Unsolved What the fastest way to draw 2d graphic in qt?

[ataryq]

What the fastest way to draw 2d graphic in qt? Where I can reading about it?
Sorry for my bad English.

[SGaist]

Hi and welcome to devnet,

It will depend on what you want to draw. Usually OpenGL is the fastest.

[ataryq]

@SGaist
Thanks. I want draw 2000 lines of different colors more than 25 time/second. What way the best for my target?

[Proteos]

Hi,

Please correct me if I'm wrong (thus, I will learn too).
If you only want to draw lines, I'm not sure that using OpenGL is necessary, and its implementation and use can be quite tricky.

Personnaly, I would go for a pure Qt approach :

• Use a QTimer to call your drawing procedure at the frequency you need.
• Draw your lines on a QImage with a loop. (QImage are optimised for direct access to pixels and are therefore made for drawing).
• Make a QPixmap from this image (QPixmap are optimised for display).
• Display you QPixmap in a widget. As far as I'm concerned, I use a QGraphicsScene to receive my QPixmap, and then I display it in a QGraphicsView (once again, I would be glad to learn a better way).

For now, with the relative knowledge I have, nothing makes me think that OpenGL would be faster for 2D, because you would have pretty much the same procedure :

• create your image on a matrix (apply a projection matrix to render it ? projection matrices are used in 3D to "flatten" the scene, but can we ignore them in 2D ? I don't know)
• save the image on a backbuffer
• swap the frontbuffer and the backbuffer
• display it in an OpenGLWidget.

The procedure is not shortened using OpenGL, so is there such a big optimization difference between Qt and OpenGL in the case of 2D drawing ?

OpenGL is more complex than Qt, so I suggest that unless you absolutely need it, use Qt instead.
Testing is the best way to know ! Try using Qt, and if the result is too slow, try using OpenGL (or should I say 'try to use OpenGL' ;-) ).

[ataryq]

@Proteos Thanks very mach! It's very intresting! I will try it.