File:Nether Portal (EW) JE2.png

Summary
It takes some effort to render this image. Each face should be combined to form a single geometry. If you don't create the front and the right first, the back will not be masked as shown in this image.

Also, please make sure to enable the following options to render:
 * glEnable( GL_DEPTH_TEST )
 * glCullFace( GL_FRONT_AND_BACK )
 * glDepthFunc( GL_LESS )