To create a shader program that will generate a set of triangles using quad tessellation from a patch of four vertices, use the following steps:

1. Use the following code for the vertex shader:

layout (location = 0 ) in vec2 VertexPosition; 
 
void main() {
    gl_Position = vec4(VertexPosition, 0.0, 1.0); 
}

2. Use the following code as the tessellation control shader:

layout( vertices=4 ) out; 
 
uniform int Outer; 
uniform int Inner; 
void main() {
    // Pass along the vertex position unmodified 
    gl_out[gl_InvocationID].gl_Position =  
               gl_in[gl_InvocationID].gl_Position; 
 
    gl_TessLevelOuter[0] = float(Outer); 
    gl_TessLevelOuter[1] = float(Outer); 
    gl_TessLevelOuter[2] = float(Outer); 
    gl_TessLevelOuter[3] = float(Outer); 
 
    gl_TessLevelInner[0] = float(Inner); 
    gl_TessLevelInner[1] = float(Inner); 
}

3. Use the following code as the tessellation evaluation shader:

layout( quads, equal_spacing, ccw ) in; 
 
uniform mat4 MVP; 
 
void main() {
    float u = gl_TessCoord.x; 
    float v = gl_TessCoord.y; 
 
    vec4 p0 = gl_in[0].gl_Position; 
    vec4 p1 = gl_in[1].gl_Position; 
    vec4 p2 = gl_in[2].gl_Position; 
    vec4 p3 = gl_in[3].gl_Position; 
 
    // Linear interpolation 
    gl_Position = 
        p0 * (1-u) * (1-v) + 
        p1 * u * (1-v) + 
        p3 * v * (1-u) + 
        p2 * u * v; 
 
    // Transform to clip coordinates 
    gl_Position = MVP * gl_Position; 
} 

4. Use the geometry shader from the Drawing a wireframe on top of a shaded mesh recipe
5. Use the following code as the fragment shader:

uniform float LineWidth; 
uniform vec4 LineColor; 
uniform vec4 QuadColor; 
 
noperspective in vec3 GEdgeDistance;  // From geom. shader 
 
layout ( location = 0 ) out vec4 FragColor; 
 
float edgeMix() {
   // ** insert code here to determine how much of the edge 
   // color to include (see recipe "Drawing a wireframe on 
   // top of a shaded mesh").  ** 
} 
 
void main() {
    float mixVal = edgeMix(); 
 
    FragColor = mix( QuadColor, LineColor, mixVal );
} 

6. Within the render function of your main OpenGL program, define the number of vertices within a patch:

glPatchParameteri(GL_PATCH_VERTICES, 4); 

7. Render the patch as four 2D vertices in counterclockwise order