Motion Blur on backplate

[Ealexander]

Hi,

I've got a simple scene with a car model, a backplate, a HDRI to light and a plane with a shadowcatcher under the car.  Easy.  I'm trying to add motion blur and have watched tutorials, searched forum, etc. but I can only get the car to blur and not the backplate.  What am I missing....

Keyframed the car at Frame 01.  Advanced to Frame 90 and moved the car forward and keyframed it.  Made the camera a child of the car.  Turned on Motion Blur for both object and Camera.  Adjusted my Shutter Speed.  Advance to frame 30ish and hit render.  Car is blurred, backplate is perfectly in focus. Level of blur looks good, it's just that I want it inverted (car focus, backplate blurred).

I'm sure I'm missing something simple.  Any thoughts?  Thanks team - the model and the backplate are behind NDA, so I can't share the file, but will make a dummy/proxy file if that is needed.  Using Corona 9 in R2023.1.0

Thanks!

-Evan

[Beanzvision]

Hi there, if you can share that ''dummy'' scene, that would be great! :)

[pokoy]

Since the camera is now moving, you probably also need to enable motion blur for the camera, too. Otherwise it won't render its motion blur against stationary objects.

[Beanzvision]

Hmm, I have also tried to reproduce this but it isn't giving the results as expected. Camera is linked to the car which is moving forward (the front wheels have correct motion blur), the bystander is stationary and also has correct motion blur. But not the ground.


With a different setup using the HDRI in dome mapping mode, the result was much better.

[pokoy]

Probably because the backplate gets reprojected continuously on the geometry when going to the next frame (assuming you are using a Shadow catcher material) so it is not stationary, hence no motion blur. You would need to map it on the geometry so it is stationary. I'm on Max where you would use camera mapping to do this, can't comment on how this would be done in C4d.

[Ealexander]

Thanks all!

Beanz - your second snowy shot is what I'm after, but alas, I have TIF backplates I need to use and the HDRI won't work (graphically) in Dome mode for what I need.

I suspect Pokoy is correct and the Shadowcatcher projection is screwing things up - I linked the camera to the moving car, but I didn't link the Shadowcatcher plane - going to try that next and see if it works.  If not, I'll recreate my setup in a client friendly file and upload.

Thanks!

[Ealexander]

So I set up a simple scene, and I'm getting the same results.  Car is blurry and backplate and Shadowcatcher are not.  You can download the files here:  https://evanalexander.digitalpigeon.com/msg/sI_rcOOuEe2tSgIULjfkoQ/Of6cw2gp1GKhXXRFMJER7g# - Motion Blur test

Also - when I turn on IR in the VFB I get the blur on the car, but when I hit RENDER in the VFB window, no blur.  in the IR mode, I have to change the Shutter Speed before the blur shows up.

Ultimately, the goal is to have the car be in focus and the backplate to be motion blurred.  I'll deal with the wheels spinning, later, now I'm just trying to get the base set up.  I can't be the first person doing car renders with backplates and MB here....?

Any thoughts or help appreciated!

[TomG]

Blurring backplates is usually done by rendering with alpha to remove any backplate, then adding it in post, using some "faked blur" in Photoshop to blur the background. Thing is that a backplate has no actual location in 3D space, so it can't "move" in any way and thus can't be blurred - it's always in the same place in relation to the camera.

[pokoy]

Blurring backplates is usually done by rendering with alpha to remove any backplate, then adding it in post, using some "faked blur" in Photoshop to blur the background. Thing is that a backplate has no actual location in 3D space, so it can't "move" in any way and thus can't be blurred - it's always in the same place in relation to the camera.

Like I suggested above, a workaround would be to map geometry with the backplate so it's stationary. It may need some tricks, like extending the frame mapped on the geometry so it covers the entire area that might come into the frame when the camera moves. It's something that's done in VFX regularly. But it can be quite some work if you have overlapping geometry, it's far easier if it's just a ground plane and a horizon plane in the background for example.

But using Shadowcatcher mtl means the backplate will be projected in every frame with the same static image which means no motion blur.

[Beanzvision]

But using Shadowcatcher mtl means the backplate will be projected in every frame with the same static image which means no motion blur.

Yep, indeed.

[Beanzvision]

So, the way I have done it here is to create a huge stage with the backplate set to frontal projection. A composite tag with only seen by camera checked. An HDRI for the lighting with a comp tag with only ''seen by camera'' unchecked. No shadow catcher used.

[Ealexander]

Thanks Ben! I'll give that a try. Here's a quick test mock up where I just blurred the backplate (poorly) in Photoshop and then animated the wheels to MB them. Seems to work great. Now I need to spend some time making the backplate blur look better.

[Juraj]

I am using https://www.virtualrig-studio.com/#index for motion blurring backplates, I am not sure if there's any alternative that is on par.

"Staging" Backplate = i.e creating fake camera mapped little world only works in super specific scenario, one that is particularly either super open (flat salt plane for example) or very clean strict geometry shapes (in front of architecture). Any more complex depth and the result would be terribly fake. Even dome-mapping looks rather terrible in most situations.

[Ealexander]

I am using https://www.virtualrig-studio.com/#index for motion blurring backplates, I am not sure if there's any alternative that is on par.

"Staging" Backplate = i.e creating fake camera mapped little world only works in super specific scenario, one that is particularly either super open (flat salt plane for example) or very clean strict geometry shapes (in front of architecture). Any more complex depth and the result would be terribly fake. Even dome-mapping looks rather terrible in most situations.

Oh this looks cool - thanks for the link.  Downloading the trial right now.  Path blur in Photoshop only took me so far.  This is why I love message boards - so much to learn!

Also - some really great work on your site!

-evan

[BigAl3D]

When blurring the backplate, remember to match the angle of the blur to the direction of the camera/car. Also, break your backplate into visual planes and apply a lesser amount of the same blur to each section with the far distance getting just a little blur. This is a very quick example using a still I found and very basic blur of the wheels.

If you want to go crazy, you can use a gradient to change the blur as the distance increases. Subtle, but adds to the realism.