Full setup of Transform Feedback(openGL)

问题

GLSL 1.50, openGL 3.3.

I've been lately trying to get my tranform feedback working but without success. I still receive error after glBeginTranformFeedback() and as I haven't found any full working code I have stacked up my knowledge with some code that I found and documentation, it should be working well by now but I am missing something. So if anybody got full code (initializing of buffers, setting up, updating, rendering, reading back) it would definitelly help and if you don't but know what's going on you could take look at my code. I excluded some benchmarking, handling of windows and it's creation:

int main()
{
    bool fullsize = false, paused = false; 
    std::string caption = "Tester";

    GLuint dataVAO,speedUpdateVBO,dataVBO;
    std::vector<vector3f> dataW;

    // Create the main rendering window

    init(); //just some camera commands

    UniShader shader; //my shader class keeps everything together
    shader.init();
    shader.addShader("test.vert");
    shader.addShader("test.frag");
    shader.newAttributeVariable("speed");
    shader.newFeedbackVarying("sp");
    shader.linkShader();
    shader.use();

    //init some data
    dataW.push_back(vector3f(0,1,0));

    //creating VAO
    glGenVertexArrays(1,&dataVAO);
    glBindVertexArray(dataVAO);
    //creating VBO
    glGenBuffers(1,&dataVBO);
    glBindBuffer(GL_ARRAY_BUFFER,dataVBO);
    glBufferData(GL_ARRAY_BUFFER, sizeof(vector3f), 0, GL_DYNAMIC_DRAW);
    glVertexAttribPointer(shader.getAttributeIndex("speed"), 3, GL_FLOAT, GL_FALSE, 0, 0);

    glGenBuffers(1, &speedUpdateVBO);
    glBindBuffer(GL_TRANSFORM_FEEDBACK_BUFFER, speedUpdateVBO);
    glBufferData(GL_TRANSFORM_FEEDBACK_BUFFER, sizeof(vector3f), NULL, GL_DYNAMIC_COPY);
    glBindBufferBase(GL_TRANSFORM_FEEDBACK_BUFFER, 0, speedUpdateVBO); 
    glBindVertexArray(0);


    while (App.IsOpened())
    {
            App.SetActive();
        benchP = Clock.GetElapsedTime();

        //update calls
        if(!paused)
            //update
        benchU = Clock.GetElapsedTime();

        //render calls
        glClearColor(1.0f, 1.0f, 1.0f, 0.0f);
        glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
        glColor3f(0.6f,0.7f,0.7f);

    GLuint query;
    GLuint count = 0;

    glGenQueries(1, &query);

    glEnableVertexAttribArray(shader.getAttributeIndex("speed"));

    glBindVertexArray(dataVAO);

    glBindBuffer(GL_ARRAY_BUFFER,dataVBO);
    glBufferData(GL_ARRAY_BUFFER, sizeof(vector3f)*dataW.size(), &dataW[0], GL_DYNAMIC_DRAW);
    glBindBuffer(GL_TRANSFORM_FEEDBACK_BUFFER, speedUpdateVBO);
    glBufferData(GL_TRANSFORM_FEEDBACK_BUFFER, sizeof(vector3f)*dataW.size(), NULL, GL_DYNAMIC_COPY);

    glBindBufferBase(GL_TRANSFORM_FEEDBACK_BUFFER, 0, speedUpdateVBO); 
    glEnable(GL_RASTERIZER_DISCARD);
    glBeginQuery(GL_TRANSFORM_FEEDBACK_PRIMITIVES_WRITTEN, query);
    printOglError(); //Until this everything OK, I think
    glBeginTransformFeedback(GL_POINTS); 
    printOglError(); //This one prints out Invalid Value

    glDrawArrays(GL_POINTS,0,dataW.size());

    glEndTransformFeedback();
    glEndQuery(GL_TRANSFORM_FEEDBACK_PRIMITIVES_WRITTEN); 
    glDisable(GL_RASTERIZER_DISCARD);

    //retrieve updated data
    glGetQueryObjectuiv(query, GL_QUERY_RESULT, &count); //count is 0 -> nothing happend

    glBindVertexArray(0);
    glDisableVertexAttribArray(shader.getAttributeIndex("speed"));

    glDeleteQueries(1, &query);

    App.Display();
    //some other benchmark stuff
}

shaders: vert

#version 150 core
in vec3 speed;

varying vec3 sp;

const float gravity_constant = 9.81f;

void main(){
    sp = speed;
    sp += vec3(0,-gravity_constant,0);
}

frag

#version 150 core
varying vec3 sp;

void main (void)
{
    vec3 c = sp;
    gl_FragColor = vec4(c,1.0);
}

Fragment shader is there just for GLSL optimalization. If sp wouldn't be used GLSL would clear it up. There may be some minor bugs as I extracted this from much larger code with multiple varyings that fails aswell.

回答1:

Take a look at OpenGL Samples Pack. You might be interested in ogl-330-transform-feedback.cpp and ogl-400-transform-feedback-object.cpp. You might even check January 2011 OpenGL drivers status whether latest drivers have any issues with the provided examples.

Good luck.

回答2:

Daniel Rakos has a demo with source code, implementing instance culling using transform feedback on OpenGL. It might help to look at it :

http://rastergrid.com/blog/2010/02/instance-culling-using-geometry-shaders/

回答3:

You might want to check out Ogre 3D: http://www.ogre3d.org/forums/viewtopic.php?p=299736

They seem to have a working implementation of transform feedback.

回答4:

I also experienced a similar problem that glBeginTransformFeedback generated "invalid operation". After I read the source code in file "gl-440-transform-feedback.cpp" from the package provided by Orhun, I found that adding