# DX11 Engine design questions...

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Hey guys,

The porting of my old code base over to SlimDX (using exclusively Dx11) has been going quite well. But I'm not liking the way some of my old, naive code is looking. A lot of things are so radically different from Dx9/XNA -> Dx11 that it facilitates throwing the old junk away and writing completely new code. So I've been posed with a few questions that I'd like to get some peer feedback on.

FYI, so you know, I'm trying my best to abstract away a lot of the pain involved with writing straight DX applications with this engine, but not so much that those who WANT lots of nit-picky features can't get to them (not an easy feat, and sometimes not possible).

1) Graphics Settings :

Any engine worth its salt has an fairly intuitive way of changing graphics settings on the fly and often by configuration files; thus enabling programmers to create nice little graphics settings/options menus for their games. I'm brand new to DX11/SlimDX API, so I'm probably missing a lot of stuff and just not seeing it. I'm from the days of "PresentationParameters" and all that jazz. :) So I'm wondering, to begin with, what range of graphics settings/options will be vital to a SlimDX/DX11-based engine? What things do I need to look up and learn about to make an effective graphics settings system? Any particular classes in the API that can help me not reinvent the wheel (as I often do when I'm ignorant of an API's features)? What things are/may be rarely used and not very important to include?

One other question that's been bugging me is about the ModeDescription property for SwapChainDescriptions, concerning their refresh rates. Obviously, when windowed, your application can effectively use any resolution which will fit in the display bounds and match the window size. But what about the refresh rate? Do we have to perfectly match the display mode's refresh rate in windowed mode, or is it only a requirement in full-screen? I remember the DX11 doc said it's vital to get the refresh rate correct in full-screen.

Also, are there any examples on the proper way to switch graphics (Device/SwapChain) settings at runtime? I'm not really sure if the way I switch from windowed to fullscreen is a "good" way of doing things lol. The limited samples and incomplete docs have left me guessing on a lot of things. :P

2) "Main Loop" and timing:

I've already implemented a new, internal clock/timer class which is very similar to the one buried in the XNA framework; although it's lighter and cleaner imho. I've tested it and it's as accurate as it gets on my system. In XNA, the "Game" class is fixed-timestep by default, and has a target update interval of 1/60th second (roughly 16.666something6667-something ms). Question is, what is the best way to ensure you stay within a reasonable tolerance of your update interval? What if the loop completes super fast and you have only a tiny bit of elapsed time between updates? Do you intentionally wait a bit and try to adjust the speed? Or should you just rock out the Update(...) calls as fast as they can go?

I'm also planning to add the ability to have parallel update and rendering loops on separate threads; each keeping their own time and handling their own problems. So I'm wondering if anyone has some advice for me on this front as well.

Wow, big post. Any input/tips/advice will be greatly appreciated. I'm just trying to figure out where I'm going with my ideas and what things I should be aware of before I make big, time consuming mistakes!

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Quote:
 Original post by keinmannWhat things do I need to look up and learn about to make an effective graphics settings system?
I'm not particularly familiar with the API, but DX11 has "feature levels" that describe what the graphics card is capable of. If you're using new features added in DX10/11, but want to have fallbacks for older-cards, make sure you check the card's feature level.
Quote:
 Question is, what is the best way to ensure you stay within a reasonable tolerance of your update interval? What if the loop completes super fast and you have only a tiny bit of elapsed time between updates? Do you intentionally wait a bit and try to adjust the speed? Or should you just rock out the Update(...) calls as fast as they can go?
A lot of games use a fixed time-step, where update is only called if 16.67ms have elapsed. If 33.3ms have elapsed, you call update twice, etc...
If less than 16ms have elapsed, you hand your time-slice back to the CPU.
On Windows there's functions like YieldProcessor, SwitchToThread, Sleep, etc for this.
Quote:
 I'm also planning to add the ability to have parallel update and rendering loops on separate threads; each keeping their own time and handling their own problems. So I'm wondering if anyone has some advice for me on this front as well.
If you go down this path, I'd keep them separated and communicate via double-buffered state. To do this, you have two 'communication' structures. At any one time, the Update thread is writing to one, and the Render thread is reading from the other. At the end of each frame, the threads synch up and swap structures with each other.

However, this isn't the most scalable approach to threading -- you're targeting a dual core processor. Quad-core is standard on PCs now, and consoles have been doing ~6 hardware threads for years now.
Another approach is the task-pool model, where you make one worker thread per core (or per hardware thread), and then break up your update and render code into lots of isolated tasks that communicate via asynchronous message passing.

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Quote:
 Original post by keinmannQuestion is, what is the best way to ensure you stay within a reasonable tolerance of your update interval? What if the loop completes super fast and you have only a tiny bit of elapsed time between updates? Do you intentionally wait a bit and try to adjust the speed? Or should you just rock out the Update(...) calls as fast as they can go?

A common way is to use an accumulator.

float accumulator;const float TimeStep = 0.2f;public void Frame(float dt) {    accumulator += dt;    while (accumulator > TimeStep) {        accumulator -= TimeStep;        Update(TimeStep); // fixed timestep update    }}

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To add to the last post, here is a full article on how to manage fixed timesteps well. It is pretty good.

http://gafferongames.com/game-physics/fix-your-timestep/

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Thanks for the answers everyone. And I like the article, Dranith. I've also used variable time-step and just let it run as fast as possible. But now I know exactly how to implement a reliable fixed-timestep system. I think it will be important to do so with the accuracy I demand of the physics, especially aerodynamics.

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