Drawing a concave polygon in OpenGL(error)

Question

I am trying to draw an L shaped polygon using OpenGL in C. According to me this code should do it but instead it doesn't give me the expected result. The shape I am trying to draw is something similar to below. The point where the error occurs is at point 2 in function polygon1.

Is there some mistake that I seem to be making here or is it a bug in OpenGL. The pic shown below is the error-red output and the diagram drawn below that, is the expected output from the code.

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void polygon1(float x,float y)
{
  glColor3f(0,1,0);
  glBegin(GL_POLYGON);
  glVertex2f(x,y);
  glVertex2f(x,y+25);
  glVertex2f(x+25,y+25);
  glVertex2f(x+25,y+75);
  glVertex2f(x+50,y+75);
  glVertex2f(x+50,y);
  glEnd();
  glFlush();
}

void mydisplay()
{
  polygon1(200.0,25.0);
}

void main(int argc,char **argv)
{
  glutInit(&argc,argv);
  glutInitDisplayMode(GLUT_SINGLE | GLUT_RGBA);
  glutInitWindowSize(400,400);
  glutInitWindowPosition(540,320);
  glutCreateWindow("my first attempt");
  glClearColor(0.0f,0.0f,0.0,0.0f);
  glClear(GL_COLOR_BUFFER_BIT);   
  glutDisplayFunc(mydisplay);
  gluOrtho2D(0.0,400.0,0.0,400.0);
  glutMainLoop();
}

Answer 1

As Drew Hall pointed out in a previous answer, GL_POLYGON cannot draw concave polygons. You can draw this shape with single triangle fan, though. Try this:

glBegin(GL_TRIANGLE_FAN);
glVertex2f(x+25,y+25);
glVertex2f(x,y+25);
glVertex2f(x,y);
glVertex2f(x+50,y);
glVertex2f(x+50,y+75);
glVertex2f(x+25,y+75);
glEnd();

Answer 2

OpenGL only renders convex polygons properly. Your "L" needs to be broken into a triangle strip. You can do this manually or via GLU tesselators (I'm assuming GLU is an option for you as you're rendering in immediate mode anyways).

Answer 3

As has been mentioned, OpenGL can't do this on its own (unless you store the triangles beforehand)

OpenGL doesn't provide a tessellator, but GLU does, as has been mentioned.

If libGLU isnt an option there are a few tessellators out there - libGDX has quite a simple implementation (in Java - but easy to port).

• http://libgdx.badlogicgames.com/nightlies/docs/api/com/badlogic/gdx/math/EarClippingTriangulator.html

Blender has a C port of this code with optimizations (using 2D kd-tree)

• https://developer.blender.org/diffusion/B/browse/master/source/blender/blenlib/intern/polyfill2d.c