How to Rotate a line?

Can anyone help me out with rotating a line about a fixed point?

say Like x,y = 100 and length of line is 10. (By rotation, It draws a circle of radius 10). To be clear, I have a circle of radius 10. I imagine it to be a wheel of a car. I need to rotate a line from the center to the circumference of the circle as the car is moving forward.

My code for moving a 2-d Car from left to right is below.

Code:

---

#include<graphics.h>
void main()
{
        int gd=DETECT,gm;
        initgraph(&gd,&gm,"e:\\lab\\tc\\bgi");
        for(i=100;i<400;i++)
        {
                setcolor(3);
                line(i,100,i+15,85);                //RISING LINE
                line(i+15,85,i+70,85);          //ST.LINE AT TOP
                line(i+70,85,i+100,100);        //FALLING LINE
                rectangle(i+100,150,i,100);        //BODY
                rectangle(i+10,130,i+30,110);  //WINDOW1
                rectangle(i+40,130,i+60,110);  //WINDOW2
                rectangle(i+70,130,i+90,110);  //WINDOW3
                circle(i+15,125,5);            //FACE1
                circle(i+45,125,5);            //FACE2
                circle(i+75,125,5);            //FACE3
                circle(i+20,160,10);            //WHEEL1
                circle(i+80,160,10);            //WHEEL2
                delay(25);
                cleardevice();
          }
          closegraph();
}

---

Please help me out :)

I want to point out that void main is undefined. You should use int main instead.

For what you need something like this should work:

Code:

---

line(x, y, x+cos(degrees)*length, y+sin(degrees)*length);

---

x, and y would be the point around which to rotate. In C you also need to convert degrees to radians tho.

@mike_g

Could you please put that in a for loop and show?

@elysia

I can see your hatred towards void main from your siggy :D, but I don't have a need to define the return type for main() in my program.

Quote:

---

Originally Posted by hariharan

@elysia

I can see your hatred towards void main from your siggy :D, but I don't have a need to define the return type for main() in my program.

---

You do no seem to understand. Whether or not you need to return something is irrelevant. Void main is undefined behavior and should not compile at all. Unfortunately, it does compile, but it is still undefined and very, very bad. So don't use it. Use int main.

I'm working with some rotating rim spokes for your car's wheels.

You should have some "bad" low-rider wheels later this evening. :D

It's not terribly sophisticated, (no math involved), but it might work.

Code:

---

#include <graphics.h>

/*
line(x, y, x+cos(degrees)*length, y+sin(degrees)*length);
x, and y would be the point around which to rotate. In C you also need to
convert degrees to radians tho.

Can we add spokes/rims to the wheels, and show them rotating?
Add a black line, same xy's, that chases the blue one, deleting them.
*/

int main()
{

  int i, msec;  //milliseconds used by delay()
  int gd=DETECT, gm, errorcode;  //int gd=DETECT,gm;

  /* what I use: initgraph(&gd,&gm,""); */
  initgraph(&gd,&gm,"e:\\lab\\tc\\bgi");
 
  /* check if it loaded OK */
  errorcode = graphresult();
  if (errorcode != grOk) {                /* an error occurred */
      printf("Graphics error: &#37;s\n", grapherrormsg(errorcode));
      printf("Press any key to halt:");
      getch();
      exit(1);                                      /* return with error code */
  }

  for(i=100;i<400;i++)   
  {
      setcolor(3);
      line(i,100,i+15,85);                        //RISING LINE
      line(i+15,85,i+70,85);                //ST.LINE AT TOP
      line(i+70,85,i+100,100);            //FALLING LINE
      rectangle(i+100,150,i,100);          //BODY
      rectangle(i+10,130,i+30,110);  //WINDOW1
      rectangle(i+40,130,i+60,110);  //WINDOW2
      rectangle(i+70,130,i+90,110);  //WINDOW3
      circle(i+15,125,5);                      //FACE1
      circle(i+45,125,5);                      //FACE2
      circle(i+75,125,5);                      //FACE3
      circle(i+20,160,10);                    //WHEEL1
      circle(i+80,160,10);                    //WHEEL2

      msec = 30;
      line(i+20, 160, i+20, 164);      //6 o'clock line
      delay(msec);                    //after a delay
      setcolor(0);                    //erase it
      line(i+20, 160, i+20, 164);
 
      setcolor(3);
      line(i+20, 160, i+10, 160);    //9 o'clock
      delay(msec);
      setcolor(0);
      line(i+20, 160, i+10, 160);

      setcolor(3);
      line(i+20, 160, i+20, 156);    //12 o'clock
      delay(msec);
      setcolor(0);
      line(i+20, 160, i+20, 156);

      setcolor(3);
      line(i+20, 160, i+30, 160);    //3 o'clock
      //delay(msec);

      delay(msec);  //was 25
      cleardevice();                  //3 o'clock erased here
  }

  closegraph();
  return 0;
}

---

The "turning wheel spokes", could have spokes at 2, 4, 8, & 10 o'clock positions, instead, or have them both, to double up on the realism.

A small circle in the center of each wheel could serve as a visual wheel hub, when the spokes would
stop at the circumference of the hub, instead of going on to the center of the wheel (which looks a bit cartoony, imo). Edit: tried this, and it
didn't look good. Went with more spokes, and a sliding scale of "visibility", below. That worked very well.

I played around with the timing a lot, the above looks good on a P3, but it may need to be adjusted for other cpu's.

This is the best rear wheel I could come up with, consistent with your design. I left the front wheel unchanged for comparison.

You're cruisin' now! :D

Code:

---

#include <graphics.h>
/*
line(x, y, x+cos(degrees)*length, y+sin(degrees)*length);
x, and y would be the point around which to rotate. In C you also need to
convert degrees to radians tho.

I didn't use the above info.
*/
int main()
{
  int i, msec;  //milliseconds used by delay()
  int gd=DETECT, gm, errorcode;  //int gd=DETECT,gm;
  //initgraph(&gd,&gm,"");      //for my system
  initgraph(&gd,&gm,"e:\\lab\\tc\\bgi");

  /* check if it loaded OK */
  errorcode = graphresult();
  if (errorcode != grOk) {  /* an error occurred */
      printf("Graphics error: &#37;s\n", grapherrormsg(errorcode));
      printf("Press any key to halt:");
      getch();
      exit(1);            /* return with error code */
  }

  for(i=100;i<400;i++)                                               
  {

        setcolor(3);
        line(i,100,i+15,85);                        //RISING LINE
        line(i+15,85,i+70,85);                        //ST.LINE AT TOP
        line(i+70,85,i+100,100);                //FALLING LINE
        rectangle(i+100,150,i,100);                //BODY
        rectangle(i+10,130,i+30,110);                //WINDOW1
        rectangle(i+40,130,i+60,110);                //WINDOW2
        rectangle(i+70,130,i+90,110);                //WINDOW3
        circle(i+15,125,5);                        //FACE1
        circle(i+45,125,5);                        //FACE2
        circle(i+75,125,5);                        //FACE3
        circle(i+20,160,10);                        //WHEEL1
        circle(i+20,160,12);                        //Wheel depth
        circle(i+80,160,10);                        //WHEEL2

        msec = 10;
        line(i+20, 160, i+20, 164);            //6 o'clock line
        //delay(msec);                                  //after a delay
        if(i % 3 == 0) {
          setcolor(0);                                //erase it
            line(i+20, 160, i+20, 164);
        }

        setcolor(3);
        line(i+20, 160, i+16, 163);                //8 o'clock line
        //delay(msec);                                //after a delay
        if(i % 4 == 0) {
                setcolor(0);                        //erase it
                line(i+20, 160, i+16, 163);
        }
        setcolor(3);
        line(i+20, 160, i+10, 160);                //9 o'clock
        //delay(msec);
        if(i % 5 == 0) {
                setcolor(0);
                line(i+20, 160, i+10, 160);
        }
        setcolor(3);
        line(i+20, 160, i+15, 158);                //10 o'clock
        //delay(msec);
        if(i % 6 == 0) {
                setcolor(0);
                line(i+20, 160, i+15, 158);
        }
        setcolor(3);
        line(i+20, 160, i+20, 156);                //12 o'clock
        delay(msec);
        if(i % 7 == 0) {
                setcolor(0);
                line(i+20, 160, i+20, 156);
        }
        setcolor(3);
        line(i+20, 160, i+25, 158);                //2 o'clock
        delay(msec);
        if(i % 8 == 0) {
                setcolor(0);
                line(i+20, 160, i+25, 158);
        }
        setcolor(3);
        line(i+20, 160, i+30, 160);                //3 o'clock
        delay(msec);
        if(i % 9 == 0) {
                setcolor(0);
                line(i+20, 160, i+30, 160);
        }
        setcolor(3);
        line(i+20, 160, i+25, 162);                //4 o'clock
        delay(msec);
        if(i % 10 == 0) {
                setcolor(0);
                line(i+20, 160, i+25, 162);
        }
        delay(msec);                                //was 25
        cleardevice();                                //last o'clock line erased here
    }

      closegraph();
      return 0;
}

---

The delay before erasing gave way to a better looking scheme involving the a sliding scale use of mod on i, to determine if a line should be erased.

Woah bro! you are damn good at this! thanks

Quote:

---

Originally Posted by hariharan

Woah bro! you are damn good at this! thanks

---

As a matter of fact, I'm a total noob at any kind of animation. I did spend a *lot* of time trying different things, to make that wheel look as good as it does.

I'm glad you like it.

Well heres my solution. Not sure if it will actually work as I never used graphics.h, and haven't tested it. But anyway:

Code:

---

#include <graphics.h>
#include <math.h>
#include <time.h>
#define DEG_TO_RAD 0.0174532925

int main()
{

  int i;
  int gd=DETECT, gm, errorcode;

  /* what I use: initgraph(&gd,&gm,""); */
  initgraph(&gd,&gm,"e:\\lab\\tc\\bgi");
 
  /* check if it loaded OK */
  errorcode = graphresult();
  if (errorcode != grOk) {                /* an error occurred */
      printf("Graphics error: &#37;s\n", grapherrormsg(errorcode));
      printf("Press any key to halt:");
      getch();
      exit(1);                                      /* return with error code */
  }
   
  // Some new variables
  int spokes, degrees = 0, speed = 3, angle;
  unsigned start_time;

  for(i=100;i<400;i++)   
  {
      start_time = clock();     

      setcolor(3);
      line(i,100,i+15,85);                        //RISING LINE
      line(i+15,85,i+70,85);                //ST.LINE AT TOP
      line(i+70,85,i+100,100);            //FALLING LINE
      rectangle(i+100,150,i,100);          //BODY
      rectangle(i+10,130,i+30,110);  //WINDOW1
      rectangle(i+40,130,i+60,110);  //WINDOW2
      rectangle(i+70,130,i+90,110);  //WINDOW3
      circle(i+15,125,5);                      //FACE1
      circle(i+45,125,5);                      //FACE2
      circle(i+75,125,5);                      //FACE3
      circle(i+20,160,10);                    //WHEEL1
      circle(i+80,160,10);                    //WHEEL2
     
      // Draw each spoke 
      for(spokes = 0; spokes < 8; spokes ++) 
      {
          angle = (degrees+spokes*45)*DEG_TO_RAD;
          line(i+20, 160, i+20+cos(angle)*10, 160+sin(angle)*10; 
      }
      // Rotate
      if((degrees+=speed) > 359) degrees -= 360;
     
      // Run at ~60fps
      if(clock()-start_time < 17) delay(17-(clock()-start_time)); 
      cleardevice();                 
  }

  closegraph();
  return 0;
}

---

Code:

---

      delay(17-(clock()-start_time));

---

Lets hope:
1. clock() returns a time compatible with delay.
2. That clock()-starttime isn't greater than 17.

--
Mats

Quote:

---

1. clock() returns a time compatible with delay.

---

It should be because its running in windows

Quote:

---

2. That clock()-starttime isn't greater than 17.

---

Good point, I'll do a quick edit. Theres probably bound to be other stuff wrong with it too ;)

Actually, the above code runs in DOS - albeit emulated inside Windows.

--
Mats

Oh, I though graphics.h was a windows thing. Guess not. Well I supposed if you ran it in DOSbox on Linux it it would be a bit screwed up then.

Quote:

---

Originally Posted by mike_g

Oh, I though graphics.h was a windows thing. Guess not. Well I supposed if you ran it in DOSbox on Linux it it would be a bit screwed up then.

---

Probably not, it's not dependant on the underlaying OS architecture, it's the implementation of "clock" that I'm concerned with - I'm just pointing out that "clock()" may not return a number of milliseconds. Unfortunately it gets a bit difficult to fix if you want to make it fully portable.

--
Mats

@mike_g : You've made my day pal! thank you so much :)

@matsp : Could you please explain your point in layman's view? I'm not getting much

Quote:

---

Originally Posted by mike_g

Well heres my solution. Not sure if it will actually work as I never used graphics.h, and haven't tested it. But anyway:

Code:

---

#include <graphics.h>
#include <math.h>
#include <time.h>
#define DEG_TO_RAD 0.0174532925

int main()
{

    // Some new variables
  int spokes, degrees = 0, speed = 3, angle;
  unsigned start_time;

  int i;
  int gd=DETECT, gm, errorcode;

  /* what I use: initgraph(&gd,&gm,""); */
  initgraph(&gd,&gm,"e:\\lab\\tc\\bgi");
 
  /* check if it loaded OK */
  errorcode = graphresult();
  if (errorcode != grOk) {                /* an error occurred */
      printf("Graphics error: %s\n", grapherrormsg(errorcode));
      printf("Press any key to halt:");
      getch();
      exit(1);                                      /* return with error code */
  }
   
 

  for(i=100;i<400;i++)   
  {
      start_time = clock();     

      setcolor(3);
      line(i,100,i+15,85);                        //RISING LINE
      line(i+15,85,i+70,85);                //ST.LINE AT TOP
      line(i+70,85,i+100,100);            //FALLING LINE
      rectangle(i+100,150,i,100);          //BODY
      rectangle(i+10,130,i+30,110);  //WINDOW1
      rectangle(i+40,130,i+60,110);  //WINDOW2
      rectangle(i+70,130,i+90,110);  //WINDOW3
      circle(i+15,125,5);                      //FACE1
      circle(i+45,125,5);                      //FACE2
      circle(i+75,125,5);                      //FACE3
      circle(i+20,160,10);                    //WHEEL1
      circle(i+80,160,10);                    //WHEEL2
     
      // Draw each spoke 
      for(spokes = 0; spokes < 8; spokes ++) 
      {
          angle = (degrees+spokes*45)*DEG_TO_RAD;
          line(i+20, 160, i+20+cos(angle)*10, 160+sin(angle)*10); 
      }
      // Rotate
      if((degrees+=speed) > 359) degrees -= 360;
     
      // Run at ~60fps
      if(clock()-start_time < 17) delay(17-(clock()-start_time)); 
      cleardevice();                 
  }

  closegraph();
  return 0;
}

---

---

It runs in Turbo C with the changes I made above - very minor. I love the way you've done this Mike, but the spokes stick 1/4 of the way outside the wheel, *4 instead of 10 fixes that, but then some spokes are too short (an aspect ratio problem maybe?), and the spokes appear "spazzy" and bunched up together around the 6 o'clock and 12 o'clock, position. Even with much longer delay times, I couldn't see spokes ever appearing at the 9 o'clock or 3 o'clock, positions.

I do love how compact it is. Much more practical than mine, if I could get it to behave itself! :D

The function "clock()" is not defined to return milliseconds. It is defined to return "a count of clock-ticks" - how big a clock-tick is depends on the implementation - you can find out how many clock-ticks per second"CLOCKS_PER_SEC", but the code from Mike_g is relying specifically on the tick-size being compatible with the function delay, which takes milleseconds as input.

In this case, it's no big deal - the code will probably run only in DOS (because it uses Turbo C specific code, including the "delay" function itself) - but in general, it's a good idea to NOT rely on how large the unit of a clock-tick is - it could be a microsecond (1000000 ticks per second) or it could be 10 ms (100 ticks per second), or something else (147138 ticks per second, perhaps?) - it all depends on for example what timing hardware there is on the machine - it may have a timer that runs at 1/10 of a 1.47138MHz crystal - then that's what you get!

Of course, if you feed delay with a number that is 1000 times larger than you want, then you get a very long delay, and if you feed delay with a number much smaller than the correct value, then you don't get a long enough delay.

--
Mats

Adak: I guess I thought the radius was 10 pixels when it was the diameter. Like I said I couldent test it to see what it came out like. I imagine the main reason it looks so bad is because the wheels are so tiny, it should look better on bigger wheels. I think the pixels sticking out problem has to do with the way that the circle is drawn. Its likely that graphics.h draws Bresenham circles because its a fast and accurate algorithm. However the curvature ends up slightly different to a circle drawn using sin and cos.

matsp: I usually use SDL_GetTicks(), maybe graphics.h has a method for getting the time in millisecs too? and yeah clock() seems to suck.

Erm.. Actually ignore that stuff I was babbling about to do with circles. The answer was painfully obvious; the circle needs to have an odd diameter. Otherwise on one half the spokes will end up one pixel too long, or short.

@Mats

Thanks for the explanation dude :)