Algorithm to roll a rectangle

[Ducky]

I'm trying to draw and pivot a rectangle.
How to get the new coordinates if I turn it 90 degree for example?

From this:


To this:


This is not good because it will reduce it in size but this is just to give you an idea.

 int size = 50;

    if(b) // draw rectangle
    {
        POINT points[4] =
           {{xCoord,      yCoord},
            {xCoord,      yCoord-size},
            {xCoord+size, yCoord-size},
            {xCoord+size, yCoord}
        };
        Polygon(hdc, points, 4);
    }
    else // turn rectangle
    {
        POINT points[4] =
           {{xCoord+(size/2),      yCoord-(size/2)},
            {xCoord+(size),        yCoord-size}, 
            {xCoord+size+(size/2), yCoord-size+(size/2)},
            {xCoord+size,          yCoord}
        };
        Polygon(hdc, points, 4);
    }

[Elysia]

I suspect you'd use something like a rotation matrix: Rotation matrix - Wikipedia, the free encyclopedia

[Alpo]

Take a look here too:

Can someone help me to math?

The code I posted there was from an analogue clock program, where the points are rotated around the window origin, which is set in the center of the client area.

[Ducky]

Thanks, yes I found that clock example from the Petzold book too, that is the answer to my question.

[Alpo]

Thanks, yes I found that clock example from the Petzold book too, that is the answer to my question.

It seems as if you are needing to move the rectangle as well (for example it moves to the right when you roll it that way). I was wondering if there was a formula to determine the correct amount to move it by as well?

The best I could think of was something like : rect.x += cos(a) * width . I thought that would move it over a width length as cos approaches 1, I haven't actually worked it out though. Let me know what you think.

[Ducky]

I thought about adding the width to the rotating point every time the rectangle turns 180 degree. Seems to be easier.

[stahta01]

I thought about adding the width to the rotating point every time the rectangle turns 180 degree. Seems to be easier.

Please state that you understand that you only rolled it 45 degrees in the posted image.
Or, state you really did roll it 90 degrees in the posted image.

I think it is 45 degrees and you had made a typo of 90; but, now you state 180 when I think 90 would be the value.

Tim S.

[Ducky]

Please state that you understand that you only rolled it 45 degrees in the posted image.
Or, state you really did roll it 90 degrees in the posted image.

I think it is 45 degrees and you had made a typo of 90; but, now you state 180 when I think 90 would be the value.

Tim S.

The posted image is just an example to show the movement. But youre right it should be 90 degrees to make it roll on a plain surface.