Question

I am trying to create some simple polygons in openGL3.3. I have 2 types of objects with the following properties :

Object 1 – 10 vertices (listed below, in order) stored in GL_ARRAY_BUFFER and use GL_TRIANGLE_FAN

v x y z w  
v 0.0 0.0 1.0 1.0  
v 0.0 1.0 0.1 1.0  
v 0.71 0.71 0.1 1.0  
v 1.0 0.0 0.1 1.0  
v 0.71 -0.71 0.1 1.0  
v 0.0 -1.0 0.1 1.0  
v -0.71 -0.71 0.1 1.0  
v -1.0 0.0 0.1 1.0  
v -0.71 0.71 0.1 1.0  
v 0.0 1.0 0.1 1.0  

Object 2 – 4 vertices (listed below, in order) stored in GL_ARRAY_BUFFER and use GL_TRIANGLE_STRIP

v x y z w  
v 0.0 0.0 0.0 1.0  
v 0.0 1.0 0.0 1.0  
v 1.0 0.0 0.0 1.0  
v 1.0 1.0 0.0 1.0  

I load 64 Object1’s and 4 Object2’s. The Object2’s are scaled using glm::scale(20, 20, 0).

When I try to render these with GL_CULL_FACE disabled but GL_DEPTH_TEST enabled with glDepthFunc(GL_LESS) everything works fine. As soon as I try to enable GL_CULL_FACE all I get is a blank window.

Some other useful information :
– Rendering order = 4 Object2’s followed by 64 Object1’s
– Camera – glm::lookAt(glm::vec3(0.0f, 0.0f, 50.0f), glm::vec3(0.0f, 0.0f, 0.0f), glm::vec3(0.0f, 1.0f, 0.0f));
– Perspective – glm::perspective(45.0f, 16.0f / 9.0f, 0.01f, 1000.0f);

I have been trying to figure out why GL_CULL_FACE doesn’t work for the past couple of days but have no idea. Any help is much appreciated.

Answer 1

This is almost always due to polygon normal direction (right-hand rule of the three vertices of each triangle)

Try switching the direction using:

glCullFace(GL_BACK or GL_FRONT)

Some more reading here