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Tips on motion blurring a road

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Umberto Uderzo
Tips on motion blurring a road
on Oct 13, 2015 at 8:16:06 am

Hello all!

I'm trying to add some motion blur to a clip shot from over a car roof. The car nose is visible on the bottom of the frame and the road is coming towards the watcher and disappears under the car (quite obviously). Well, some artifacts are visible on the boundary between the road and the car, as the optical flow algorithm evidently lacks enough information for a good motion detection.

I'm planning to reduce or remove the motion blur effect from this shot because it's too easy to spot the artifacts in this case. But before doing that, i'd like to know if there is some tip to alleviate such artifacts that i can apply on my footage.

Thank you! Umberto


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Pierre Jasmin
Re: Tips on motion blurring a road
on Oct 13, 2015 at 3:16:24 pm

Two things to try :

Set sensitivity to a much lower number where it fails (it's animatable)

Apply two times, changing Track frame to Next the second time, set Amount to 1/2 of normal and blend the two results 50%

Pierre



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Umberto Uderzo
Re: Tips on motion blurring a road
on Oct 14, 2015 at 7:40:29 am

Thank you for the hint, Pierre.

I had a run to it on my clip and results are different:
If i apply the two effects in sequence, i get a slightly better result, but the artifact is still visible.
If i apply them in parallel and merge them with 50% opacity each, i get a lot of aliasing due to details being blurred in opposite directions, i guess this is due to the fact that the perspective changes the amount of blurring of the same clip area when applied on previous and next frames.

Looks like the smart blur doesn't interpolate fluidly the blurred frames, but has some "micro steps" that are evident when merged in such a way.

I also noticed this trying to blur rolling credits titles: while in some frames the title movement is perfectly smoothed, in some others i notice the "smoothing steps", that leave a 1 pixel gap between each other.

Is it to be expected?

Umberto


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Pierre Jasmin
Re: Tips on motion blurring a road
on Oct 14, 2015 at 7:49:57 pm

In RSMB Pro you could use a matte input to separate the front of the car (assuming it does not move in frame much so it's easy), unfortunately multiple input is not currently well supported (if at all) by Resolve.

Yes for simple text scroll (like flat white text) sometimes animating a vertical blur works better.

Pierre



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Umberto Uderzo
Re: Tips on motion blurring a road
on Oct 15, 2015 at 9:27:22 am

Too bad Resolve doesn't publish all the parameters, i have access only to few of them, i guess they are only the numeric parameters.
Would a foreground mask in RSMB produce a different result than masking the RSMB result after the processing?

And... about the Pro version, i tought the only additional feature over the Standard version were the motion blur based on 3d velocity pass, useful for 3D rendering.


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Pierre Jasmin
Re: Tips on motion blurring a road
on Oct 17, 2015 at 8:21:41 pm

No Pro also adds support for layering (there is when Pro is available 3 plug-ins).

RE: Splitting in two - with a matte on one and the reverse matte on the other and merging back

The way I understand your shot is there is a part in front that does not move, for that part you could isolate it with a matte - (assuming it does not move). The issue comes from the fast background motion that gets contaminated by the foreground not moving. This splitting would not help the background.

Conclusion:

Let me document this publically here so when discussing with Resolve engineer there is a clear example of this and let's call this the Dashcam issue - the camera is inside the car moving fast, the foreground as a result does not move or little and the street moves real fast.

1) With support for an additional matte channel (single channel) we could use this to do the Motion Blur segmentation - internally by having that information we can prevent the static foreground from affecting the background motion. That alpha channel must not have any impact on compositing within Resolve, it's just to guide this filter.

2) To keep the same example having option for a full second input RGBA image would allow for example to composite something in the car (for example showing speed in burmese system - kawtha per hour? for the version for that market ). Using the original image sequence as Motion Source allows the composited image to get matching realistic motion blur when the car hit bumps on the road etc.

To do both at once would require 3 inputs, but that's rare you need that and might be too hard for Resolve right now so let's ignore such special case, plus they have another product, Fusion, which at some point becomes a better place for things like that.

Pierre



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