QGLBuffer VBO implementation

[saman_artorious]

I want to transfer vertices & colors variables in my drawing GPU. Unfortunately, it cannot render when the program runs.

definitions is as follow:

QVector<QVector3D> vertices;
    float* colors;
 
    QGLBuffer* m_bufferData;
    QGLBuffer* m_colorData;
    float*     m_color;

QVector<QVector3D> vertices;
    float* colors;
    
    QGLBuffer* m_bufferData;
    QGLBuffer* m_colorData;
    float*     m_color;

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Here is what I do iniside initializeGL(), As vertices is built once and used forever, I do not allocate and map it. However, colors change regularly in the code, so I mapped colors to m_color in the code.

void PlanPositionIndicator::QtVBO()
{
    m_bufferData = new QGLBuffer(QGLBuffer::VertexBuffer);
    m_bufferData->create();
    m_bufferData->bind();
    m_bufferData->setUsagePattern(QGLBuffer::DynamicDraw);
    m_bufferData->allocate(6*sizeof(float)* ANGLE_COUNT*RANGE_COUNT);
    m_bufferData->release();
    //m_data = (QVector<QVector3D>*)m_bufferData->map (QGLBuffer::ReadWrite);
 
 
    m_colorData = new QGLBuffer(QGLBuffer::VertexBuffer);
    m_colorData->create();
    m_colorData->bind();
    m_colorData->setUsagePattern(QGLBuffer::DynamicDraw);
    m_colorData->allocate(6*sizeof(float)* ANGLE_COUNT*RANGE_COUNT);
    m_colorData->release();
    m_color = (float*)m_colorData->map(QGLBuffer::ReadWrite);
}

void PlanPositionIndicator::QtVBO()
{
    m_bufferData = new QGLBuffer(QGLBuffer::VertexBuffer);
    m_bufferData->create();
    m_bufferData->bind();
    m_bufferData->setUsagePattern(QGLBuffer::DynamicDraw);
    m_bufferData->allocate(6*sizeof(float)* ANGLE_COUNT*RANGE_COUNT);
    m_bufferData->release();
    //m_data = (QVector<QVector3D>*)m_bufferData->map (QGLBuffer::ReadWrite);


    m_colorData = new QGLBuffer(QGLBuffer::VertexBuffer);
    m_colorData->create();
    m_colorData->bind();
    m_colorData->setUsagePattern(QGLBuffer::DynamicDraw);
    m_colorData->allocate(6*sizeof(float)* ANGLE_COUNT*RANGE_COUNT);
    m_colorData->release();
    m_color = (float*)m_colorData->map(QGLBuffer::ReadWrite);
}

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So far so good, next, in the paintGL(), after setting up matrices and binging the shader program, I call the render function to render my drawing as following:

void PlanPositionIndicator::render(QGLShaderProgram* shaderProgram)
{
    glEnableClientState(GL_VERTEX_ARRAY);
    if (m_bufferData->bind ()) {
 
        m_bufferData->write(0,vertices.constData(),sizeof(vertices));
 
        glEnableClientState(GL_VERTEX_ARRAY);
        glVertexPointer( 3, GL_FLOAT, 0, &vertices);
 
        shaderProgram->setAttributeArray("vertex", vertices.constData());
        shaderProgram->enableAttributeArray("vertex");
        shaderProgram->setAttributeArray("color", GL_FLOAT,colors,1);
 
        shaderProgram->enableAttributeArray("color");
        glEnable    (GL_BLEND);
        glBlendFunc (GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
        glDrawArrays(GL_TRIANGLES, 0, vertices.size());
 
        glDisableClientState(GL_VERTEX_ARRAY);
        m_bufferData->release();
    }
}

void PlanPositionIndicator::render(QGLShaderProgram* shaderProgram)
{
    glEnableClientState(GL_VERTEX_ARRAY);
    if (m_bufferData->bind ()) {

        m_bufferData->write(0,vertices.constData(),sizeof(vertices));

        glEnableClientState(GL_VERTEX_ARRAY);
        glVertexPointer( 3, GL_FLOAT, 0, &vertices);

        shaderProgram->setAttributeArray("vertex", vertices.constData());
        shaderProgram->enableAttributeArray("vertex");
        shaderProgram->setAttributeArray("color", GL_FLOAT,colors,1);

        shaderProgram->enableAttributeArray("color");
        glEnable    (GL_BLEND);
        glBlendFunc (GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
        glDrawArrays(GL_TRIANGLES, 0, vertices.size());

        glDisableClientState(GL_VERTEX_ARRAY);
        m_bufferData->release();
    }
}

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Finally, I attempt to change values of colors array. Firstly I did it with map() & unmap() but it gave me segmentation fault. Next, I used write(), it runs well, but does not render anything.

for(int i = 0; i < RANGE_COUNT; ++i)
    {
        float rnd = ((rand() + segmentIndex) % 10000) / 10000.0f;
 
        QGLBuffer* glBuffer = ppi->Get_mColorData();
        float*     m_color  = ppi->Get_mColor();
 
//        m_color = (float*)glBuffer->map (QGLBuffer::ReadWrite);
 
        float a[5];
        glBuffer->write(0,m_color,100);
 
//        m_color[k++] = rnd;
//        m_color[k++] = rnd;
//        m_color[k++] = rnd;
//        m_color[k++] = rnd;
//        m_color[k++] = rnd;
//        m_color[k++] = rnd;
 
//        glBuffer->unmap ();
    }

for(int i = 0; i < RANGE_COUNT; ++i)
    {
        float rnd = ((rand() + segmentIndex) % 10000) / 10000.0f;

        QGLBuffer* glBuffer = ppi->Get_mColorData();
        float*     m_color  = ppi->Get_mColor();

//        m_color = (float*)glBuffer->map (QGLBuffer::ReadWrite);

        float a[5];
        glBuffer->write(0,m_color,100);

//        m_color[k++] = rnd;
//        m_color[k++] = rnd;
//        m_color[k++] = rnd;
//        m_color[k++] = rnd;
//        m_color[k++] = rnd;
//        m_color[k++] = rnd;

//        glBuffer->unmap ();
    }

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Any suggestions would be appreciable.

[saman_artorious]

I still do not understand why the allocated mapped m_color buffer above gives me a segmentation. I am dubious if my approach is correct. Has anyone done this before?

[wysota]

Your drawing code doesn't use the buffers anywhere.

[saman_artorious]

what you mean!?

[wysota]

I mean you create the buffers but don't use them anywhere. You are using the original QVector:

shaderProgram->setAttributeArray("vertex", vertices.constData());

shaderProgram->setAttributeArray("vertex", vertices.constData());

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[saman_artorious]

I mean you create the buffers but don't use them anywhere.

Here, I set up their relation:

m_bufferData->write(0,vertices.constData(),sizeof(vertices));

m_bufferData->write(0,vertices.constData(),sizeof(vertices));

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And pass the original variable in the function you mentioned above.

[wysota]

No, here you copy data from the vector to the buffer. But then you use the vector again instead of the buffer. You should use setAttributeBuffer and not setAttributeArray.