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Question about Motion Blur functionality

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Daniel Leary

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Question about Motion Blur functionality

PostSat Oct 23, 2021 8:03 pm

Maybe I am missing something with enabling motion blur on nodes. So hoping someone can shed some light on it. Can’t seem to find any in-depth section about blur noes in the fusion manual.

What I am running into is when I create an animated object (say for example a white square across the frame over a transparent bg) and enable motion blur, the motion blur looks fine in the viewers. When I export it out to use as a graphic element in resolve (making some motion graphics in my case) the motion blur is there but the color of the motion blur is not correct. Instead of the color of the motion blurred item fading away into complete transparency, it seems as though it is fading away into a back background before getting cut off by the alpha channel. See attached photo for example.
Motion Blur example.jpg
motion blur example
Motion Blur example.jpg (137.01 KiB) Viewed 1026 times


I have found out that the color of the bg layer I am using to set the frame size and have a transparent layer to begin with plays a role in the color of motion blur I get. Although the bg layer is set to alpha 0, the color of the layer seems to "bleed" into the motion blur. So setting the bg to for example red with an alpha of 0, creates motion blur that has red in it.

It seems that if I use the actual video the motion graphic will play over as a transparent background layer, it would be desirable for a better motion blur but is an issue when exporting a motion graphic layer to be used multiple times in an edit.
Also can see that being able to control the color of the motion blur based on the background color could have some unique uses so wasn't sure if this was normal or unintended behavior or if I am doing something wrong in making motion graphics that have motion blur.

So my two questions are:
    1. Am I missing something obvious about motion blur?
    2. Is this how these tools work?
    3. If so, is there a different way to do motion graphics that doesn’t “color” my motion blur when exported with an alpha channel?

Things I ruled out:
    It does not seem to be an issue of export format or settings or operating systems, they all behave consistently. I have tried exporting in dpx, tiff, ProRes4444 and DNxHR w/alpha and it all behaves the same way. From both Windows and Mac operating systems. I am using Fusion 17.1. And also this behaves the same way in the fusion tab of Resolve.
    It also is not an issue of editing software that the exported file is brought into. Seems to look the same on different editing programs.
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Bryan Ray

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Re: Question about Motion Blur functionality

PostSat Oct 23, 2021 8:19 pm

From the description, I'd say your RGB is getting multiplied by the alpha twice. Try adding an AlphaDivide node right before the MediaOut or Saver to disassociate RGB from A. It will look plain awful in the Fusion viewport, but it might help elsewhere.

I can't say for certain that that's what the problem is, but it's certainly what it looks like.
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peterbaumann

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Re: Question about Motion Blur functionality

PostTue Nov 02, 2021 6:08 pm

I have also noticed this behaviour when working in Fusion in DR17
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Daniel Leary

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Re: Question about Motion Blur functionality

PostMon Apr 18, 2022 10:06 pm

Bryan Ray wrote:From the description, I'd say your RGB is getting multiplied by the alpha twice. Try adding an AlphaDivide node right before the MediaOut or Saver to disassociate RGB from A. It will look plain awful in the Fusion viewport, but it might help elsewhere.

I can't say for certain that that's what the problem is, but it's certainly what it looks like.



Your AlphaDivide node solution fixed the issue. Not sure what exactly is happening in the background that causes this in some cases but the AlphaDivide trick seems to fix it every time I have run into it so far.

Thanks Bryan!
Rocky9.2, Intel X299 Platform, 64GB Memory, RTX3090, DeckLink Studio 4K, Chelsio T580
MacStudio M1 Ultra 64GB
Resolve Version: 18.5 & 18.6

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