30. OpenGL Circle Drawing
Here we are going to create a circle. This is something I don’t believe
you can do unless you create it on your own. In other words, it is not
preset.
Now because a circle is just a bunch of lines. That is what we are going
to make. A bunch of lines, close together that go around in a circle.
Now first of all I am going to create a typedef to hold our circle properties
as we call them
typedef struct
{
This will hold the x values as we need them
float x;
This will hold the y values as we need them
float y;
Call it CIRCLE
}CIRCLE;
Then I am setting the property circle to act like the typedef CIRCLE
CIRCLE circle;
Now for the meat of this tutorial, the circle creation function
The properties this takes are:
k – the translation on the y axis
r – the radius of the circle
h – the translation on the x axis
void createcircle (int k, int r, int h) {
Now we begin drawing our lines
glBegin(GL_LINES);
Here we are setting up the vertices all in one line function, this
sets every 2 vertices to a line.
First we begin our loop, this loop will cycle through, constantly
changing our x and y values for our lines. It cycles through until
it reaches 180, increasing in intervals of 1
for (int i = 0;i < 180;i++)
{
Now to set up the current x value that we need for our vertex
I am setting it to the radius of the circle, times by the cosine of
the current value of i, then I am taking h to translate it
circle.x = r * cos(i) – h;
Then to set up the current y value that we need for our vertex
I am setting it to the radius of the circle, times by the sine of
the current value of i, then I am adding k to translate it
circle.y = r * sin(i) + k;
Then I am drawing the vertex
glVertex3f(circle.x + k,circle.y – h,0);
Now for the second part of the line, I am doing the same as above, only
I am moving it 0.1 units so that I am not down to points. This also
Lowers the chance of any holes occuring in the circle
circle.x = r * cos(i + 0.1) – h;
circle.y = r * sin(i + 0.1) + k;
Then I am drawing the vertex
glVertex3f(circle.x + k,circle.y – h,0);
}
glEnd();
}
Then to call it, use something like this:
createcircle(0,10,0);
That would create a circle that is originated around (0,0) and with
a radius of 10 units.
Changing GL_LINES to another shape can come up with some other little
effects if you would like to give it a go.
But other than that, this is all you need for a circle.
If you have any queries, please email me at swiftless@gmail.com
1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42. 43. 44. 45. 46. 47. 48. 49. 50. 51. 52. 53. 54. 55. 56. 57. 58. 59. 60. 61. 62. 63. 64. 65. 66. 67. 68. 69. 70. 71. 72. 73. 74. 75. 76. 77. 78. 79. 80. 81. 82. 83. 84. 85. 86. 87. 
#include <GL/gl.h> #include <GL/glut.h> #include <math.h> typedef struct CIRCLE circle; float rot = 0; void createcircle (int k, int r, int h) { void display (void) { void reshape (int w, int h) { int main (int argc, char **argv) { glutDisplayFunc (display);

i need to know how to draw scaling line inside the circle how to do that?
Bob,
How did you arrive at the constant 6.28f in the below lines
onst int kSegments = 30;
const GLfloat kRadianIncr = 6.28f/kSegments;
6.28f is simply 2 * PI. PI is 3.14159…. which you will see in geometry books. It is the constant that maps a circle radius to its circumference.
very nice presentation.
i try this code
”
#include
#include
#include
GLfloat d;
float p1x=0.0;
float p1y=0.0;
float p1z=0.0;
const float p1radius = 8.0;
const float p2radius = 8.0;
float yRotationAngle = 0.0f;
int k=0,l=0;
float p2x=0.0;
float p2y=0.0;
float p2z=0.0;
void collision (void) {
d = sqrt(((p1x – p2x) * (p1x – p2x)) + ((p1y – p2y) * (p1y – p2y))+ ((p1z – p2z) * (p1z – p2z)));
}
void pointz (void) {
glPushMatrix();
if (d 2.0)
k=1;
if(p1x2.0)
l=1;
if(p2y 360.0f) // If we have rotated beyond 360 degrees (a full rotation)
yRotationAngle = 360.0f; // Subtract 360 degrees off of our rotation
}
void reshape (int w, int h) {
glViewport (0, 0, (GLsizei)w, (GLsizei)h);
glMatrixMode (GL_PROJECTION);
glLoadIdentity ();
gluPerspective (60, (GLfloat)w / (GLfloat)h, 1.0, 100.0);
glMatrixMode (GL_MODELVIEW);
}
void keyboard (unsigned char key, int x, int y) {
if (key==’q’) {
p1z = p1z – 0.1;
}
if (key==’z’) {
p1z = p1z + 0.1;
}
if (key==’w’) {
p1y = p1y + 0.1;
}
if (key==’s’) {
p1y = p1y – 0.1;
}
if (key==’a’) {
p1x = p1x – 0.1;
}
if (key==’d’) {
p1x = p1x + 0.1;
}
if (key==’i’) {
p2y = p2y + 0.1;
}
if (key==’k’) {
p2y = p2y – 0.1;
}
if (key==’j’) {
p2x = p2x – 0.1;
}
if (key==’l’) {
p2x = p2x + 0.1;
}
if (key==27) { //27 is the ascii code for the ESC key
exit (0); //end the program
}
}
int main (int argc, char **argv) {
glutInit (&argc, argv);
glutInitDisplayMode (GLUT_DOUBLE); //set up the double buffering
glutInitWindowSize (500, 500);
glutInitWindowPosition (100, 100);
glutCreateWindow (“A basic OpenGL Window”);
glutDisplayFunc (display);
glutIdleFunc (display);
glutReshapeFunc (reshape);
glutKeyboardFunc (keyboard);//the call for the keyboard function.
glutMainLoop ();
return 0;
}
but there is no collision?
plz clear me this topic.
thanks in advance.
“
I would stray away from everything GLUT related.
You are not passing in radians to the trig functions. I made your algorithm more mathematically clear. For example the number of segments now can be be adjusted for circle resolution (30 segments is good enough for my example). kSegments = 5 gives a pentagon ! Also only one glVertex call using GL_LINE_LOOP. I took out the translation h, k for simplicity:
static void drawCircle (int r) {
CIRCLE circle;
glBegin(GL_LINE_LOOP);
int i = 0;
const int kSegments = 30;
const GLfloat kRadianIncr = 6.28f/kSegments;
for (i = 0; i < kSegments; i++)
{
GLfloat angleRadians = kRadianIncr*i;
circle.x = r * cos(angleRadians);
circle.y = r * sin(angleRadians);
glVertex3f(circle.x,circle.y,0);
}
glEnd();
}
I believe using a GL_TRIANGLE_FAN drawing mode reuses the center vertex for better performance.
Hi
Nice post.
You can get the same result with gluDisk(gluNewQuadric(), inner, outer, slices, loops), for more details see
http://www.talisman.org/opengl1.1/Reference/gluDisk.html
and if you need to draw part of a circle you can use gluPartialDisk.
Cheers, o.
Hi Olgis,
Yes you can, but the point of this tutorial is to show you don’t need a third party library to make these shapes. This also gives you more control, you can put this inside of a VBO easily, unlike glu methods.
Cheers,
Swiftless
Sorry, missed glBegin(GL_LINE_LOOP); in the beginning =]
Hi,
I think following code will yield in smoother circle approximation even when the number of segments is very small, say 20. What do you think ?
const float PI = 3.1415926535897932;
for (float i = 0.0; i < 2.0*PI; i += 2.0*PI/30.0){ float x = radius * cos(i) + center.x + SCREEN_WIDTH / 2.0; float y = radius * sin(i) + center.y + SCREEN_HEIGHT / 2.0; glVertex3f(x,y,0); } glEnd();
Would it be better if you use GL_LINE_LOOP so you need not to redundanty called glVertex for the tip of each line ?
Anyway, Thank you for your tutorial — I learn a lot from many of your posts.
Hey Pisit,
Sure, just make sure you add a glVertex call before and after the loop to set the start and end points.
Cheers,
Swiftless
Hi Cyp,
I see what you mean, I have combined both the creation and the rendering of the sphere into one method, which does make the struct pointless.
The efficient way to do this, would be to store all the vertices into the struct and then create another method with a loop for rendering the circle.
However when it comes to k and h, you can remove them if you go with the above implementation and just use translate, and you can even do this in the second example I just gave, but they are there because of the mathematics behind creating a circle involves k and h for setting the location.
Cheers,
Swiftless
I think that use of struct circle is pointless its working same way with local varibles.
Adding k, h varibles seems strange u can acomplish same thing with use of translate functions. less simple math ?
I’d rather add segmentation varible than this two.
Anyways i’m not pro opengl coder and i can be wrong but all in all this one rly helped me thx ^_^