Saturday, July 9, 2011

wxDC DrawPoint broken under wxMAC

To follow up on my previous post, I had written my own routines for handling alpha channel support under wxWidgets, and finally everything seemed to be working properly. Both Win32 and Linux/GTK builds seemed to work exactly as expected using my alpha channel bitmaps where I needed them. For most other places, I still used wxDC primitives, as wxDC direct drawing seemed to be a bit faster than going through a bitmap/blit interface, and I still needed to use wxDC to render fonts.

I figured porting to MacOS X would be a snap, and for the most part it wasn't too bad. There were some issues with Repaint() not working the same, but adding a few explicit paint calls in the right place fixed that up.

However, something was wrong with some of the graphics in the various windows. The sky bar, normally with lots of little stars in it, still had stars in it, but instead of one pixel there were two: one at offset (0, 0), and one at offset (1, 1). I also noticed 'fuzz' on the corners of various windows and buttons, and in a handful of other locations.

Long story short, wxDC::DrawPoint() draws two pixels under wxMAC. Not one, but two. I found a couple bug reports regarding this from a few years ago, where it was basically acknowledged as a "verified bug", and the ticket was never closed. Advice was "these routines are slow and shouldn't be used for high performance drawing". Thanks guys.

I have some mighty fine choice words to say to the developers regarding this, but I'll try to keep this post on topic.

If something as fundamental as DrawPoint() is malfunctioning on a major development platform, I figured it would probably be a good idea to transition away from the classes that implement it in favor of my own drawing routines. So, I started using my bitmap class, which just happens to have a DrawPixel() function that works properly.

As expected, this fixed the drawing problems on all three platforms. But alas, using my bitmap class exposed another set of very nasty wx bugs, which I'll discuss in the next post.

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