Bare minimal QOpenGLFramebufferObject render to texture: Rendered triangle contains "noise"

axeljaeger17

Hello,
I am trying to make a minimal example that renders the famous red triangle to a texture. I tried this using the following code:

void GLWidget::initializeGL()
{
    initializeOpenGLFunctions();
    glClearColor(0.0f, 0.0f, 0.4f, 0.0f);
    m_vao = new QOpenGLVertexArrayObject(this);
    m_vao->create();
    m_vao->bind();
    static const GLfloat g_vertex_buffer_data[] = {
       -1.0f, -1.0f, 0.0f,
       1.0f, -1.0f, 0.0f,
       0.0f,  1.0f, 0.0f,
    };
    m_vertexBuffer.create();
    m_vertexBuffer.bind();
    m_vertexBuffer.allocate(g_vertex_buffer_data, sizeof(g_vertex_buffer_data));
    m_program = new QOpenGLShaderProgram(this);
    m_program->addShaderFromSourceFile(QOpenGLShader::Vertex, ":/vertexshader.glsl");
    m_program->addShaderFromSourceFile(QOpenGLShader::Fragment, ":/fragmentshader.glsl");
    m_program->link();
    m_program->enableAttributeArray(0);
    m_program->setAttributeBuffer(0, GL_FLOAT,0,3,0);
    m_framebuffer = new QOpenGLFramebufferObject(QSize(512,512), GL_TEXTURE_2D);
    m_filename = "C:/Users/Axel/Desktop/phased-array/trunk/testcases/build-rendertotexture-Desktop_Qt_5_8_0_MinGW_32bit2-Debug/test.png";
    glDisable(GL_DEPTH_TEST);
}

void GLWidget::paintGL()
{
    m_framebuffer->bind(); // <= uncomment this one to draw on screen, which works perfectly
    glViewport(0,0,512,512);

    m_vao->bind();
    m_program->bind();

    glDrawArrays(GL_TRIANGLES, 0, 3); // Starting from vertex 0; 3 vertices total -> 1 triangle
    glFinish();

    QImage fbo_img(m_framebuffer->toImage());

    fbo_img.save(m_filename);
}

When I uncomment the first two lines of my drawing routine, e.g. m_framebuffer->bind(); and glViewport(0,0,512,512); I get my red triangle as expected on the screen. When I render to the FBO and look into the resulting file, only about maybe half of the pixels are red, the others stay black. The black and red pixel follow a noise-like pattern and on each drawing calls, different pixels belonging to the triangle are red or black.

I see two possible explainations:

There is some kind of uninitialized buffer involved. Do you have any idea which one?
The graphics card is still busy drawing all the fragments while I save the image. I tried fixing this by calling glFinish, but was not successfull.

Do you have any idea what might went wrong in my case?

Best regards

Axel

johngod

Just a shot in the dark, in case the problem is number 2) maybe you should try to save to the file outside the paintGL()

Lassson

Just to be clear, are you using QGLWidget?

I might be wrong, but I have a feeling that the FBO isn't fully updated at the time of m_framebuffer->toImage().
If that's the case, you need to find a way to execute that at a point where the rendering (paintGL) is done. Maybe updateGL()?

Ps. I just saw johngod pointing in the same direction. Maybe this answer give some more value. Ds.

johngod

Any way, if all you want to do is create a image file from the window and if you using QOpenGLWidget I just remember I have done it before, check the following snipet that you can adatp to your needs:

void XXXXX::SaveImageAs()
{
    QString filename;
    filename = QFileDialog::getSaveFileName( this,
                                            tr("Save File"),
                                            getenv( "HOME" ), tr(" Image (*.png)") );
                                            //getenv( "HOME" ), fileformat );

    if( !filename.endsWith( ".png" ) )
    {
        filename.append( ".png" );
    }

    QPixmap pixmap( size() );
    render( &pixmap );
    QImage image = grabFramebuffer();
    QImageWriter imageWriter( filename, "png" );

    //imageWriter.setQuality(100);

    if( imageWriter.canWrite() )
    {
        imageWriter.write(image);
    }
    else
    {
        QMessageBox::warning( this, tr("Save Image"), tr("Error saving image.") );
    }
}