Draw a curved line from an arc edge

Question

Here\'s the screenshot of what I am doing. Currently, I\'m stuck from drawing a curved borders into this rectangle.

Answer 1

GL_POLYGON is only for convex polygons.

Link together the vertices on your inner and outer radii to form quads/triangles:

#include 
#include 

void Torus2d
    ( 
    float angle,            // starting angle in radians
    float length,           // length of arc in radians, >0
    float radius,           // inner radius, >0
    float width,            // width of torus, >0
    unsigned int samples    // number of circle samples, >=3
    )
{
    if( samples < 3 ) samples = 3;
    const float outer = radius + width;
    glBegin( GL_QUAD_STRIP );
    for( unsigned int i = 0; i <= samples; ++i )
    {
        float a = angle + ( i / (float)samples ) * length;
        glVertex2f( radius * cos( a ), radius * sin( a ) );
        glVertex2f( outer * cos( a ), outer * sin( a ) );
    }
    glEnd();
}

void display()
{
    glClear( GL_COLOR_BUFFER_BIT );

    glMatrixMode( GL_PROJECTION );
    glLoadIdentity();
    double w = glutGet( GLUT_WINDOW_WIDTH );
    double h = glutGet( GLUT_WINDOW_HEIGHT );
    double ar = w / h;
    glOrtho( -4 * ar, 4 * ar, -4, 4, -1, 1);

    glMatrixMode( GL_MODELVIEW );
    glLoadIdentity();

    glColor3ub( 255, 0, 0 );
    Torus2d( 0, 1.57079633, 2, 1, 20 );

    glutSwapBuffers();
}

int main( int argc, char **argv )
{
    glutInit( &argc, argv );
    glutInitDisplayMode( GLUT_RGBA | GLUT_DOUBLE );
    glutInitWindowSize( 640, 480 );
    glutCreateWindow( "GLUT" );
    glutDisplayFunc( display );
    glutMainLoop();
    return 0;
}