FPS display error

[Terry]

hello folks,

Maybe it's a easy problem,i'd like to compute fps by my own.but it displays error.

Right code:

Code: Select all

fps = driver->getFPS();
if (lastFPS != fps) 
{ 
core::stringw str = L"Collision detection example - Irrlicht Engine [";
str += driver->getName();
str += "] FPS:";
str += (int)fps;
device->setWindowCaption(str.c_str());
lastFPS = fps;
} 

but if i change first sentence to:

Code: Select all

now = device->getTimer()->getTime();
fps = (int)(1000.0f/(now-last));

"last"has been computed before,it could not display the fps infor,
what's wrong?

Thank you very much!

[CuteAlien]

Maybe now-last is zero? Then you would try dividing by that.

[Terry]

Maybe now-last is zero? Then you would try dividing by that.

No i've traced the code step by step,it's not zero.and fps is 19.

[Valmond]

If you want 'perfect' fps you need to save off the reminder of that division (so this frame when fps=19.5 and shows 19, the 0.5 should go to the next frames calculation).

Otherwise, what's the problem with the code (crashes, slightly wrong fps, not at all correct etc.)?

[Terry]

wow,i mean it displays like "…………FPS：@#！"
19 could not be displayed.

[greenya]

I see "now" assignment, but "last" -- i don't.
So, after this:

Code: Select all

now = device->getTimer()->getTime(); 
fps = (int)(1000.0f/(now-last));

you have to do also

Code: Select all

last = now;

But to make sure never have devision by zero, i would wrote: last = now - 1;

Maybe that is a problem.

[DtD]

Since you said it is displaing random characters, my guess is it is because you are adding the FPS in like it is text, do this instead:
str += irr::core::stringw(fps);

Edit: However, the divide by zero is an important issue as well. I output the time since the last frame along with the FPS in my game and I notice it is very common it is 0 on my machine.