 # To Draw a 3D Circle and Applying Texturing .

Hi…

``````   I want to draw a 3D Circle and applying texturing to it using OpenGL + VC++ [Esp in a MFC ActiveX Control]. Especially I want to texture the 3D Circle in a Square Shape Panel.So Please give me a fine solution to solve this problem.
``````

Thanks,

Regards,
Balamurugan.

Here is some code that will draw you a circle (from this site):

``````
void DrawCircle(float cx, float cy, float r, int num_segments)
{
float theta = 2 * 3.1415926 / float(num_segments);
float tangetial_factor = tanf(theta);//calculate the tangential factor

float x = r;//we start at angle = 0

float y = 0;

glBegin(GL_LINE_LOOP);
for(int ii = 0; ii < num_segments; ii++)
{
glVertex2f(x + cx, y + cy);//output vertex

//calculate the tangential vector
//remember, the radial vector is (x, y)
//to get the tangential vector we flip those coordinates and negate one of them

float tx = -y;
float ty = x;

x += tx * tangetial_factor;
y += ty * tangetial_factor;

}
glEnd();
}

``````

Yes, it is 2D. There is no such thing as a 3D circle. A circle is a 2D primitive only (can only exist with all points coplanar and still be called a circle).

Texturing the circle means computing texture coordinates for each vertex of the circle. That is where it gets tricky.

The Algorithm that MarkS give you draw only the outline of the circle. If you want a circle filed with a texture, you can use triangle fan. First, draw the vertex at the center of the circle. Then draw the vertex on the contour of the circle, use cos(angle)*radius for x and sin(angle)*radius for y. Since texture coordinates s and t are in the range [0 1] => s = (cos(angle)+1.0)*0.5 and t = (sin(angle)+1.0)*0.5 . The texture coordinate for the vertex at the center of the circle is (0.5,0.5).

Here is an unoptimized sample code that you can test:

``````
GLvoid draw_circle(const GLfloat radius,const GLuint num_vertex)
{
GLfloat vertex;
GLfloat texcoord;

const GLfloat delta_angle = 2.0*M_PI/num_vertex;

glEnable(GL_TEXTURE_2D);
glBindTexture(GL_TEXTURE_2D,texID);
glTexEnvi(GL_TEXTURE_ENV,GL_TEXTURE_ENV_MODE,GL_REPLACE);
glBegin(GL_TRIANGLE_FAN);

//draw the vertex at the center of the circle
texcoord = 0.5;
texcoord = 0.5;
glTexCoord2fv(texcoord);

vertex = vertex = vertex = 0.0;
vertex = 1.0;
glVertex4fv(vertex);

for(int i = 0; i < num_vertex ; i++)
{
texcoord = (std::cos(delta_angle*i) + 1.0)*0.5;
texcoord = (std::sin(delta_angle*i) + 1.0)*0.5;
glTexCoord2fv(texcoord);

vertex = 0.0;
vertex = 1.0;
glVertex4fv(vertex);
}

texcoord = (1.0 + 1.0)*0.5;
texcoord = (0.0 + 1.0)*0.5;
glTexCoord2fv(texcoord);

vertex = 0.0;
vertex = 1.0;
glVertex4fv(vertex);
glEnd();

glDisable(GL_TEXTURE_2D);

}

``````

Notice that only the portion of a circle inside the square texture image will be displayed to avoid deformation.

Correct. I didn’t finish the post before hitting submit. :eek: