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On The Coordinate System Of QGraphicsScene In Qt

December 17, 2012 8 comments

When it comes to Qt’s fancy 2D engine embedded in QGraphicsScene – usually attached to a QGraphicsView – a common question is what the coordinate system of it is like. Basically this means how the X and Y axes of its grid are oriented. The official Qt documentation is of little use here as they probably never consider this to be important, let alone for exchanging data with other coordinate systems, such as that of OpenGL.

For a 2.5D Arcade game my company Nyanko is currently working on I had to create a level editor. I chose Qt and QGraphicsScene for the simple reason that it is very easy to set up a kind of editor inside a fancy UI with it. When it comes to interoperability with our in-house 3D engine which is based upon OpenGL, the question was how compatible these two coordinate systems of Qt and OpenGL would be. As it turned out they’re almost the same, but also different. See the below illustrations:

opengl_coordinate_system
OpenGL’s coordinate system

qt_coordinate_system
QGraphicsScene’s coordinate system

As you can see, QGraphicsScene has an inverted Y axis in comparison to OpenGL, with the Y axis growing downwards instead of upwards. This makes the starting point of OpenGL bottom-left and that of QGraphicsScene top-left. It’s unfortunate that this orientation was chosen, as in the level editor I had to invert the Y axis prior to saving to a level file and again upon loading. It also means that the OpenGL functionality in Qt doesn’t have the smooth interoperability with QGraphicsScene it otherwise would have had.

Fortunately it’s not hard to invert the Y axis, it’s just a shame that it had to be done like this, and with so little documentation. Hopefully this article will help someone out there avoid a few pitfalls 🙂

 

Maya

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