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View Full Version : Motion Blur "Shutter Open" Setting Problem



kcole
02-11-2013, 09:59 AM
I've opened a support ticket on this, but thought someone may have a nice workaround idea for me.

A "brief" description of the problem:

The new "Shutter Open" setting is designed to alter the time window through which the frame is rendered, and for objects in motion this works exactly as expected. With a blur length of 50% and a shutter open of -25%, the object's apparent position with motion blur is centered on the current frame's position rather than 25% into the future (as with the default setting of 0% shutter open).

However, it appears that camera motion is not affected by the shutter open setting. The net result is that in scenes with primarily camera motion (as with motion tracked shots), the "Shutter Open" setting has zero effect. So, renders without motion blur track perfectly while renders with motion blur appear to float, anticipating the camera motion. This can be quite apparent with sudden shifts.

Until/If NewTek is able to correct this, the only workaround I can think of is to 'invert' the motion between the tracked nulls and the camera, locking the camera in space and having the nulls move in reverse, to match the shot. If that could be accomplished, the Shutter Open setting should work fine. Any hope this can be accomplished without becoming a Python guru?

EDIT: Here's an example of the issue, with a telephoto perspective camera to reduce perspective. In the top shots the red cube moves, and in the bottom shots the camera and background plane move, in reverse. If all were working as expected, the top and bottom shots should match.

https://www.dropbox.com/s/3lxlx401onn5m67/ShutterOpen.png

Slartibartfast
02-12-2013, 03:55 AM
Would it work if you turn on fractional frames and shifted all keys by a quarter frame? Just an idea. Probably a stupid suggestion :-)

jwiede
02-12-2013, 04:04 AM
Would it work if you turn on fractional frames and shifted all keys by a quarter frame? Just an idea. Probably a stupid suggestion :-)
Hehe, I almost posted that exact same suggestion earlier today. Has some problems, though, esp. if motion modifiers/expressions/etc. are being used anywhere in the scene. What's really needed is for the internal representation of time to shift by same amount as "Shutter open", if populated, but that might be a nightmare to implement. If all anim is keyframed, the subframe adjust might be an acceptable workaround, though.

kcole
02-12-2013, 06:04 AM
Would it work if you turn on fractional frames and shifted all keys by a quarter frame? Just an idea. Probably a stupid suggestion :-)

You are absolutely correct - this works perfectly in simple scenes, with the limits jwiede posted and probably more we're not thinking of. I would guess frame-based volumetrics such as TurbulenceFD wouldn't match up either.

I rendered a test last night with all frames except frame 0 shifted forward 1/4 frame. Motion blurred objects are now spot-on.

It's possible that a combination of these two techniques may work - move only the camera's keyframes forward 1/4 frame, then set 'Shutter Open' to -25%. If that setting works with everything except the camera (as it appears to), that would be a full workaround until the issue is fixed (assuming NewTek agrees it should be changed).

One other oddness I noticed. To reproduce, just add an object to a blank scene and set an X position of 10m at frame 10. Move to frame 5 and you should see your object at 5m. Now enable motion blur and set Shutter Open to -25%. Nothing changes. However, render a frame and your object will now report a position of 4.75m. All keyframes are shifted (although they don't show on the timeline as shifted). Worse, if you try to set a keyframe for 5m at frame 5, it will actually set that keyframe at frame 4.75. To revert the scene, change Shutter Open back to 0% and render a frame. Your old keyframes revert but the newly added one stays at frame 4.75. I hope this is a bug and not the permanent behavior for this setting.

vncnt
06-06-2015, 02:10 AM
Can someone confirm if this has been solved in LW2015?

jwiede
06-06-2015, 03:39 AM
Can someone confirm if this has been solved in LW2015?

Hope so, but expecting not: From the behavior kcole described in their last paragraph, LW is internally doing that exact fractional keyframe trick proposed, but hiding it by faking the time value display. Unfortunately, the symptoms described are intrinsic side effects of a disconnect between render current fractional time and displayed current fractional time (I'm betting others exist along lines of what I said).

vncnt
06-06-2015, 02:22 PM
In that case Ill move the Camera keys by myself. That worked fine in the past.
Noticed that my backplates were shot with a shutter angle of 90 so motionblur is not really needed in my case (and it solved really fast in SynthEyes because of the lack of motionblur).