CODEWITHSUNDEEP
qtqt4embedded-linux
dan
dan

Reputation: 229

qt performance – OpenGL

i wan't to use Native OpenGL in the paint function of my widgets(QPainter), to improve performance. i saw that there is function QPainter::begin/endNativePainting(), that can help me. but i can't find examples for that... i wanted to know if those functions are low cost, or evry use of them reduce performance? 2.can i define beginNativePainting() and endNativePainting(), in general for all the widgets i use, instead of using that in every paint function i have.

tnx for any help....

Upvotes: 1

Views: 1656

Answers (2)

Steffen
Steffen

Reputation: 2948

There is some basic example code right in the documentation: http://doc.qt.io/qt-4.8/qpainter.html#beginNativePainting

The functions themselves should be fairly low-cost, but calling them might still cause a noticeably overhead, because Qt has to flush its internal painting queue on the beginNativePainting() call and probably has to assume that everything is changed as soon as endNativePainting() is called.

For the second part I am not sure if I understand what you are aiming at. Basically if you have a QPainter object, you can call beginNativePainting() once. But you have to match it with an endNativePainting() call. So the usual place would be the paint() method.

Upvotes: 1

kuroi neko
kuroi neko

Reputation: 8641

Qt is using a range of OpenGL functionalities to implement its 2D painting, including custom shaders and various frame buffers. It puts OpenGL into a pretty messy state.

beginNativePainting / endNativePainting are there to allow Qt's drawing engine to save this context and retrieve it once the user is done drawing.

It would have been nice to have the xxxNativePainting methods do the contrary (i.e. automatically save and restore user configuration of OpenGL), but since Qt allows to call OpenGL primitives directly, saving the global state is nigh impossible without tons of code and potential serious performance hit.

Instead, these methods simply save Qt's internal OpenGL state and, rather than having user code start in a configuration that would be meaningless anyway (and likely to change with each new Qt release), reset OpenGL to a "neutral" state.
It means that, inside a begin/end section, you will start with a clean slate: no shader linked, no vertex array, most of global parameters reset, etc.

Contrary to a simple QGLWidget / PaintGL scenario where you can afford to setup the global OpenGL state once and for all and simply call the rendering primitives each frame, you will have to restore pretty much everything just after the call to beginNativePainting (link/bind your shaders, set global parameters, select and enable various buffers, etc).

It also means that you should use native painting sparringly. Having each single widget do custom painting might soon bring your rendering to its knees.

Upvotes: 0

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