qt opengl coloring cube example-vbo edited

**saman_artorious** · 30th June 2013, 12:12

To write a vbo sample I edited the qt opengl coloring cube example. What I did is to create a vao and two vbos for vertices and colors variables. Unfortunately, when program runs, it doesn't render anything. Attached file is the program, as I am only using Linux, you may not worry about any virus detections.
I would appreciate if you run this example on your system and try if you can make it give the desired output.

**wysota** · 30th June 2013, 15:58

You are not linking the buffers to the shader program you are using anywhere. Are you sure this is just a modified version of an example?

**saman_artorious** · 30th June 2013, 17:52

Originally Posted by wysota

You are not linking the buffers to the shader program you are using anywhere.

I changed the coloring example and added vbo to it. I do not understand your statement. I already linked vertices and colors variables to vertex and color respectively in the shader program. I do not understand where I am doing wrong.
could get more into the details please.

**wysota** · 30th June 2013, 18:02

Originally Posted by saman_artorious

I already linked vertices and colors variables to vertex and color respectively in the shader program.

Where exactly did you do that? Which file, which line?

**saman_artorious** · 30th June 2013, 18:14

Please check the file

glwidget.cpp

.
The linking of shader programs is happening inside

initializeGL()

Rendering and initialization(if not initialized)is done in the

paintGL()

function.

Qt Code:

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if(!initialized)
    {
        GLuint vertexId = shaderProgram.attributeLocation("vertex");
 
        GLuint colorId  = shaderProgram.attributeLocation("color");
 
        glGenVertexArrays(1, &VAO_ID);
        glBindVertexArray(VAO_ID);
 
        glBindBuffer(GL_ARRAY_BUFFER, VBO_ID[0]);
        glVertexAttribPointer(vertexId, 3, GL_FLOAT, GL_FALSE, sizeof(vertices), 0);
        glBufferData(GL_ARRAY_BUFFER, sizeof(QVector3D)*vertices.size(), vertices.constData(), GL_STATIC_DRAW);
 
        glBindBuffer(GL_ARRAY_BUFFER, VBO_ID[1]);
        glVertexAttribPointer(colorId, 3, GL_FLOAT, GL_FALSE, sizeof(colors), 0);
        glBufferData(GL_ARRAY_BUFFER, sizeof(QVector3D)*colors.size(), colors.constData(), GL_STATIC_DRAW);
 
        initialized = true;
    }

if(!initialized)
    {
        GLuint vertexId = shaderProgram.attributeLocation("vertex");

        GLuint colorId  = shaderProgram.attributeLocation("color");

        glGenVertexArrays(1, &VAO_ID);
        glBindVertexArray(VAO_ID);

        glBindBuffer(GL_ARRAY_BUFFER, VBO_ID[0]);
        glVertexAttribPointer(vertexId, 3, GL_FLOAT, GL_FALSE, sizeof(vertices), 0);
        glBufferData(GL_ARRAY_BUFFER, sizeof(QVector3D)*vertices.size(), vertices.constData(), GL_STATIC_DRAW);

        glBindBuffer(GL_ARRAY_BUFFER, VBO_ID[1]);
        glVertexAttribPointer(colorId, 3, GL_FLOAT, GL_FALSE, sizeof(colors), 0);
        glBufferData(GL_ARRAY_BUFFER, sizeof(QVector3D)*colors.size(), colors.constData(), GL_STATIC_DRAW);

        initialized = true;
    }

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**wysota** · 30th June 2013, 22:13

Again, which line in your opinion binds the VBO to the shader? Because none of what you posted in this post does. This code only fills the buffer.

**saman_artorious** · 1st July 2013, 09:45

Originally Posted by wysota

Again, which line in your opinion binds the VBO to the shader? Because none of what you posted in this post does. This code only fills the buffer.

Here:

Qt Code:

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int vertexLocation = shaderProgram->attributeLocation("vertex");
        shaderProgram->enableAttributeArray(vertexLocation);
        int colorLocation = shaderProgram->attributeLocation("color");
        shaderProgram->enableAttributeArray(colorLocation);

int vertexLocation = shaderProgram->attributeLocation("vertex");
        shaderProgram->enableAttributeArray(vertexLocation);
        int colorLocation = shaderProgram->attributeLocation("color");
        shaderProgram->enableAttributeArray(colorLocation);

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**wysota** · 1st July 2013, 10:03

I think you forgot to read the docs for "enableAttributeArray". This only tells the shader to expect an array instead of a single value. It does not tell it what data to assign to this array. I already told you in the other thread a couple of days ago which function you need to use. Now RTFM for enableAttributeArray and read the other thread again.

**saman_artorious** · 1st July 2013, 12:20

I added the two lines concerning setAttributeBuffer() to my code. However, it gives me segmentation fault when after glDrawArray() is called. Though it executes fine with

Qt Code:

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glDrawElements(GL_TRIANGLES, vertices.size(), GL_FLOAT, vertices.constData());

glDrawElements(GL_TRIANGLES, vertices.size(), GL_FLOAT, vertices.constData());

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but does not render anything on the screen.

Qt Code:

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shaderProgram->setAttributeBuffer(vertexLocation, GL_FLOAT , 0, 3, 0);
    shaderProgram->setAttributeBuffer(colorLocation , GL_FLOAT , 0, 1, 0);

shaderProgram->setAttributeBuffer(vertexLocation, GL_FLOAT , 0, 3, 0);
    shaderProgram->setAttributeBuffer(colorLocation , GL_FLOAT , 0, 1, 0);

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whole render part:

Qt Code:

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if(!initialized)
    {
        GLuint vertexId = shaderProgram->attributeLocation("vertex");
 
        GLuint colorId  = shaderProgram->attributeLocation("color");
 
        glGenBuffers(2, VBO_ID);
        glGenVertexArrays(1, &VAO_ID);
        glBindVertexArray(VAO_ID);
 
        glBindBuffer(GL_ARRAY_BUFFER, VBO_ID[0]);
        glBufferData(GL_ARRAY_BUFFER, sizeof(QVector3D)*vertices.size(), vertices.constData(), GL_STATIC_DRAW);
        glVertexAttribPointer(vertexId, 3, GL_FLOAT, GL_FALSE, 0, 0);// param - 1 ->0
        glEnableVertexAttribArray(vertexId);
 
        glBindBuffer(GL_ARRAY_BUFFER, VBO_ID[1]);
        glBufferData(GL_ARRAY_BUFFER, sizeof(float)*vertices.size(), colors, GL_DYNAMIC_DRAW);
        glVertexAttribPointer(colorId, 1, GL_FLOAT, GL_FALSE, 0, 0);
        glEnableVertexAttribArray(colorId);
 
        glBindBuffer(GL_ARRAY_BUFFER, 0);
    }
 
    glBindVertexArray(VAO_ID);
 
    int vertexLocation = shaderProgram->attributeLocation("vertex");
    int colorLocation = shaderProgram->attributeLocation("color");
 
    shaderProgram->enableAttributeArray(vertexLocation);
    shaderProgram->enableAttributeArray(colorLocation);
 
    shaderProgram->setAttributeBuffer(vertexLocation, GL_FLOAT , 0, 3, 0);
    shaderProgram->setAttributeBuffer(colorLocation , GL_FLOAT , 0, 1, 0);
 
    glEnable    (GL_BLEND);
    glBlendFunc (GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
    glDrawArrays(GL_TRIANGLES, 0, numVerts);

if(!initialized)
    {
        GLuint vertexId = shaderProgram->attributeLocation("vertex");
        
        GLuint colorId  = shaderProgram->attributeLocation("color");
        
        glGenBuffers(2, VBO_ID);
        glGenVertexArrays(1, &VAO_ID);
        glBindVertexArray(VAO_ID);
        
        glBindBuffer(GL_ARRAY_BUFFER, VBO_ID[0]);
        glBufferData(GL_ARRAY_BUFFER, sizeof(QVector3D)*vertices.size(), vertices.constData(), GL_STATIC_DRAW);
        glVertexAttribPointer(vertexId, 3, GL_FLOAT, GL_FALSE, 0, 0);// param - 1 ->0
        glEnableVertexAttribArray(vertexId);
        
        glBindBuffer(GL_ARRAY_BUFFER, VBO_ID[1]);
        glBufferData(GL_ARRAY_BUFFER, sizeof(float)*vertices.size(), colors, GL_DYNAMIC_DRAW);
        glVertexAttribPointer(colorId, 1, GL_FLOAT, GL_FALSE, 0, 0);
        glEnableVertexAttribArray(colorId);
        
        glBindBuffer(GL_ARRAY_BUFFER, 0);
    }
    
    glBindVertexArray(VAO_ID);
    
    int vertexLocation = shaderProgram->attributeLocation("vertex");
    int colorLocation = shaderProgram->attributeLocation("color");
    
    shaderProgram->enableAttributeArray(vertexLocation);
    shaderProgram->enableAttributeArray(colorLocation);
    
    shaderProgram->setAttributeBuffer(vertexLocation, GL_FLOAT , 0, 3, 0);
    shaderProgram->setAttributeBuffer(colorLocation , GL_FLOAT , 0, 1, 0);
    
    glEnable    (GL_BLEND);
    glBlendFunc (GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
    glDrawArrays(GL_TRIANGLES, 0, numVerts);

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could you please run this code on your system, I don't know exactly what I am missing. Although, I added setArrtibuteBuffers.

**wysota** · 1st July 2013, 15:48

Make sure you are setting up the buffers correctly. I would also advise to use Qt classes for VBO manipulation.

**saman_artorious** · 4th July 2013, 10:49

Originally Posted by wysota

Make sure you are setting up the buffers correctly. I would also advise to use Qt classes for VBO manipulation.

Thanks for your guidance. I solved the problem. The correct code for what I aimed for is:

Qt Code:

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if(!initialized)
    {
        GLuint vertexId = shaderProgram->attributeLocation("vertex");
        GLuint colorId  = shaderProgram->attributeLocation("color");
 
        glGenBuffers(2, VBO_ID);
 
        glGenVertexArrays(1, &VAO_ID);
        glBindVertexArray(VAO_ID);
 
        glBindBuffer(GL_ARRAY_BUFFER, VBO_ID[0]);
        glBufferData(GL_ARRAY_BUFFER, sizeof(QVector3D)*verticesSize, vertices.constData(), GL_STATIC_DRAW);
        glVertexAttribPointer(vertexId, 3, GL_FLOAT, GL_FALSE, 0, 0);// param - 1 ->0
        glEnableVertexAttribArray(vertexId);
 
        glBindBuffer(GL_ARRAY_BUFFER, VBO_ID[1]);
        glBufferData(GL_ARRAY_BUFFER, sizeof(float)*verticesSize, colors, GL_STATIC_DRAW);
        glVertexAttribPointer(colorId, 1, GL_FLOAT, GL_FALSE, 0, 0);
        glEnableVertexAttribArray(colorId);
 
        glBindBuffer(GL_ARRAY_BUFFER, 0);
 
        vertices.clear();
 
        free(colors);
 
        initialized = true;
    }
 
    glBindVertexArray(VAO_ID);
 
    int vertexLocation = shaderProgram->attributeLocation("vertex");
    int colorLocation = shaderProgram->attributeLocation("color");
 
    shaderProgram->enableAttributeArray(vertexLocation);
    shaderProgram->enableAttributeArray(colorLocation);
 
    glEnableClientState(GL_VERTEX_ARRAY);
 
    glEnable    (GL_BLEND);
    glBlendFunc (GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
    glDrawArrays(GL_TRIANGLES, 0, 6*numVerts);
 
    glFlush();
 
    glBindVertexArray(0);
 
    glDisableVertexAttribArray(0);
    glDisableVertexAttribArray(1);

if(!initialized)
    {
        GLuint vertexId = shaderProgram->attributeLocation("vertex");
        GLuint colorId  = shaderProgram->attributeLocation("color");

        glGenBuffers(2, VBO_ID);

        glGenVertexArrays(1, &VAO_ID);
        glBindVertexArray(VAO_ID);

        glBindBuffer(GL_ARRAY_BUFFER, VBO_ID[0]);
        glBufferData(GL_ARRAY_BUFFER, sizeof(QVector3D)*verticesSize, vertices.constData(), GL_STATIC_DRAW);
        glVertexAttribPointer(vertexId, 3, GL_FLOAT, GL_FALSE, 0, 0);// param - 1 ->0
        glEnableVertexAttribArray(vertexId);

        glBindBuffer(GL_ARRAY_BUFFER, VBO_ID[1]);
        glBufferData(GL_ARRAY_BUFFER, sizeof(float)*verticesSize, colors, GL_STATIC_DRAW);
        glVertexAttribPointer(colorId, 1, GL_FLOAT, GL_FALSE, 0, 0);
        glEnableVertexAttribArray(colorId);

        glBindBuffer(GL_ARRAY_BUFFER, 0);

        vertices.clear();

        free(colors);

        initialized = true;
    }

    glBindVertexArray(VAO_ID);

    int vertexLocation = shaderProgram->attributeLocation("vertex");
    int colorLocation = shaderProgram->attributeLocation("color");

    shaderProgram->enableAttributeArray(vertexLocation);
    shaderProgram->enableAttributeArray(colorLocation);

    glEnableClientState(GL_VERTEX_ARRAY);

    glEnable    (GL_BLEND);
    glBlendFunc (GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
    glDrawArrays(GL_TRIANGLES, 0, 6*numVerts);

    glFlush();

    glBindVertexArray(0);

    glDisableVertexAttribArray(0);
    glDisableVertexAttribArray(1);

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