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

[Matt Bailey]

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