This article continues our series on what is new in Qt 5.1 with respect to OpenGL. If you haven’t already seen them, you may be interested in reading Part 1 and Part 2.
Timer Queries
OpenGL on the desktop exposes a very useful tool in the shape of timer query objects. These can be used to record the amount of time taken by the GPU to process sequences of commands. If we couple this with the usual CPU profiling techniques then we can get a very complete understanding of the bottlenecks in our rendering code.
One obvious place that could take advantage of this is the Qt Quick 2 renderer. Although the Qt Quick 2 renderer does have options to enable timings of various stages of rendering, this only tells us half of the story. Namely, the CPU half. Using OpenGL timer queries would allow us to fill in the gap and gain an understanding of how the GPU is performing.
Another common use for OpenGL timer queries is to provide feedback to adaptive rendering routines in order to maintain more consistent frame rates. For example, if we instrument our rendering function and find at runtime that our application is rendering too slowly, we can perhaps increase the frame rate by using less complex shaders or by using lower resolution meshes etc. Conversely, if we find that we have GPU cycles to spare then we could improve rendering quality by perhaps using higher levels of tessellation.
Qt 5.1 will make such tasks much easier with the introduction of two new classes developed by KDAB engineers: QOpenGLTimerQuery and QOpenGLTimeMonitor. The QOpenGLTimerQuery class is a simple wrapper around a single OpenGL timer query object and can be used if you wish to have total control. It is expected that QOpenGLTimeMonitor will be the more commonly used class as this is a wrapper around a sequence of timer query objects and makes it trivial to measure the GPU time of the various stages of our rendering functions:
void Scene::initialize()
{
// Create a timer query object
m_timeMonitor = new QOpenGLTimeMonitor( this );
m_timeMonitor->setSampleCount( 5 );
if ( !m_timeMonitor->create() )
qWarning() << "Failed to create timer query object";
// Generate some names for the various stages of rendering
m_renderStages << "Clear Buffer" << "VAO Binding"
<< "Shader Program Binding" << "Drawing";
}
void Scene::render()
{
// Start the timer query
m_timeMonitor->recordSample();
// Do some rendering
m_funcs->glClear( GL_COLOR_BUFFER_BIT );
m_timeMonitor->recordSample();
m_vao.bind();
m_timeMonitor->recordSample();
m_shaderProgram.bind();
m_timeMonitor->recordSample();
m_funcs->glDrawArrays( GL_TRIANGLES, 0, 3 );
// End the timer query
m_timeMonitor->recordSample();
...read more
Source: FULL ARTICLE at Planet KDE
In the next weeks I will post more specific informations and I will update everyone with my progresses. Any comment is welcome! If you want to contribute to this you’re welcome too of course.
