Tag: Managed DirectX

Ah, the good ol’ render loop.  Everyone’s favorite topic of conversation.  As I’m sure everyone is aware, the Managed DirectX samples that shipped with the DirectX9 SDK as well as the Summer 2003 update used the ‘dreaded’ DoEvents() loop I speak so negatively about at times.  People also probably have realized my book used the ‘infamous’ Paint/Invalidate method.  I never really made any recomendations in the earlier posts about which way was better, and really, I don’t plan on it now.  So why am I writing this now?!?!

If you read David’s post about the upcoming 2004 Update, you may have noticed that he mentions the DoEvents() methods that samples used to employ is gone.  In all reality, along with the new sample framework, the samples themselves actually never use the Windows Forms classes anymore either.  The actual render window and render loop are all run through P/Invoke calls into win32, and I figured I’d take a quick minute to explain the reasoning behind it.

Obviously the idea of using DirectX is for game development.  Sure, there are plenty of other non-game development scenarios that DirectX is great for (data visualization, medical imaging, etc), but what drives our API are the game developers.  If you know any game developers (or are one yourself), you’re probably vastly aware that while the game is running (and rendering), things need to happen quickly, and predictably.  With all the benefits of managed code, one thing that can be hard to achieve is that ‘predictability’, particularly when you’re dealing with the garbage collector.

So let’s say you decided to use Windows Forms for your rendering window, and you wanted to watch what the mouse was doing, so you hook the MouseMove event.  Aside from the ‘cost’ of the Invoke call to call into your handler, a managed object (the mouse event arguments) is created.  *Every* time.  Now, the garbage collector is quite efficient, and very speedy, so this alone could be easily handled.  The problem arises when your own ‘short lived’ objects get promoted to a new generation due to the extra collections these events are taking.  Generation 0 collections won’t have any effect on your game, generation 2 collections on the other hand will.

Thus the new sample framework doesn’t rely on these constructs at all.  This is probably one of the ‘most efficient’ rendering loop available in the managed space currently, but the code doesn’t necessarily follow many of the constructs you see in the managed world.  So, when deciding on the method you want to use to drive your rendering, you need to ask yourself what’s more important?  Performance, or conformance?  In the case of the updated sample framework, we’ve chosen performance.  Your situation may be different.

So I asked before what types of features you would like to see in Managed DirectX (and the feedback was awesome – I’m still interested in this topic)..  What I’m also interested in that I didn’t ask about back then though was what types of things people are using it for currently?

Are you using it to write some tools?  Game engines?  Playing around on the weekends?  What experiences have you had working with the API?

It seems that at least once a week i’m answering questions directly regarding the performance of managed code, and Managed DirectX in particular.  One of the more common questions i hear is some paraphrase of “Is it as fast as unmanaged code?”.

Obviously in a general sense it isn’t.  Regardless of the quality of the Managed DirectX API, the fact remains that it still has to run through the same DirectX API that the unmanaged code does.  There is naturally going to be a slight overhead for this, but does it have a large negative impact on the majority of applications?  Of course not.  No one is suggesting that one of the top of the line polygon pushing game coming out today (say, Half Life 2 or Doom 3) should be written in Managed DirectX, but that doesn’t mean that there isn’t a whole slew of games that could be.  I’ll get more to that later.

I’m also asked quite a bit things along the lines of “Why is it so slow?”  Sometimes the person hasn’t even ran a managed application, they just assume it has to be.  Other times, they may have run numerous various ‘scenarios’ comparing against the unmanaged code (including running the SDK samples) and have found that in some instances there is large differences.

Like I’ve mentioned earlier in this blog, all of the samples in the SDK use the dreaded ‘DoEvents’ loop, which can artificially slow down the application due to allocations and the subsequent large amounts of colllections.  The fact that most of the samples run with similar frame rates as the unmanaged API is a testament to the speed of the API to begin with.

The reality is that for many of the developers out there today, they simply don’t know how to write well performing managed code.  This isn’t through any shortcoming of the developer, but rather the newness of the API, combined with not enough documentation on performance, and how to get the best out of the CLR. Luckily, this is changing, for example, see Rico Mariani’s blog (or his old blog). For the most part, we are all newbies in this area, but things will only get better.

It’s not at all dissimilar to the change from assembler to C++ code for games.  It all comes down to a simple question.  Do the benefits outweigh the negatives?  Are you willing to sacrifice a small bit of performance for the easier development of managed code?  The quicker time to market?  The greater security?  The easier debugging?

Like i said earlier, there are certain games today that aren’t good fits for having the main engine written in managed code, but there are plenty of titles that are.  The top 10 selling PC games a few weeks ago included two versions of the Sims, Zoo Tycoon (+ expansion), Age of Mythology, Backyard Basketball 2004, Uru: Ages beyond myst, any of which could have been written in managed code.

Anyone who’s take the time to write some code in one of the managed languages normally realizes the benefits pretty quickly.

Wow. I wouldn’t have thought that my blog on the render loop and doevents would spark as much discussion as it did. Invariably everyone wanted to know what i thought the ‘best’ way to do this was.

Actually, the answer is (naturally) ‘It Depends’. It wasn’t actually an oversight on my part to leave out a recommendation at the end of the post, it was done intentionally. I had hoped to spark peoples interest in learning the cost of the methods they were calling, and pointing out a common scenario where the method had side effects that many people weren’t aware of.

However, since I’ve been asked quite a few times on alternatives, I feel obligated to provide some. =)

Here are some alternatives, in no particular order.

• Set your form to have all drawing occur in WmPaint, and do your rendering there. Before the end of the OnPaint method, make sure you do a this.Invalidate(); This will cause the OnPaint method to be fired again immediately.
• P/Invoke into the Win32 API and call PeekMessage/TranslateMessage/DispatchMessage. (Doevents actually does something similar, but you can do this without the extra allocations).
• Write your own forms class that is a small wrapper around CreateWindowEx, and give yourself complete control over the message loop.
• Decide that the DoEvents method works fine for you and stick with it.

Each of these obviously have benefits and disadvantages over the others. Pick the one that best suits your needs.