Hi Arthur, Sorry you did not get any feedback for your issue after so long, shows there must be a minimal use or understanding of this class. Did you get help or find solution you can share. I'm also trying to find help using this class and lack of doc is still there I'm afraid. One possibility is to look at KoTextDocumentLayout.h and cpp files, these might help with some information. https://github.com/JeremiasE/KFormula/blob/gsoc/libs/kotext/KoTextDocumentLayout.h

@Nico1564 said in How to draw on Image loaded in a QGraphicsView: Yeah i know that this work but i need to draw with fonctions. I successed to do that but I don't want to have the GraphicsView at the beginning of the program, and i want to add draws like an Ellipse (and the image) when i click on buttons. These lines could just as well happen inside a button click QGraphicsEllipseItem* testItem = new QGraphicsEllipseItem(20,30,120,70,0); m_pScene->addItem(testItem); In that case, you would create an ellipse every time you pressed that button. I don't quite understand the "I don't want to have the GraphicsView at the beginning of the program" - maybe you could elaborate on that.

Hi Clockwork I didnt read your code with much attention, but for starters if all you want is draw a triangle, check this tutorials I did https://bitbucket.org/joaodeusmorgado/opengltutorials I'm using vbos, to draw triangles. I'm using all OpenGL native calls, but should be pretty easy to convert to Qt OpenGL functions. I also recommend you this book wich is great, it has a lot of details that usually are ommited in other books http://alfonse.bitbucket.org/oldtut/index.html

You could use a QRubberBand to setup the line and once you release it add the QGraphicsLineItem to your scene.

Thank you! Just perfect! :D

@AlvaroS ok. seems a bit messy. Do you really need a QGraphicsScene then ? You could also draw it as a custom control. you need zooming etc ? In any case, did u see this example http://doc.qt.io/qt-5/qtwidgets-graphicsview-diagramscene-example.html it even has arrowhead class :)

The sample http://doc.qt.io/qt-5/qtwidgets-graphicsview-diagramscene-example.html has arrow class. Maybe it can provide inspiration.

@SysTech Thanks again! You're very welcome. Those 3D displays are kind of handy for visualizing the radio output but they are right now pretty CPU intensive. If I may insert yet another suggestion here. While I don't believe you'd gain much by using OpenGL painting for the "waterfall" data, I think switching to it for the 3D displays would work better. I nice side effect would be that you can also render OpenGL from different threads, provided the appropriate locking mechanisms are in place. You could, as the most simple test, try using QOpenGLWidget for those FFT displays. Kind regards.

@as2388 Yes there is, do the drawing in another thread on a QImage and then trigger the update with that image. I'd add to @SGaist's great point - even convert the image to a QPixmap in the worker (after drawing it), and in the paint event just splat it onto the widget.

@Ramin Actually I have never seen such a library. What I have done in the past (3 years ago) was to draw some elements connected to each other using Qt Quick (QML) and it worked pretty good. Another option is to look at the new modelling plugin that will be part of Qt Creator 3.6. And since Qt Creator is open source, you can look at how it is done and perhaps use that as a starting point.