From: "Sly" <sly AT aussie DOT net>
Newsgroups: comp.os.msdos.djgpp
Subject: Re: Rotation problem
Date: 6 Mar 1997 04:23:59 GMT
Organization: Sly
Lines: 76
Message-ID: <01bc29e6$bc0e4540$8a081ecb@sly>
References: <Pine DOT GSO DOT 3 DOT 95 DOT 970305131547 DOT 2365A-100000 AT explorer2 DOT clark DOT net>
NNTP-Posting-Host: max0ppp08.bne.aussie.net
To: djgpp AT delorie DOT com
DJ-Gateway: from newsgroup comp.os.msdos.djgpp

Guess <times9 AT clark DOT net> wrote in article
<Pine DOT GSO DOT 3 DOT 95 DOT 970305131547 DOT 2365A-100000 AT explorer2 DOT clark DOT net>...
> Hello again!
> I have a small (?) problem. I'm plotting 100 points in mode 13h and
> rotating them in increments of 1 degree. It works, only the rotating
> pixels get smaller and smaller until it becomes a little dot on the
> screen. What am I doing wrong? :) I'm figuring it's due to the
> rounding error...or am I wrong? It seemed to work fine when i was
> rotating a triangle with the line() function. *Shrug* Here is the
> code..thank you in advance for any help!
> --

I figure it would most probably be due to cumulative rounding errors.  That
is, the longer the program runs, the larger the rounding error gets, as it
is adding *all* the previous rotations rounding errors.  Try using two
arrays of pixels, where one is the original pixels and the other is the
rotated pixels.  I have made some suggested changes to the following
code...

> 
> struct {
>    float x, y;
> } pix[100];
> 

struct {
   float x, y;
} pix[100], rotated_pix[100];

/* Used later in rotate() */
int angle = 0;

> void rotate()
> {
>    int i;
>    for(i = 0; i < 100; i++) {
>       pix[i].x = pix[i].x * cos_t[1] - pix[i].y * sin_t[1];
>       pix[i].y = pix[i].y * cos_t[1] + pix[i].x * sin_t[1];
>    }
> }

void rotate()
{
   int i;

   /* Increase rotation by one degree */
   angle++;
   if (angle >= 360)
      angle = 0;
   /* Rotate all the points into the rotated pixel array */
   for(i = 0; i < 100; i++) {
      rotated_pix[i].x = pix[i].x * cos_t[angle] - pix[i].y * sin_t[angle];
      rotated_pix[i].y = pix[i].y * cos_t[angle] + pix[i].x * sin_t[angle];
   }
}

> void draw()
> {
>    int i;
>    for(i = 0; i < 100; i++)
>       putpixel(screen, 160+pix[i].x, 100+pix[i].y, 13);
> }

void draw()
{
   int i;
   for(i = 0; i < 100; i++)
      putpixel(screen, 160+rotated_pix[i].x, 100+rotated_pix[i].y, 13);
}


Hope this helps...

-- 
TTFN
Sly (Steve)