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Thread: transform feedback does NOT work in Qt

  1. #1
    Join Date
    Jul 2020
    Posts
    3
    Qt products
    Qt5
    Platforms
    Unix/X11

    Default transform feedback does NOT work in Qt

    I am trying to set up transform feedback on my Qt OpenGL code.

    Qt with OpenGL code:

    GLfloat vPosition[24] = {
    -0.75, -0.25, -0.7, 1.0 ,
    -0.25, -0.25, -0.7, 1.0 ,
    0.25, 0.25, -0.7, 1.0 ,
    0.75, -0.5, -0.8, 1.0 ,
    0.0, -0.5, -0.8, 1.0 ,
    0.75, 0.5, -0.8, 1.0
    };

    GLfloat vColor[24] = {
    1.0, 0, 0, 1.0 ,
    0, 1.0, 0, 1.0 ,
    0, 0, 1.0, 1.0 ,
    1.0, 1.0, 0, 1.0 ,
    0, 1.0, 1.0, 1.0 ,
    1.0, 0, 1.0, 1.0
    };

    GLfloat vMatrix[16] = {
    1.0, 0, 0, 0,
    0, 1.0, 0, 0,
    0, 0, 1.0, 0,
    0, 0, 0, 1.0
    };

    void feedback::initialize() {

    initializeShader();

    glEnable(GL_DEPTH_TEST);
    GLint matrix = glGetUniformLocation(program, "vMatrix");
    glUniformMatrix4fv(matrix, 1, GL_TRUE, vMatrix);//second parameter indicates the number of matrices

    cf->glGenVertexArrays(2, VAOs);

    GLuint vboHandles[2];
    glGenBuffers(2, vboHandles);
    GLuint positionBufferHandle = vboHandles[0];
    GLuint colorBufferHandle = vboHandles[1];

    glBindBuffer(GL_ARRAY_BUFFER, positionBufferHandle);
    glBufferData(GL_ARRAY_BUFFER, 24 * sizeof(GLfloat), vPosition, GL_STATIC_DRAW);

    glBindBuffer(GL_ARRAY_BUFFER, colorBufferHandle);
    glBufferData(GL_ARRAY_BUFFER, 24 * sizeof(GLfloat), vColor, GL_STATIC_DRAW);

    cf->glBindVertexArray(VAOs[0]);

    glEnableVertexAttribArray(0); // Vertex position,
    glEnableVertexAttribArray(1);

    glBindBuffer(GL_ARRAY_BUFFER, positionBufferHandle);
    glVertexAttribPointer( 0, 4, GL_FLOAT, GL_FALSE, 0, (GLubyte *)NULL );

    glBindBuffer(GL_ARRAY_BUFFER, colorBufferHandle);
    glVertexAttribPointer( 1, 4, GL_FLOAT, GL_FALSE, 0, (GLubyte *)NULL );

    // glVertexAttribFormat(position, 4, GL_FLOAT, GL_FALSE, 0);
    // glVertexAttribBinding(position, 0);
    // glVertexAttribFormat(color, 4, GL_FLOAT, GL_FALSE, 0);
    // glVertexAttribBinding(color, 0);
    cf->glBindVertexArray(0);

    cf->glBindVertexArray(VAOs[1]);

    cf->glVertexAttribFormat(0, 4, GL_FLOAT, GL_FALSE, 0);
    cf->glVertexAttribBinding(0, 0);
    cf->glVertexAttribFormat(1, 4, GL_FLOAT, GL_FALSE, 4 * sizeof(GL_FLOAT));
    cf->glVertexAttribBinding(1, 0);
    glEnableVertexAttribArray(0);
    glEnableVertexAttribArray(1);
    cf->glBindVertexArray(0);

    glGenBuffers(2, VBOs);

    glBindBuffer(GL_ARRAY_BUFFER, VBOs[0]);
    glBufferData(GL_ARRAY_BUFFER, sizeof(vPosition) + sizeof(vColor), 0, GL_STATIC_DRAW);
    glBufferSubData(GL_ARRAY_BUFFER, 0, sizeof(vPosition), vPosition);
    glBufferSubData(GL_ARRAY_BUFFER, sizeof(vPosition), sizeof(vColor), vColor);
    glBindBuffer(GL_ARRAY_BUFFER, 0);

    glBindBuffer(GL_ARRAY_BUFFER, VBOs[1]);
    //glBufferStorage(GL_ARRAY_BUFFER, sizeof(vPosition) + sizeof(vColor), 0, 0);
    glBufferData(GL_ARRAY_BUFFER, sizeof(vPosition) + sizeof(vColor), 0, GL_STREAM_READ);
    glBindBuffer(GL_ARRAY_BUFFER, 0);

    cf->glGenTransformFeedbacks(1, TFBs);

    cf->glBindTransformFeedback(GL_TRANSFORM_FEEDBACK, TFBs[0]);
    cf->glBindBufferBase(GL_TRANSFORM_FEEDBACK_BUFFER, 0, VBOs[1]);
    cf->glBindTransformFeedback(GL_TRANSFORM_FEEDBACK, 0);

    glPolygonMode(GL_FRONT_AND_BACK, GL_FILL);
    }

    void feedback::render() {
    m_context->makeCurrent( this );
    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
    glViewport(0, 0, this->width() * devicePixelRatio(), this->height() * devicePixelRatio());

    float q[48];
    glBindBuffer(GL_ARRAY_BUFFER, VBOs[0]);
    cf->glGetBufferSubData(GL_ARRAY_BUFFER, 0, sizeof(vPosition) + sizeof(vColor), q);

    glEnable(GL_RASTERIZER_DISCARD);
    cf->glBindVertexArray(VAOs[0]);
    cf->glBindVertexBuffer(0, VBOs[0], 0, 4 * sizeof(GL_FLOAT));
    cf->glBindTransformFeedback(GL_TRANSFORM_FEEDBACK, TFBs[0]);
    cf->glBeginTransformFeedback(GL_TRIANGLES);
    glDrawArrays(GL_TRIANGLES, 0, 3);
    glDrawArrays(GL_TRIANGLES, 3, 3);
    cf->glEndTransformFeedback();
    cf->glBindTransformFeedback(GL_TRANSFORM_FEEDBACK, 0);
    cf->glBindVertexArray(0);

    float p[48];
    glBindBuffer(GL_ARRAY_BUFFER, VBOs[1]);
    cf->glGetBufferSubData(GL_ARRAY_BUFFER, 0, sizeof(vPosition) + sizeof(vColor), p);

    // print out the content
    for (int i = 0; i < 48; i++)
    printf("%f\n", p[i]);
    printf("\n");

    glDisable(GL_RASTERIZER_DISCARD);
    cf->glBindVertexArray(VAOs[1]);
    cf->glBindVertexBuffer(0, VBOs[1], 0, 8 * sizeof(GL_FLOAT));
    cf->glDrawTransformFeedback(GL_TRIANGLES, TFBs[0]);
    cf->glBindVertexArray(0);

    m_context->swapBuffers( this );
    }

    This is my Qt code. It is copied almost exactly line by line from a GLEW/GLUT based openGL program that works as expected: two triangles. However, in Qt it does NOT: I only see a black screen. in my code $cf->$ means QOpenGLFunctions_4_5_Core*. When I print out the content obtained from the feedback buffer ("// print out content " in render function) it differs from the same content printed out in the original code using GLEW/GLUT.

    I have no real clue why the same code imported to Qt and using QWindow does not work. I guess somehow the creation and display of QWindow object interfere with feedback transformation. BTW, I had similar experience when I reimplement FramebufferObjects in Qt.
    Last edited by zixuan; 25th July 2020 at 18:24.

  2. #2
    Join Date
    Jul 2020
    Posts
    3
    Qt products
    Qt5
    Platforms
    Unix/X11

    Default Re: transform feedback does NOT work in Qt

    I found out this out:

    The InitializeShaders (which I didn't show), in it I initialized program as an local variable, But program is a global variable.

    Silly mistake.

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