Forums > Jitter

nonrealtime issues part 2

March 31, 2006 | 4:18 pm

Removing sfrecord~ or replacing it with record~/buffer~ solved the
non-realtime slowdown with render_node, but I discovered that the video
frame recordings are not accurate anymore. Frames get dropped in the
process. In a 1minute render, the video is gradually lagging behind to
about 2 seconds at the end.

To me it looks like the the non-realtime mode, which is now pushing my cpu
to 100%, is killing the whole idea behind jit.qfaker. Can someone think of a
way to ensure that all frames are rendered and written to disk, without
having to slow it down 30x with sfrecord~?

T_


March 31, 2006 | 8:38 pm

Difficult to say. Off the top of my head I can’t think of why just
changing sfplay/record to a buffer based model would affect this
(unless you aren’t running scheduler in audio interrupt, which is a
pretty critical part of this patch if I remember correctly).
Unfortunately i don’t have the time to pick the patch apart, perform
tests, and comment. I will mention a few things:

- A good way to debug items such as frame drop and application
behavior huring movie rendering is to write debug information in each
frame recorded (e.g. frame number, assumed running time, any relevant
parameters, etc with jit.gl.text2d or jit.lcd). This might prove
insightful

- I believe the NRT model employed by this patch is only necessary
for synchronizing audio/video during the rendering process. If this
is not necessary, you can avoid the audio driver, scheduler, and
qfaker and drive the entire system with a synthesized clock generated
from qmetro->counter or something of that form.

Hope this helps.

-Joshua


April 2, 2006 | 3:14 pm

Here’s a little movie that demonstrates the problem. It is supposed to
be rendered at 25 fps, but quicktime shows 26.09. The numbers on the bottom
indicate: world-time / smtpe / dsptime. The dsptime is what the smpte and
world-time is generated from, in the smpte subpatcher javascript.

http://www.sndwrx.com/tmp/test_time3.mov

Open the movie and navigate by frame. See the 3 frames from 9402 woldtime.
With the 3 squares in the middle, its obvious that a frame is skipped from
the movie. (the objects are supposed to move at constant speed) The smpte
code is normal (always 1frame incr) while the world and dsp time show a
2 frame increase (9402-> 9468).

It’s obvious that the objects don’t move smoothly at a fixed velocity. This
is always the problem in non-realtime mode, whether recording to disk or
just playing, so I guess it has nothing to do with disk access.

I have no idea what to make of these findings. Anyone a clue? Are there
other things I can investigate? Maybe porting the smpte clock script to java
or c?

Best, Thijs


April 2, 2006 | 3:53 pm

I just discoverd that the jump is occuring exactly every 2 seconds, so I
looked at the signal vector and it seems related. Setting the vector to 2
makes it a little smoother, 1 makes the whole ogl dissapear and bigger sizes
make the animation more screwed up.

I turned off scheduler at audio interrupt, and this lookes a lot better
(maybe perfect, but that’s hard to say because its playing faster than
realtime). Problem is, I can’t record non-realtime with audio interrupt off.
Max hangs.

??

T_


April 2, 2006 | 6:29 pm

In nonrealtime mode I am using qfaker in a very specific way. If
done in realtime mode, or with audio interrupt off, this will cause
Max to hang, as noted in the big red warnings in the patch.

It looks like you are running into some math issues in the clock
which might happen more at 25fps. I have not tried this frame rate
much. I’ll look at it in a bit.

-Randy


April 2, 2006 | 7:42 pm

Thanks Randy for looking into it. I don’t think the problem is specifically
with 25fps. I’ve used other framerates too. All rates worked well before,
until I got rid of sfrecord~. I’ll just use the slow way for now.

Thijs


April 2, 2006 | 10:37 pm

Hi Randy,

I think I found the problem. 29,97fps is hard coded in some parts of the
smpte_clock javascript. 29,97 is the only render_node fps setting that
works, giving a 30fps output (?).

25 fps generates a 26.09fps movie.
30 fps output a 31.58fps movie.

So the non-realtime mode wasn’t the problem after all. Glad it works.

Best, Thijs


April 2, 2006 | 11:58 pm

if you read through the javascript, you will find that gFPS is
initialized with a default value of 29.97, but this global is changed
when the script receives the fps() message. If the script is not
receiving the message, this will cause problems. If changing the
default in the Javascript fixed your problem, it’s likely that the
message is not getting to the script for some reason.

-Randy


April 3, 2006 | 12:16 am

Maybe I did something stupid while hacking the patch. I’ll have a closer
look at it tomorrow. Thanks.

Thijs


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