Using glPrimitiveRestartIndex to declare multiple geometries in the same VBO

Hi,

I have a VBO storing a collection of polygons - i.e. triangle fans. I am willing to use glPrimitiveRestartIndex to declare when a polygon finishes and when a new one start.

I draw with

GL2 gl2 = gl.getGL2();
gl2.glEnable(GL2.GL_PRIMITIVE_RESTART);
gl2.glPrimitiveRestartIndex(PRIMITIVE_RESTART_VALUE); // 0xffffffff
...
gl2.glPolygonMode(GL.GL_FRONT_AND_BACK, GL2GL3.GL_FILL);
gl2.glDrawElements(glGeometryType, elementSize, GL.GL_UNSIGNED_INT, firstCoordOffset);

I generate VBO with a restart value every 4 point

protected void append(int[] hexahedronPoints, int p1, int p2, int p3, int p4) {
    geometries[cursor++] = hexahedronPoints[p1];
    geometries[cursor++] = hexahedronPoints[p2];
    geometries[cursor++] = hexahedronPoints[p3];
    geometries[cursor++] = hexahedronPoints[p4];
    geometries[cursor++] = hexahedronPoints[p1];
   
    if(DrawableVBO2.PRIMITIVE_RESTART)
      geometries[cursor++] = DrawableVBO2.PRIMITIVE_RESTART_VALUE;
}

OpenGL does not consider the restart value and instead see it as a vertex.

This show a VBO fed with a closed loop p1, p2, p3, p4, p1, then p5, ... No primitive restart value is used, so OpenGL continue the line to point p5 when the loop is closed. (I am surprised to have the need to define the closing point, but anyway, the problem is mainly to see a line between p1 and p5 (which are labeled 3 and 0 in the chart).



When activating primitive restart, OpenGL simply continue reading and seem to take the value as a coordinate.

I have never used this feature but I think that you have to pass a real index. You pass 4 vertices and you indicate that the 5th vertex is the primitive restart vertex by passing its index to glPrimitiveRestartIndex. Maybe I'm wrong.

I got the explanation for the failure : I am using GL2 core profile. Primitive restart is only supported in core profiles as of OpenGL 3.1. Before 3.1, one should have the NV_primitive_restart extension available, which is not the case on my MacOS 10.12.3.

I indeed get these information

Capabilities  : GLCaps[rgba 8/8/8/8, opaque, accum-rgba 0/0/0/0, dp/st/ms 16/0/0, dbl, mono  , hw, GLProfile[GL2/GL2.hw], offscr[fbo]]
GL_VENDOR     : NVIDIA Corporation
GL_RENDERER   : NVIDIA GeForce GT 650M OpenGL Engine
GL_VERSION    : 2.1 NVIDIA-10.17.5 355.10.05.45f01
GL_EXTENSIONS :
        GL_ARB_color_buffer_float
        GL_ARB_depth_buffer_float
        GL_ARB_depth_clamp
        GL_ARB_depth_texture
        GL_ARB_draw_buffers
       ...


If you want to get the same information, use this

GL gl = ...

StringBuffer sb = new StringBuffer();
sb.append("Capabilities  : " + caps + "\n");
sb.append("GL_VENDOR     : " + gl.glGetString(GL.GL_VENDOR) + "\n");
sb.append("GL_RENDERER   : " + gl.glGetString(GL.GL_RENDERER) + "\n");
sb.append("GL_VERSION    : " + gl.glGetString(GL.GL_VERSION) + "\n");

String ext = gl.glGetString(GL.GL_EXTENSIONS);

if(ext!=null) {
  sb.append("GL_EXTENSIONS : " + "\n");
  for(String e: ext.split(" ")) {
    sb.append("\t" + e + "\n");
  }
}


More detailed answers can be found here.