Graphics (Rotation in 2D)

Question

I am now trying for 8+ hours to solve this but cant figure it out, please help find whats wrong with my code.

int main() {
    int gd = DETECT, gm;
    float ANGLE = 360/10 * M_PI/180;

    initgraph(&gd, &gm, NULL);
    int cx = getmaxx() / 2;
    int cy = getmaxy() / 2;
    int p[] = {cx-50,cy-150, cx+50,cy-150, cx+50,cy-50, cx-50,cy-50, cx-50,cy-150};

    outtextxy(cx, cy, "*");
    setcolor(G);

    int n, i; 
    for (n = 0; n < 10; n++)
    {
            drawpoly(5, p);
            for (i = 0; i < 10; i+=2)
            {
                    p[i] = p[i]*cos(ANGLE) - p[i+1]*sin(ANGLE) + cx - cx*cos(ANGLE) + cy*sin(ANGLE);
                    p[i+1] = p[i]*sin(ANGLE) + p[i+1]*cos(ANGLE) + cy - cx*sin(ANGLE) - cy*cos(ANGLE);
            }
    }

    getch();
    closegraph();
    return (0);
}

But i need like this.

it doesn't matter because it will stop at 8 because of i+=2. — Dpetrov, May 18 '18 at 12:40
5 points, the fith connects with first in other words its the same(its required for drawpoly function). — Dpetrov, May 18 '18 at 12:42
Note that in `p[i+1] = p[i]*...` the `p[i]` is the _new_ `p[i]`, not the old one. Shouldn't it use the old one? — Paul Ogilvie, May 18 '18 at 12:45
Whoever is teaching you to use BGI graphics in the year 2018 should be hit in the head with a MS DOS computer. — Lundin, May 18 '18 at 12:51
Haha, true, it's my University, aparently its not their interest to progress — Dpetrov, May 18 '18 at 12:56

Dpetrov · Accepted Answer · 2018-05-18T12:58:03.523

Thanks to Paul Ogilvie. I was using p[i] with modified value instead of old one in order for transformation to work. thanks again!

New code:

int main() {
    int gd = DETECT, gm;
    float ANGLE = 360/10 * M_PI/180;

    initgraph(&gd, &gm, NULL);
    int cx = getmaxx() / 2;
    int cy = getmaxy() / 2;
    int p[] = {cx-50,cy-150, cx+50,cy-150, cx+50,cy-50, cx-50,cy-50, cx-50,cy-150};

    outtextxy(cx, cy, "*");
    setcolor(G);

    int n, i, save;
    for (n = 0; n < 10; n++)
    {
            drawpoly(5, p);
            for (i = 0; i < 9; i+=2)
            {
                    save = p[i];
                    p[i] = p[i]*cos(ANGLE) - p[i+1]*sin(ANGLE) + cx - cx*cos(ANGLE) + cy*sin(ANGLE);
                    p[i+1] = save*sin(ANGLE) + p[i+1]*cos(ANGLE) + cy - cx*sin(ANGLE) - cy*cos(ANGLE);
            }
    }

    getch();
    closegraph();
    return (0);
}

And output:

Instead of repeated rotation by `ANGLE`, you should try to increase `ANGLE` in constant steps and transform the intial points. Your present approach will start showing distortions because of loss of precision. — Ajay Brahmakshatriya, May 18 '18 at 13:03

Graphics (Rotation in 2D)

1 Answers1