		theta = 1;
	
			//vsync();

	//readkey();	

	blit(dblbuffer, screen, 0, 0, 0, 0, SCREEN_W, SCREEN_H);
	frame_count++;
	clear(dblbuffer);

	} while (!key[KEY_ESC]);

	// Now, remember to FREE the memory you previously allocated

    destroy_bitmap(dblbuffer);

    return 0;
}
END_OF_MAIN();
 

I'm looking for a solution that will allow me to rotate any graph of points. when i posted this on the allegro.cc forum I was told:

quote:

---

Try replacing the above with just: //calculate x, y for circle elements plotx = 300* cos((x+theta)*PI/180); ploty = 300* sin((x+theta)*PI/180);

---

I understand that by adding theta to x it makes the circle rotate but that isn't neccisarilly a solution for rotating other types of graphs. Say you wanted to graph y = 100*(sinx) or something like that if you put, ploty = 100*(sin(x+theta*PI/180)); then you would see the sine graph phase shifting correct? I tried nearly the same program as the original source listed with the graph of a sine wave and it also got all distoreted as I tried to rotate it. What I'd like to figure out is a way to manipulate the x and y values of the points on a graph to rotate it. Thanks so much for any help with this. -Jake [edited by - jakerrz on January 10, 2004 3:14:56 AM]