win32 make RECT spin

Question

3.00/5 (2 votes)

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I'm trying to get a simple spin effect for an object defined by RECT structure and I'm having a little bit of trouble with the math of it.

Here is what i have so far

C++

int moveAroundHimself(RECT &r, RECT limit, double step, int id) {
	static double info[100][10] = {};
	if(!info[id][0]) {
		info[id][0] = r.left;
		info[id][1] = r.right;
		info[id][2] = r.top;
		info[id][3] = r.bottom;
		info[id][4] = 0;
		info[id][5] = (r.right + r.left)/2; // xCenter
		info[id][6] = (r.bottom + r.top)/2; // yCenter

	}
	// xRot = xCenter + cos(Angle) * (x - xCenter) - sin(Angle) * (y - yCenter)
	// yRot = yCenter + sin(Angle) * (x - xCenter) + cos(Angle) * (y - yCenter)
	r.left = info[id][5] + cos(info[id][4]) * (info[id][0] - info[id][5]) - sin(info[id][4]) * (info[id][2] - info[id][6]);
	r.right = info[id][5] + cos(info[id][4]) * (info[id][1] - info[id][5]) - sin(info[id][4]) * (info[id][3] - info[id][6]);
	r.top = info[id][6] + sin(info[id][4]) * (info[id][0] - info[id][5]) + cos(info[id][4]) * (info[id][2] - info[id][6]);
	r.bottom = info[id][6] + sin(info[id][4]) * (info[id][1] - info[id][5]) + cos(info[id][4]) * (info[id][3] - info[id][6]);


	info[id][4] += step;

	return 0;

}

You can ignore the limit rect for now.

Any idea on what is wrong ?

Posted 4-Apr-13 5:16am

ionutvmi

Updated 4-Apr-13 9:20am

v3

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Comments

Captain Price 4-Apr-13 11:23am

Do you want to rotate the RECT ?

ionutvmi 4-Apr-13 11:25am

yes around his center like he is spinning

Captain Price 4-Apr-13 11:34am

You mean only 4 rotations; to left, to top, to right & to bottom ?

ionutvmi 4-Apr-13 11:44am

no i mean like this http://i.imgur.com/uBlN7Uc.png

the left,top,right,bottom are it's coordinates http://msdn.microsoft.com/en-us/library/windows/desktop/dd162897(v=vs.85).aspx

nv3 4-Apr-13 11:47am

You are rotating around the origin of the coordinate system and not the center of the rect.

pasztorpisti 4-Apr-13 12:04pm

In order to rotate around the center of the rect first you have to calculate the center point, subtract the center point from all 4 points of the rect, make the rotation and then add the center point.

Sergey Alexandrovich Kryukov 4-Apr-13 13:17pm

That's the answer, my 5. Will you formally answer?
—SA

pasztorpisti 4-Apr-13 16:03pm

Thank you! As I see I'm late with an answer. :-)

Sergey Alexandrovich Kryukov 4-Apr-13 16:10pm

:-)

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nv3 · Accepted Answer · 2013-04-04T08:15:00

Solution 1

Here is a version that calculates the rotation in relation to the rectangle's center.

C++

RECT RotateRect (const Rect& r, double angle)
{
    double sina = sin (angle);
    double cosa = cos (angle);

    double xc = (r.left + r.right) * 0.5;
    double yc = (r.top + r.bottom) * 0.5;

    double w2 = r.right - xc;   // half the width
    double h2 = r.bottom - yc;  // half the height

    double dx = cosa * w2 - sina * h2;
    double dy = sina * w2 + cosa * h2;

    RECT q;
    q.left   = xc - dx;
    q.top    = yc - dy;
    q.right  = xc + dx;
    q.bottom = yc + dy;
    return q;
}

I have removed that static array, which actually belongs into the calling code. The function returns the rotated rectangle. I didn't take the time to debug that code, leaving that as an exercise for you.

Note that movement of the corners is symmetrical. Hence we need to calculate the new corner vector only once, and then add repsectively subtract it from the center.

Posted 4-Apr-13 8:15am

nv3

Updated 4-Apr-13 9:01am

v3

Comments

ionutvmi 4-Apr-13 14:47pm

hello, thanks for your reply but that seems to shrink it down till it's gone and it's not rotating at all...

nv3 4-Apr-13 15:02pm

The sign in the line dy = ... was inverted.

ionutvmi 4-Apr-13 15:17pm

it's changing it's shape and it moves it to the right corner,
i also got to something similar witch seems to be stationary but instead of rotating it's changing it's shape (width and height) exactly like yours(i updated the question).

i just can't figure out why is doing that...

nv3 4-Apr-13 15:49pm

The code looks mathematically correct. My guess is that is has to do with the code outside, which you haven't shown.

Also, the initial test "if (!info[id][0]) ..." is a great way to get caught. What if r.left is initially zero?

I would just step through it with de debugger. If you are not familiar with that, do at least a test print of a couple of iteration, each with in- and out-coords. Then it should become obvious what's happening.

Sergey Alexandrovich Kryukov 4-Apr-13 15:59pm

I trust it works correctly, the idea is right... a 5.
—SA

nv3 4-Apr-13 16:03pm

Thanks, Sergey!

pasztorpisti 4-Apr-13 16:03pm

+5

nv3 4-Apr-13 17:31pm

Thank you!