[simpits-tech] "Big sim" visit, and 25 frames per second.............

[wledzian1 at comcast.net]

The sims may only show 25 frames per second, but one of the requirements for certification of sim visuals is that this frame rate is rock-solid and reliable. Our home flight simulators don't have this requirement, so they can get away with stutters. You may see the frame counter telling you that you've got 40 fps, but you may get a few fast frames that look really smooth, with moments that it drops to 10-15 fps. Your eyes most definitely notice the difference. I typically run my sim locked at 15 fps even though it will routinely display 30FPS peaks, knowing that at 15 it will be steady, and will be an even divisor of the 60hz display frequency. It looks choppy when I'm low and fast, but then I'm not usually flying low and fast. 

Can I see a difference above 25fps? Sure. I used to play a lot of FPS games, and I could easily tell the difference between 60 and 100 fps. (aside - I really miss my ViewSonic CRT that could reliably do 1024x768 at 120hz). For the simulation world, slow and steady beats fast and choppy. 

-Wayne 
----- Original Message -----
From: "Matt Bailey" <dabigboy at cox.net> 
To: simpits-tech at simpits.org 
Sent: Sunday, August 4, 2013 2:54:59 PM 
Subject: [simpits-tech] "Big sim" visit, and 25 frames per second............. 

So this past week I spent probably an hour and a half climbing around 
some big sims with the senior sim tech. It was REALLY cool, I gained a 
lot of insight and learned a lot about how these sims operate. The one 
thing that had me dumbfounded, however, was some of the frequencies the 
sims were running at. Most processes were running 7 to 15hz (which is 
fine for something like radio tuning, or updating cockpit switch 
positions, for instance). The graphics, however, are running at a 
blazing 25fps. The graphics computer for this particular sim was 
running at 2 ghz (I didn't catch how many CPUs it had) and had 2g RAM. 
The sim tech was insistent that 25 is all the human eye can really see, 
and that yes, indeed, this was literally only putting 25 unique images 
on the screen per second. 

Now I have heard folks say no one can see the increase in rates above 
30fps or so, but I have always dismissed this, as I can assure you that 
25fps on a PC flight simulator looks not-very-good. 40 is nice, but 60+ 
is what I really like to see. Above that, yep, can't tell much 
difference. I will say, however, that I have seen animations running at 
extremely high FPS that seem incredibly real because of their high 
refresh rates....this may have something to do with other graphic 
effects (like good antialiasing), but it seems to happen only with very 
high FPS animations (like the DirectX graphics tests, for instance). 

This has me stumped. Is it possible that our computers are lying to us? 
Could it have to do with the inconsistency/variation in frame rate 
(that is, not a consistent 25fps)? I have flown some of these big sims, 
and I don't recall the graphics looking slow or stuttery at all. 

I considered one other option, and this could be a clever way to gain 
FPS: could it be that the actual graphic generators for these sims are 
intelligently adding frames? In other words, your main graphics 
processor only has to calculate geometry, lighting, etc 25 times per 
second, but the final piece of the graphics pipeline could be looking 
at the most recent frames, and generating new frames based on the trend 
of the past few frames. Could this explain it? It doesn't seem likely, 
as the sim tech was quite insistent that 25fps is about all a human 
could detect, and was all they were running. Puzzling! 

Or, it could just be that the big sims would indeed look slow and 
stuttery if they were stuck on a sharp, bright monitor instead of over 
a big projection system that blurs the image just a little bit...... 

Matt 
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