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Thread: Tessellation vertices/triangles count

  1. #1
    Newbie Newbie
    Join Date
    Aug 2013

    Tessellation vertices/triangles count

    Hello folks,

    i've got the following problem:

    I actually working on my bachelorthesis. My title is "Displacement Mapping with Tessellation".

    I am almost finished, but i need the numbers of triangles/vertices to do some comparisons.

    I am working with the JMonkey3-Engine(JME3) and OpenGL under windows. JME3 provides a statsView, the problem there is that it only gives you the number of your object you created.
    For example: I create a triangle so I have 3 Vertices and 1 Triangle, now I start my programm and raise the TessellationLevel, but the counts are not increasing because the Tessellation is in the GPU.
    Now there is my question, how can I get the numbers?

    Here are my Shader Stages (a simple one, without DisplacemetMapping):

    Code :
    uniform mat4 g_WorldMatrix;
    in vec3 inPosition;
    out vec3 vPosition;
    void main() {
    	vec4 position = g_WorldMatrix * vec4(inPosition, 1.0);
    	position /= position.w;
    	vPosition =;		
    layout(vertices = 3) out;
    in vec3 vPosition[];
    out vec3 tcPosition[];
    uniform float m_InnerTess;
    uniform float m_OuterTess;
       void main()
            tcPosition[gl_InvocationID] = vPosition[gl_InvocationID];
        if (gl_InvocationID == 0) {
            gl_TessLevelInner[0] = m_InnerTess;
            gl_TessLevelOuter[0] = m_OuterTess;
            gl_TessLevelOuter[1] = m_OuterTess;
            gl_TessLevelOuter[2] = m_OuterTess;     
    layout(triangles, equal_spacing, ccw) in;
    in vec3 tcPosition[];
    out vec3 tePosition;
    uniform mat4 g_ViewProjectionMatrix;
    vec3 interpolate3D(vec3 v0, vec3 v1, vec3 v2)
       	 return vec3(gl_TessCoord.x) * v0 + vec3(gl_TessCoord.y) * v1 + vec3(gl_TessCoord.z) * v2;
    void main()
       	 tePosition = interpolate3D(tcPosition[0], tcPosition[1], tcPosition[2]);	
    	gl_Position = g_ViewProjectionMatrix * vec4(tePosition, 1.0);
    out vec4 outFragColor;
    void main() {
            vec4 color = vec4(1.0);
            outFragColor = color;       

    That is my code for a single triangle to tessellate, by giving him the number of the tessellationlevel.
    I guess i have to look at the TES because he gives me the vertices to draw.

    If you have any more question, feel free to ask, I hope you guys can help me.

  2. #2
    Advanced Member Frequent Contributor
    Join Date
    Apr 2010
    Hmm, I'm not terribly familiar with tessellation, but isn't the number of triangles produced a function of (number of input triangles, m_InnerTess, m_OuterTess), in other words can't you calculate the information you need if you have the former three values? Alternatively you could use an atomic counter to count triangles/vertices.

  3. #3
    Advanced Member Frequent Contributor arekkusu's Avatar
    Join Date
    Nov 2003
    GL can tell you how many triangles were generated. Read up about the GL_PRIMITIVES_GENERATED query.

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