# Tessellation won't render

I am starting to work with tessellation and have some problems right in the beginning. I try to render a quad made of two triangles with tessellation, but nothing appears on the screen. But as soon as I remove the tessellation shaders and change GL_PATCHES to GL_TRIANGLESagain, it starts working again, without tessellation obviously.

So here is my code to setup the tessellation shaders:

glPatchParameteri(GL_PATCH_VERTICES, 3);
{

}


   #version 400

layout(location=0) in vec4 vertPosition;
layout(location=1) in vec2 vertUV;
layout(location=2) in vec3 vertNormal;

void main(void){
gl_Position = vertPosition;
}


The TCS:

#version 400
layout (vertices = 3) out;

void main()
{
gl_out[gl_InvocationID].gl_Position = gl_in[gl_InvocationID].gl_Position;

gl_TessLevelOuter[0] = 3.0;
gl_TessLevelOuter[1] = 3.0;
gl_TessLevelOuter[2] = 3.0;
gl_TessLevelInner[0] = 3.0;
}


And the TES:

#version 400
layout(triangles, equal_spacing, cw) in;

void main()
{
gl_Position = (gl_TessCoord.x * gl_in[0].gl_Position +
gl_TessCoord.y * gl_in[1].gl_Position +
gl_TessCoord.z * gl_in[2].gl_Position);
}


And this is how I draw the quad:

glPolygonMode( GL_FRONT_AND_BACK, GL_LINE );

I tried using in and out variables instead of gl_in/out, but that doesn't work either. I also tried it without the TCS. I was also using Apitrace to look up any errors, but nothing. The function loadShader() looks for compile errors. So what could cause this problem? I really have no idea how to debug this thing any further.