Adding motion blur to still background object

I'm curious about what's basically the inverse of adding motion blur to a composited image that moves across the screen - Is there a way to add some kind of motion blur to a stationary composite that is occluded by a foreground object with a healthy amount of in-camera motion blur? Like if you were to add an image to a wall in a shot behind where someone runs across the frame. I can key the object easily (thanks magic mask), but it looks wrong when it passes in front of an image with hard edges.

What you need is proper soft alpha mask of the foreground object. Indeed it can be generated from hard masks using oflow and this is pretty common in vfx when hard roto is supplied. To do that with magic mask probably needs mask to be baked (pre-rendered) into RGB channels first, although you could try if you can force motion blur on top of it in downstream nodes.

What you need is proper soft alpha mask of the foreground object.

Are you essentially saying that it's not about motion blurring the background object, but rather just getting a good alpha roto on the motion blurred object? Like more or less "treat it like hair" or something like that?

Well, yes, because if someone moves fast over the bg it won't make bg itself blurry, it is the fg edge that is blurred. Unless I misunderstand your description of what you want to do: static shot with some picture on bg wall that someone walks past fast on fg. Mblur needs to be added to objects moving (static image on bg doesn't) and included in roto masks (fg object moves fast, thus its edge is mblurred).

Yeah, I'm probably overthinking it. For what it's worth, here's what I'm working with:

Screenshot 2022-10-06 063432.png (204.75 KiB) Viewed 276 times

The skateboard is rolling left to right, and the edges just look wrong when played at speed. I think part of it is that it's so thin and the composite behind it is so much brighter, you get a little lateral inhibition that makes the edges look off. There's still not that much motion blur from the skateboard to begin with, but the the comp behind it still feels like it's too sharp. I might just have a matte follow the skateboard that adds a little blur to the comp behind it.

Yes, that skateboard mask would need a bit more of mblur, when you compare the width of semitransparent area on original plate and where skateboard is over new graphic you can see that composite soft edges are not quite as wide.

Blending the graphics into your plate is another topic, as you wrote it is too bright and you should also try to match the black point and overall tint, your plate has a warm tint and graphics arepretty neutral. Match black outline of text to underside of skateboard or that dark area in the background and reduce overall gain, this should make it sit better.

If you want to keep the text as it is now, you can increase edge blur even more (overcrank it=, this will make it more visible on bright bg where otherwise it gets eaten into by bright backing.

Ah, great ideas. I'm using a transition effect on it too though that kind of depends on the RGB values, so I'll try those suggestions while trying to keep my transition looking nice. Thank you.