OpenGL draw Polygon with GL_LINES and angle between lines - opengl

Question

I need to draw a Polygon iteratively. for example, I want to draw a Polygon with 8 corners. I need to draw the first line with GL_LINES and then draw the second line with the same length and an angle of 135° between them, the third line has also an angle of 135° to the second line, etc.
I want to make a loop to render it but I don't know how. I have an approach, but it doesn't work properly.
the second point of line n-1 should be the first point of n and so on...
At the end, I need to get a closed Polygon. the last point of the last line should be the first point of the first line.

Answer 1

Use GL_LINE_LOOP, that will connect your last vertex to your first automatically:
#include <GL/glut.h>
#include <cmath>
void glPolygon( unsigned int sides )
{
if( sides < 3 ) return;
const float PI = 3.14159;
const float step = ( 2 * PI ) / static_cast< float >( sides );
glBegin( GL_LINE_LOOP );
for( unsigned int i = 0; i < sides; ++i )
{
glVertex2f( cos( i * step ), sin( i * step ) );
}
glEnd();
}
void display()
{
glClear( GL_COLOR_BUFFER_BIT );
glMatrixMode( GL_PROJECTION );
glLoadIdentity();
double ar = glutGet( GLUT_WINDOW_WIDTH ) / (double)glutGet( GLUT_WINDOW_HEIGHT );
glOrtho( -2 * ar, 2 * ar, -2, 2, -1, 1);
glMatrixMode( GL_MODELVIEW );
glLoadIdentity();
glColor3ub( 255, 0, 0 );
glPolygon( 8 );
glutSwapBuffers();
}
int main( int argc, char **argv )
{
glutInit( &argc, argv );
glutInitDisplayMode( GLUT_RGBA | GLUT_DOUBLE );
glutInitWindowSize( 640, 480 );
glutCreateWindow( "Polygons" );
glutDisplayFunc( display );
glutMainLoop();
return 0;
}
If you're dead-set on GL_LINES this works:
void glPolygonLines( unsigned int sides )
{
if( sides < 3 ) return;
const float PI = 3.14159f;
const float step = ( 2 * PI ) / static_cast< float >( sides );
glBegin( GL_LINES );
for( unsigned int i = 0; i < sides; ++i )
{
unsigned int cur = ( i + 0 ) % sides;
unsigned int nxt = ( i + 1 ) % sides;
glVertex2f( cos( cur * step ), sin( cur * step ) );
glVertex2f( cos( nxt * step ), sin( nxt * step ) );
}
glEnd();
}