Ellipse drawing algorithm

• Download drwelpsalg_src.zip - 55.7 KB

Introduction

First of all, I must mention that I'm an old English language beginner, and this article may contains many mistakes!

I'm now working on a graphic library that is totally independent of the system. I wrote many routines that draw lines, points, rectangles, etc. All the routines now are basic so I can later optimize them for extra performance and speed.

I was looking today morning for an algorithm that draws ellipses. I visited www.codeproject.com and I searched for an article explaining how to draw ellipses without using the Windows GDI. I didn't find anything! Either there's really nothing about this subject or I missed up something during the search process. I said to myself, why not, write an article about the subject and maybe some one read it and suggest me an other way to make things better!

Using the code

The routine I wrote last week draws an ellipse of N segments on the screen. I'll show here the basic one. MyEllipseDraw calculate the coordinate of N points and link them with lines that is in the end an ellipse.

MyDrawEllipse has two parameters which are the handle to the window where the ellipse will be drown and the device context used to draw on the window using GDI+.

It starts by preparing objects and data needed by the draw process. nSeg is a global variable that specifies the number of segments of the ellipse to be drawn.

//A basic algorithm to draw an N segments ellipse on a window

// hWnd +> Handle to the window where the ellipse will be drawn

// hDC +> Device context to the window where the ellipse will be drawn

// nSeg +> A global variable contains the number of segments or points linked each other to form the ellipse

void MyDrawEllipse(HWND hWnd, HDC hDC)
{
    double x, y, _x, _y, __x, __y, dx, dy, z, wx, hy;
    RECT rc;

    //Prepare objects and data

    Graphics g(hDC);
    Pen p(Color(0xff, 0, 0), 1.0); //Red

    GetClientRect(hWnd, &rc);
    z = 0.99; //Point coordinate affinity

    dx = double(rc.right) / 2.0 - 1.0; //Half window height

    dy = double(rc.bottom) / 2.0 - 1.0; //Half window width

    wx = double(rc.right) / 2.0; //Ellipse center

    hy = double(rc.bottom) / 2.0; //Ellipse center

    for(int i = 0; i < int(nSeg); i++) {
        x = wx * sin((double(i) / double(nSeg)) * (pi*2.0));
        y = hy * cos((double(i) / double(nSeg)) * (pi*2.0));
        if(i > 0) {
            //Draw a line connecting the last point to the one being calculated now

            g.DrawLine(&p, int(dx+_x+z), int(dy+-_y+z), int(dx+x+z), int(dy+-y+z));
        } else {
            //Save the first point coordinate to link it with the last one

            __x = x;
            __y = y;
        }
        //Save the actual point coordinate to link it with the next one

        _x = x;
        _y = y;
    }
    //Draw a line between the last point and the first one

    g.DrawLine(&p, int(dx+x+z), int(dy+-y+z), int(dx+__x+z), int(dy+-__y+z));

    /*
    //I use the GDI+ DrawEllipse function to compare my algorithm with
    //the one used by GDI+. Only the visual result is important for me now!
    p.SetColor(Color(0, 0xff, 0));
    g.DrawEllipse(&p, 0, 0, rc.right-1, rc.bottom-1);
    */
}
 

After downloading the source files extract the files to a directory, look for drwelpsalg.cpp file where the routine MyDrawEllipse is written. The file contains the standard code generated by the Win32 project AppWizard. I deleted all the comments so only the code I added will be, that way you can read the source file easily.

When the demo program is executed, it draws an ellipse adjusted to fill all the client area of the main window. The title bar show informations about the client area and the number of segments of the drawn ellipse. You can click with left button to increase the number of segments or click with the right button to decrease it. Try values smaller than 16 segments.

In the end, I must say sorry for all mistakes you'll find in this 'article' since it's my first using English language!

In the end, I hope that you'll find something useful and interesting while reading those lines!

In the end, thank you for reading till this word :)

History

None for the moment.