Question

I’m sorry if my question is somewhat vague. It’s been a few years since I did anything with Qt, and back then I never did any fancy image stuff. What I’m asking for below is just some general suggestions on which classes to consider using. I’m trying to avoid barking up the wrong tree from the very start.

The situation: I’m writing a Qt-based program in which I need to display a somewhat large (let’s say 5000×5000) raster image. The user should be able to zoom (quickly) in and out, and pan around the image in a way similar to for example Google maps. So far, this is not very different from the Qt ImageViewer example, except perhaps for the requirement that zooming happens quickly. However, I need to draw on the order of 50k simple geometric shapes (let’s say circles) on top of the image, and be able to add and remove some of these in a simple way. The circles should have the same size no matter the zoom level, and should thus either be redrawn whenever the user zooms, or should be drawn with vector graphics. Think of the circles as map annotations. These should look the same at any zoom level, and also behave nicely with respect to panning.

I guess my question is twofold:

1. Can Qt draw vector graphics on top of a raster image?
2. In general, which classes should I consider for the above?

Thanks in advance. I don’t like answering vague questions myself, but maybe someone with experience with Qt’s graphics capabilities has an answer.

Answer 1

I suggest you use QGraphicsView and friends for this. It helps handling all the view/world transformation and the vector items can be achieved with various QGraphicsItems.

You can change the sizes of the items whenever the zoom level changes to maintain constant apparent sizes.