Question

I am trying to rotate a rectangle using the matrix (x cos θ – y sin θ, x sin θ + y cos θ). The problem is that the rectangle is getting smal and reaching the origin point per time (I am using timer). Here is my code.

void WINAPI Rotate(POINT arr[5])
{
    static POINT origin = { 400, 400 };
    static int i;
    static const double angle = 0.1;
    for (i = 0; i < 5; ++i)
    {
            // translate
        arr[i].x -= origin.x;
        arr[i].y -= origin.y;
            // rotate
        arr[i].x = arr[i].x * cos(angle) - arr[i].y * sin(angle);
        arr[i].y = arr[i].x * sin(angle) + arr[i].y * cos(angle);
            // translate
        arr[i].x += origin.x;
        arr[i].y += origin.y;
    }
}

So I want the points in arr to be fixed distance from the origin point after rotation. I don’t want them to reach the origin point per time.

Initially:

    arr[0].x = 200;
    arr[0].y = 100;
    arr[1].x = 100;
    arr[1].y = 100;
    arr[2].x = 100;
    arr[2].y = 200;
    arr[3].x = 200;
    arr[3].y = 200;
    arr[4].x = arr[0].x;
    arr[4].y = arr[0].y;

If this is the wrong way for rotation, does someone know a correct method to rotate a rectangle about its origin, without affecting its size?

Here are snapshots while it rotates:

Answer 1

In your rotation,

arr[i].x = arr[i].x * cos(angle) - arr[i].y * sin(angle);
arr[i].y = arr[i].x * sin(angle) + arr[i].y * cos(angle);

you use the new x-coordinate to compute the new y-coordinate, but you should use the old one. To fix it, use a temporary,

double temp = arr[i].x * cos(angle) - arr[i].y * sin(angle);
arr[i].y = arr[i].x * sin(angle) + arr[i].y * cos(angle);
arr[i].x = temp;