Simple problems in tessellation shader implementation

Question

I tried to render a triangle with a tessellation shader. Now, without the tessellation shaders, the triangle renders fine. As soon as I add the tessellation shaders, I get a blank screen. I have also written glPatchParameteri(GL_PATCH_VERTICES,3) before glDrawArrays(GL_PATCHES,0,3).

Here are the shaders:

VERTEX SHADER:

#version 440 core

layout (location = 0) in  vec2 apos;
out vec2 pos;

void main()
{
    //gl_Position = vec4(apos,1.0f,1.0f);  without tessellation shaders
    pos = apos;
}

TESSELLATION CONTROL SHADER

#version 440 core

layout (vertices = 3) out;
in vec2 pos[];
out vec2 EsPos[];
void main()
{
    EsPos[gl_InvocationID] = pos[gl_InvocationID];
    gl_TessLevelOuter[0] = 3.0f;
    gl_TessLevelOuter[1] = 3.0f;
    gl_TessLevelOuter[2] = 3.0f;
    gl_TessLevelInner[0] = 3.0f;
}

TESSELLATION EVALUATE SHADER

#version 440 core

layout (triangles, equal_spacing, ccw) in;
in vec2 EsPos[];
vec2 finalpos;

vec2 interpolate2D(vec2 v0, vec2 v1);

void main()
{
    finalpos = interpolate2D(EsPos[0],EsPos[1]);
    gl_Position = vec4(finalpos,0.0f,1.0f);
}
vec2 interpolate2D(vec2 v0, vec2 v1)
{
    return (vec2(gl_TessCoord.x)*v0 + vec2(gl_TessCoord.y)*v1);
}

FRAGMENT SHADER

#version 440 core

out vec4 Fragment;

void main()
{
    Fragment = vec4(0.0f,1.0f,1.0f,1.0f);
}

EDIT:

I made changes in the interpolate2D function, but still I am getting a blank screen.

Answer 1

The output patch size of the Tessellation Evaluation shader is 3:

layout (vertices = 3) out;

Thus the length of the input array to the Tessellation Control Shader is 3, too. Furthermore, the abstract patch type is triangles,

layout (triangles, equal_spacing, ccw) in;

thus the tessellation coordinate (gl_TessCoord) is a Barycentric coordinate. Change the interpolation:

vec2 interpolate2D(vec2 v0, vec2 v1, vec2 v2)
{
    return v0*gl_TessCoord.x + v1*gl_TessCoord.y + v2*gl_TessCoord.z;
}

void main()
{
    finalpos = interpolate2D(EsPos[0], EsPos[1], EsPos[2]);

    // [...]
}