Tiling a large QGraphicsItem in a QGraphicsView

Question

I am currently using a QGraphicsItem that I am loading a pixmap into to display some raster data. I am currently not doing any tiling or anything of the sort, but I have overriden my QGraphicsItem so that I can implement features like zooming under mouse, tracking whick pixel I am hovering over, etc etc.

My files that are coming off the disk are 1 - 2GB in size, and I would like to figure out a more optimal way of displaying them. For starters - it seems like I could display them all at once if I wanted - because the QImage that I am using (Qpixmap->QImage->QgraphicsItem) seems to fail at any pixel index over 32,xxx (16 bit).

So how should I implement tiling here if I want to maintain using a single QGraphicsItem? I dont think I want to use multiple QGraphicsItems to save the displayed data + neighboring data "about" to be displayed. This would require me to scale them all when the person moused over and tried to scale a single tile, and thus causing me to also have to reposition everything, right? I guess this will also require having some knowledge about what data to exactly get from the file.

I am however open to ideas. I also suppose it would be nice to do this in some kind of threaded way, that way the user can keep panning the image or zooming even if all the tiles are not loaded yet.

I looked at the 40000 chip demo, but I am not sure that is what I am after - it looks like it basically still displays all of the chips like you normally would in a scene, just overrode the paint method to supply less level of detail...or did I miss something about that demo?

Answer 1

It's not too surprising that there would be difficulty handling images that size. Qt just isn't designed for it and there are possibly other contributing factors due to the particular OS and perhaps the way memory is managed.

You very clearly need (or at least, should use) a tiling mechanism. Your main issue is that you need a way to access your image data that does not involve using a QImage (or QPixmap) to load the entire thing and provide access to that image data since it has already been determined that this fails.

You would either need to find a method (library) that can load the entire image into memory and allow you to pull regions of image data out of it, or load only a specific region from the file on disk. You would also need the ability to resize very large regions to lower resolution sections when trying to "zoom" out on any part of the image. Unfortunately, I have never done image processing like this so am unfamiliar with what library options are available, Qt likely won't be able to help you directly with this.

One option you might explore however is using an image editing package to break your large image up into more manageable chunks. Then perhaps a QGraphicsView solution similar to the chip demo would work.