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Extending Selections In QGraphicsView

posted by Andy Maloney

Overall, the Qt library has done a fantastic job of maintaining relatively simple interfaces with well-named functions that do what they say they’re going to do [unlike MFC for example] and of having objects react the way you would expect them to without much modification. I’ve used it for several cross-platform Windows/Mac OS X/Linux applications and I can usually bend it to my will fairly easily because they’ve provided the right hooks.

One exception I’ve run into recently – and an oversight that is really kind of surprising – is using rubber band selections on a QGraphicsView. What would be natural – and what most people would expect – is that if the user is holding down the “modify selection” key – Control on Windows, Command on Mac OS X – then:

  • clicking a selected object will deselect it without modifying the rest of the selection
  • clicking an unselected object will select it without modifying the rest of the selection
  • clicking and dragging will give the user a way to select several objects to add to the current selection (known as rubber band or drag selection).

QGraphicsView With Selection

QGraphicsView With Objects Selected In Red

But with QGraphicsView – by design – if you have some objects selected (as pictured above), there’s no way to extend the selection by using “rubber band selection” to add to it. Clicking the mouse clears the current selection regardless of whether the user is holding the “modify selection” key. This is a UX design bug. It is counter-intuitive and confusing for users.

QGraphicsView Selection Method

QGraphicsView Selection Method - Holding Down The Control/Command Key Resets Selection

Not only is the default behaviour confusing, there’s no way to correct it without either inheriting from QGraphicsView and writing a new rubber band selection system or hacking the Qt source. I chose to fix the problem by doing the latter. Fortunately this turned out to be easier than I originally thought it would be…

(19 Jan 2015): I submitted a patch for this which has been merged into the Qt codebase. This fix will be part of Qt 5.5.

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Point Cloud Library on Mac OS X: A Build Diary

posted by Andy Maloney

I’ve been using and contributing to various open source projects for almost 20 years. I love the fact that we can leverage each other’s work and help other developers save time instead of reinventing the wheel or fighting the same problem a thousand times over. Open source tools and libraries make what I do possible.  Whenever I use open source, I like to compile from the source so I can contribute back to the projects – even if only little patches for fixing compilation warnings, clarifying error messages, or adding to documentation.  I used to enjoy configuration and build challenges – such as tweaking and compiling my own Linux kernels – but that’s behind me now.  I just want things to work.

I’m a solo developer working on a commercial cross-platform Mac OS X/Windows application.  This means that I have to be careful how I allocate my development time since I also have research to conduct and a business to run.  Part of this is maintaining control of my development “ecosystem”.  The more moving pieces there are – tools, 3rd party libs, etc. – the more overhead, and the less time I have for actual development.  Keeping it simple means saving myself a lot of time.  It also makes it quick and easy to bring short-term contractors on board.

I started to investigate open source tools for processing point clouds for a future version of my software – I want to handle scan data from FARO and Leica scanners.  I quickly zeroed in on the Point Cloud Library which is a BSD-licensed library to read, visualize, manipulate, and process point clouds. It looks like exactly what I need and includes a bunch of capabilities I don’t understand in the least, but seem like they might be useful in the future as I learn about it. Most of the capabilities are too advanced for me to contribute much to them directly, but I thought I’d be able to contribute “around the edges”, so I wanted to build it from source.

Point Cloud Library

Point Cloud Library

This article is a blow-by-blow account of the steps I took while trying to build the Point Cloud Library (PCL) on Mac OS X 10.6 starting with the information from PCL’s Compiling from Source page. The first thing I tried to do is build and link PCL with only the mandatory dependencies. After that, I’ll want to look at the optional pieces and build them if I need them.

As I was working through it, I put together notes for myself on what I did in case I had to do it again or have a contractor do it.  As with most other things I have on this site, I thought at least one other person might benefit through the power of Google, so I decided to put the notes up here verbatim.  Knowing what I know now, I could shorten this dramatically into “Steps to Build PCL on Mac OS X” [even though I haven’t been successful yet], but I decided it might be interesting to show the action-packed sequences of what actually happened…

I hope this is taken in the spirit it’s intended – documentation of some of the problems just getting PCL to configure and build, along with what I hope are some constructive recommendations [at the end of this voluminous tome]. [Warning: I get a little rant-like in the last commentary section – it’s not directed specifically at the PCL, but software development projects in general.]

I have to point out that the PCL guys have been great, answering my initial question quickly, and they’ve been encouraging [so far – hope they still are after this!]. This doesn’t always happen in open source projects, so it’s great to see. They deserve a lot of credit for their work, which is of great value to many  developers and companies worldwide.  All the developers and supporting companies are super-awesome for making this library available as open source. Very much appreciated!

[If you want to skip the gruesome details – don’t forget they’re action-packed! – I list some bugs and recommendations near the end.]

So… here we go!

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