I’ve been trying to track down a bug in my program. I noticed that whenever I am holding down the up and left arrow keys and press the spacebar, a WM_KEYDOWN message with a WPARAM of VK_SPACE is never received or processed by my WndProc.
I was a bit curious, so I opened up Spy++ and started viewing the messages sent to my window. I noticed that when I am holding down the left and up arrow keys and press another key, say A, the WM_KEYDOWN (as well as a WM_CHAR) message is sent to my window.
But if I’m holding down the up and left arrow keys and press the spacebar, a WM_KEYDOWN message for the spacebar hit is not sent to my window.
What is the reason for this? Could anyone confirm this behavior?
This happens because your keyboard is doing what is called ghosting. This is normal for the majority of keyboards used by people today. It’s wired up so that it’s a lot cheaper to produce, and as a result, things like this happen consistently.
Basically when three keys in the same row/column are pressed at once, they cannot all be uniquely identified, causing at least one not to go through. The only thing you can do is buy an anti-ghosting keyboard, or redesign your controls to avoid conflicts.
On a personal level, I found out about this a day or two before I presented my game and ended up having to make the controls something like P1: SZXC and space and P2: /789 and +. It really is a good thing to know beforehand.