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JeZ-l-Lee

DirectX 9: Best Method To Limit FPS?

13 posts in this topic

Hi,

 

I'm developing a DirectX 9.0c 2D game engine now code named: "X-Caliber Engine"...

 

What is the best method to limit Frames Per Second?

 

I am currently doing this but I don't like it:

#include <mmsystem.h>
//...
static DWORD LastFrameTime = 0;
DWORD FPSLimit = 60;
//...

//-MAIN-LOOP--------------------------------------------------------------------
{
    render();

    DWORD currentTime = timeGetTime();
    if ( (currentTime - LastFrameTime) < (1000 / FPSLimit) )
    {
        Sleep(currentTime - LastFrameTime);
    }
    LastFrameTime = currentTime;
}

//--------------------------------------------------------------------MAIN-LOOP-

Above code adds an additional LIB dependency: "winmm.lib" which I don't like.

Any suggestions would be appreciated, thanks!

 

JeZ+Lee

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Why do you want to limit fps? To avoid rendering faster than 60fps? To save energy (i.e. battery/electricity bill)? to be friendly with other programs?

Hi,

 

I just want the game engine to run at a constant 60 Frames Per Second on any computer.

I've been programming 2D video games for 15+ years and I have always limited the FPS.

(I realize this may *not * be the best method, but it's what I feel comfortable with)

 

JeZ+Lee

 

Click HERE for developer screenshot!

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You can just enable VSYNC. It will limit you to the refresh rate of the monitor, and prevent tearing.

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I just want the game engine to run at a constant 60 Frames Per Second on any computer.

If all you care about is 60FPS, then call Present() with the D3DPRESENT_INTERVAL_ONE flag. This will enable vsync, limiting your FPS to the monitor refresh rate.

Edited by satanir
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You can just enable VSYNC. It will limit you to the refresh rate of the monitor, and prevent tearing.

Hi,
 
Thank you for the reply.
 
This sounds good, but can the video card driver override the game's VSYNC setting?
(meaning if the VSYNC is turned off in the video card driver then it won't be on in the game?)
 
Thanks!
 
JeZ+Lee
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This sounds good, but can the video card driver override the game's VSYNC setting?
(meaning if the VSYNC is turned off in the video card driver then it won't be on in the game?)

Yes it can, but by default it's application controlled, and most users don't even know this option exists, let alone change it.

 

It is implied from your question that your game logic relies on a specific frame rate. It's not a good design - you should use timers to measure the amount of time passed between frames, then use that value for you calculations. This will ensure your game logic will work with any FPS, even when it drops below 60.

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Using Sleep to control framerate is also not good; either use vsync as advised or spin in a loop until the current time has reached the time for the next frame to run at.

 

Nothing wrong with the winmm library dependency either; it's absolutely standard and will be statically linked to your program anyway: no concerns about whether or not the user has it are needed.  In fact, if you look at the documentation for D3DPRESENT, you'll see that D3DPRESENT_INTERVAL_ONE uses timeBeginPeriod itself, so the D3D runtime is also using that lib (so in fact if you're calling timeBeginPeriod in your program, and also using vsync, you're better off with D3DPRESENT_INTERVAL_DEFAULT).

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I just want the game engine to run at a constant 60 Frames Per Second on any computer.

If all you care about is 60FPS, then call Present() with the D3DPRESENT_INTERVAL_ONE flag. This will enable vsync, limiting your FPS to the monitor refresh rate.

 

This probably isn't the most reliable method if he intends on getting 60fps across the board. The number of 120hz monitors are increasing rapidly, so those individuals will be getting locked at twice the speed of others with 60hz displays. 

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You should decouple FPS from game logic so that the game behave the same way regardless of FPS. For each frame you find the fElapsedTime and pass it down to all your logic. Every parameter you want to move/rotate/scale/advance you multiply with fElapsedTime.

void Logic(float fElapsedTime){
.
.
.
.
.
PlayerPosition += PlayerVelocity*fElapsedTime;
.
.
.
.
.
}
Edited by Tispe
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I wrote a lenghty post about doing this: http://www.gamedev.net/topic/647960-extreme-framerates/#entry5096093 .

 

I don't think it's very clear form that post what I did, but if you need help understanding what I did, please let me know.

 

You still have to use the winmm timeBeginPeriod API for it, though - otherwise, the frames will not be rendered exactly at the 60 FPS mark - they'll be renderd at somewhere around the 60 FPS marks (+/-15ms, because 15ms is the default system-timer resolution), but this wouldn't cause the average framerate to go down - it will still be 60FPS.

Edited by tonemgub
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