C++ OpenGL Transform Feedback schreibt keine Daten

glUseProgram(m_constructionProgram);

GLuint query;
glGenQueries(1, &query);

// Perform transform feedback
glEnable(GL_RASTERIZER_DISCARD);

glBindBuffer(GL_ARRAY_BUFFER, m_vbo[0]);
glBindBufferBase(GL_TRANSFORM_FEEDBACK_BUFFER, 0, m_vbo[1]);

glBeginQuery(GL_TRANSFORM_FEEDBACK_PRIMITIVES_WRITTEN, query);
glBeginTransformFeedback(GL_TRIANGLES); //writes to transform_feedback_buffer
glDrawArrays(GL_TRIANGLES, 0, m_vertexCount); //takes input from array buffer
glEndTransformFeedback();
glEndQuery(GL_TRANSFORM_FEEDBACK_PRIMITIVES_WRITTEN);

//swap vbo and tbo (vertex_buffer gets generated data from feedback_buffer)
std::swap(m_vbo[0], m_vbo[1]);

// Fetch and print results
GLuint primitiveCount;
glGetQueryObjectuiv(query, GL_QUERY_RESULT, &primitiveCount);

m_vertexCount = primitiveCount * 3;

glBindBufferBase(GL_TRANSFORM_FEEDBACK_BUFFER, 0, 0); //unbind
glDisable(GL_RASTERIZER_DISCARD);

in vec3 constrPosition;
in vec3 constrNormal;
in float constrLength;

out vec3 vNormal;
out float vLength;

void main() 
{
	gl_Position = vec4(constrPosition, 1.0);
	vNormal = constrNormal;
	vLength = constrLength;
}

#version 430

layout(triangles) in;
layout(triangle_strip, max_vertices = 1024) out;

in vec3 vNormal[];
in float vLength[];

out vec3 outPos;
out vec3 outNormal;
out float outLength;

void main()
{
	for (int i = 0; i < gl_in.length(); i++)
	{
              ...
              EmitVertex();
              ...
        }
}

m_constrPosAttrib = glGetAttribLocation(m_constructionProgram, "constrPosition");
glVertexAttribPointer(m_constrPosAttrib, 3, GL_FLOAT, GL_FALSE, 7 * sizeof(float), 0);
glEnableVertexAttribArray(m_constrPosAttrib);

m_constrNormalAttrib = glGetAttribLocation(m_constructionProgram, "constrNormal");
glVertexAttribPointer(m_constrNormalAttrib, 3, GL_FLOAT, GL_FALSE, 7 * sizeof(float), (void*)(3 * sizeof(float)));
glEnableVertexAttribArray(m_constrNormalAttrib);

m_constrLengthAttrib = glGetAttribLocation(m_constructionProgram, "constrLength");
glVertexAttribPointer(m_constrLengthAttrib, 1, GL_FLOAT, GL_FALSE, 7 * sizeof(float), (void*)(6 * sizeof(float)));
glEnableVertexAttribArray(m_constrLengthAttrib);

const GLchar* feedbackVaryings[] = { "outPos", "outNormal", "outLength" };
glTransformFeedbackVaryings(m_constructionProgram, 3, feedbackVaryings, GL_INTERLEAVED_ATTRIBS);

glGenBuffers(2, m_vbo);

// render buffer
glBindBuffer(GL_ARRAY_BUFFER, m_vbo[0]);
glBufferData(GL_ARRAY_BUFFER, 1024 * 1024, nullptr, GL_STATIC_DRAW); 
glBufferSubData(GL_ARRAY_BUFFER, 0, m_vertexCount * 7 * sizeof(GLfloat), vertices);
glBindBuffer(GL_ARRAY_BUFFER, 0); //unbind

//construction buffer (transform feedback)
glBindBuffer(GL_ARRAY_BUFFER, m_vbo[1]);
glBufferData(GL_ARRAY_BUFFER, 1024 * 1024, nullptr, GL_STATIC_DRAW); 
glBindBuffer(GL_ARRAY_BUFFER, 0); //unbind