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View Full Version : Draw Circle, MidPoint Circle Algorithm

Trivialnight
09-25-2004, 11:23 AM
Im taking a OpenGL class right now and our first project is to draw a pie chart with OpenGL using the MidPoint circle algorithm. I got a semi correct circle to display. It shows about the bottom quarter and the top quarter but the rest it isn't connected. Is there something wrong with this algorithm and is just drawing those 2 parts.

#include <windows.h>
#include <GL/gl.h>
#include <GL/glut.h>

const GLdouble twoPi = 6.283185;
GLsizei winWidth = 640, winHeight = 480;

struct scrPt
{
GLint x,y;
};

class screenPt
{
private:
GLint x,y;

public:
screenPt()
{
x = y = 0;}
void setCoords(GLint xCoordValue, GLint yCoordValue)
{
x = xCoordValue;
y = yCoordValue;}

GLint getx() const
{
return x;}

GLint gety() const
{
return y;}

void incrementx()
{
x++;}

void decrementy()
{
y--;}
};

void circlePlotPoints(GLint,GLint,screenPt);
void circleMidPoint(GLint, GLint);
void setPixel(GLint,GLint);
void PieChart();
void DrawCircle();
void init();

void init()
{
glClearColor (1,1,1,1);
glMatrixMode(GL_PROJECTION);
gluOrtho2D(0,600,0,500);
}

{
GLint xc = circCtr.x;
GLint yc = circCtr.y;

screenPt circPt;
GLint p = 1 - radius;

circlePlotPoints(xc,yc,circPt);
while(circPt.getx() < circPt.gety())
{
circPt.incrementx();
if(p < 0)
p += 2 * circPt.getx();
else
{
circPt.decrementy();
p += 2 * (circPt.getx() - circPt.gety()) + 1;
}
circlePlotPoints(xc,yc,circPt);
}
}

void setPixel(GLint xCoord, GLint yCoord)
{
glBegin(GL_POINTS);
glVertex2i (xCoord,yCoord);
glEnd();
}

void circlePlotPoints(GLint xc, GLint yc, screenPt circPt)
{
setPixel(xc + circPt.getx(), yc + circPt.gety());
setPixel(xc - circPt.getx(), yc + circPt.gety());
setPixel(xc + circPt.getx(), yc - circPt.gety());
setPixel(xc - circPt.getx(), yc - circPt.gety());
setPixel(xc + circPt.getx(), yc + circPt.gety());
setPixel(xc - circPt.getx(), yc + circPt.gety());
setPixel(xc + circPt.getx(), yc - circPt.gety());
setPixel(xc - circPt.getx(), yc - circPt.gety());
}

void PieChart()
{
scrPt circCtr;
GLint radius = winWidth / 4;

circCtr.x = winWidth / 2;
circCtr.y = winHeight / 2;
}
void DrawCircle()
{ glClear(GL_COLOR_BUFFER_BIT);
glColor3f(0.0,0.0,1.0);
PieChart();
glFlush();
}
int main(int argc, char **argv)
{
glutInit(&amp;argc, argv);
glutInitWindowSize(640, 480);
glutInitDisplayMode(GLUT_SINGLE | GLUT_RGB);
glutInitWindowPosition(100,100);
glutCreateWindow("Circle");
init();
glutDisplayFunc(DrawCircle);
glutMainLoop();
return(0);
}

09-25-2004, 02:14 PM
Dude, this is your homework not ours. Your teacher wants you to understand what you are doing.

Doesnt look like you have any problem with OpenGL because your program compiles and runs fine, well except for the part where your homework kicks in.

This is not an OpenGL problem.