OpenGL Transform Feedback not returning anything

Question

I am working on a Project to create Geometry by using a Geometry Shader and Transform Feedback.

I am currently trying to have the Geometry Shader return its input (1 triangle) without changing/adding anything, but its not working.

I would appreciate any help/advice I can get. Here are parts of my code:

The creation of the Program for Transform Feedback:

//==========================================
// Create the Transform Program
//==========================================

int check = LoadShader("Shaders//transformVS.glsl", GL_VERTEX_SHADER, transformVS);

//TODO: check for fail

check = LoadShader("Shaders//transformGS.glsl", GL_GEOMETRY_SHADER, transformGS);

//TODO: check for fail

transformProgram = glCreateProgram();

glAttachShader(transformProgram, transformVS);
glAttachShader(transformProgram, transformGS);

glBindAttribLocation(transformProgram, 0, "position_in");
glBindAttribLocation(transformProgram, 1, "normal_in");
glBindAttribLocation(transformProgram, 2, "length_in");

static const char* varyings[] = { "position_out", "normal_out", "length_out" };
glTransformFeedbackVaryings(transformProgram, 3, varyings, GL_INTERLEAVED_ATTRIBS);

glLinkProgram(transformProgram);

if (CheckProgram(transformProgram) == -1){
    glDetachShader(transformProgram, transformVS);
    glDetachShader(transformProgram, transformGS);
    glDeleteShader(transformVS);
    transformVS = 0;
    glDeleteShader(transformGS);
    transformGS = 0;
    glDeleteProgram(transformProgram);
    transformProgram = 0;

    return -1;
}

error = glGetError();

The Creation of the VBOs, VAOs and TFOs:

//=====================================
// Create VBOs
//=====================================

glGenBuffers(2, VBOID);

glBindBuffer(GL_ARRAY_BUFFER, VBOID[0]);
glBufferData(GL_ARRAY_BUFFER, 3 * sizeof(TVertex_VNL), vertices, GL_DYNAMIC_COPY);
glBindBuffer(GL_ARRAY_BUFFER, 0);

error = glGetError();

glBindBuffer(GL_ARRAY_BUFFER, VBOID[1]);
glBufferData(GL_ARRAY_BUFFER, 3 * sizeof(TVertex_VNL), NULL, GL_DYNAMIC_COPY);
glBindBuffer(GL_ARRAY_BUFFER, 0);

error = glGetError();

//=====================================
// Create VAOs
//=====================================

glGenVertexArrays(2, VAOID);

glBindVertexArray(VAOID[0]);
    glBindBuffer(GL_ARRAY_BUFFER, VBOID[0]);
    glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, sizeof(TVertex_VNL), BUFFER_OFFSET(0)); //position
    glVertexAttribPointer(1, 3, GL_FLOAT, GL_FALSE, sizeof(TVertex_VNL), BUFFER_OFFSET(sizeof(float) * 3)); //normal
    glVertexAttribPointer(2, 1, GL_FLOAT, GL_FALSE, sizeof(TVertex_VNL), BUFFER_OFFSET(sizeof(float) * 6)); //length
    glBindBuffer(GL_ARRAY_BUFFER, 0);

    glEnableVertexAttribArray(0);
    glEnableVertexAttribArray(1);
    glEnableVertexAttribArray(2);
glBindVertexArray(0);

error = glGetError();

glBindVertexArray(VAOID[1]);
    glBindBuffer(GL_ARRAY_BUFFER, VBOID[1]);
    glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, sizeof(TVertex_VNL), BUFFER_OFFSET(0)); //position
    glVertexAttribPointer(1, 3, GL_FLOAT, GL_FALSE, sizeof(TVertex_VNL), BUFFER_OFFSET(sizeof(float) * 3)); //normal
    glVertexAttribPointer(2, 1, GL_FLOAT, GL_FALSE, sizeof(TVertex_VNL), BUFFER_OFFSET(sizeof(float) * 6)); //length
    glBindBuffer(GL_ARRAY_BUFFER, 0);

    glEnableVertexAttribArray(0);
    glEnableVertexAttribArray(1);
    glEnableVertexAttribArray(2);
glBindVertexArray(0);

error = glGetError();



//=====================================
// Create TFOs
//=====================================

glGenTransformFeedbacks(2, TFOID);

glBindTransformFeedback(GL_TRANSFORM_FEEDBACK, TFOID[0]);
glBindBufferBase(GL_TRANSFORM_FEEDBACK_BUFFER, 0, VBOID[0]);
glBindTransformFeedback(GL_TRANSFORM_FEEDBACK, 0);

error = glGetError();

glBindTransformFeedback(GL_TRANSFORM_FEEDBACK, TFOID[1]);
glBindBufferBase(GL_TRANSFORM_FEEDBACK_BUFFER, 0, VBOID[1]);
glBindTransformFeedback(GL_TRANSFORM_FEEDBACK, 0);

error = glGetError();

The render Method:

//=========================================
// Clear Screen
//=========================================

//Clear all the buffers
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT | GL_STENCIL_BUFFER_BIT);

//========================================
// Transform Feedback
//========================================

glEnable(GL_RASTERIZER_DISCARD);

glUseProgram(transformProgram);

glBindVertexArray(VAOID[0]);
glBindTransformFeedback(GL_TRANSFORM_FEEDBACK, TFOID[1]);

glBeginTransformFeedback(GL_TRIANGLES);

glDrawArrays(GL_TRIANGLES, 0, 3);

glEndTransformFeedback();

glBindTransformFeedback(GL_TRANSFORM_FEEDBACK, 0);

glDisable(GL_RASTERIZER_DISCARD);

//========================================
// Draw Triangle
//========================================

//Bind the shader that we want to use
glUseProgram(renderProgram);

//Setup all uniforms for your shader
glUniformMatrix4fv(renderMVP, 1, FALSE, &MVP[0][0]);

//Bind the VAO
glBindVertexArray(VAOID[1]);
glDrawArrays(GL_TRIANGLES, 0, 3);

//glDrawTransformFeedback(GL_TRIANGLES, TFOID[1]);

//========================================
// Swap Buffers
//========================================

glutSwapBuffers();

The Vertex Shader:

#version 330

in vec3 position_in;
in vec3 normal_in;
in float length_in;

out vec3 vs_position;
out vec3 vs_normal;
out float vs_length;

void main()
{
    vs_position = position_in;
    vs_normal = normal_in;
    vs_length = length_in;
}

The Geometry Shader:

#version 330

layout(triangles) in;
layout(triangle_strip, max_vertices = 3) out;

in vec3 vs_position[];
in vec3 vs_normal[];
in float vs_length[];

out vec3 position_out;
out vec3 normal_out;
out float length_out;

void main()
{
    for(int i = 0; i < 3; i++){
        position_out = vs_position[i];
        normal_out = vs_normal[i];
        length_out = vs_length[i];

        EmitVertex();
    }

    EndPrimitive();
}

during intialization, you create two TFOs, buit initialize the first one twice. During rendering, you only use the second. — derhass
@derhass thanks again for the answer. I fixed that one too but its still not working. Could there be something missing in the TFO initialisation, like telling opengl were to write the different output variables? — Omanidos
Since you use interleaved output, all outputs will be written to the same buffer (unless you use separators, but that is a completely differen thing which should not be relevant here). I'm not sure about not using a fragment shader at all. Since you use RASTERIZER_DISACRD, it probably shouldn't be necessary, but I haven't looked that up in the spec. I think it can't hurt to add some error checks to the render method, though. — derhass

derhass derhass · Accepted Answer · 2015-08-17T12:46:46

Your geometry shader is not emitting any vertices because the for loop body is never entered:

for(int i = 0; i >= 3; i++){

OpenGL Transform Feedback not returning anything

1 Answers