motion bur issues

[stevesideas]

I'm having major problems with matching motion blur from a rendered comp with motion blur at render time with a background with velocity pass using RSMB.

If you check out the attached image you will see that the motion blur at render time (the helicopters) does not match the velocity vector render blur using RSMB in aftereffects.

Also if you check out the 2nd image with the blacked out half section you will see I have another problem when using motion vectors when close up to an phoenixocean plane.

[Joelaff]

Which app are you compositing in? It looks like RSMB is set to NOT be premultiplied (when it should be premultiplied).

What is the problemn in the second image?

[Joelaff]

Also, are you referring to the weird lack of motion blur in the background on the first shot?

Can you post your motion vectors? You rendered the vectors in floating point format?

The weird lack of motion blur almost looks like your rotor is a circle, rather than individual blades, and thus it is blocking the vectors in the background.

(Love the oil rig, or whatever it is, model.)

[stevesideas]

I'm using aftereffects. I'm not sure what you mean by setting RSMB to premult. The background is using the motion vectors. The helicopters are composited over the top using motion blur at render time so the motion blur for the helis should be spot on. The rear rotor is 4 blades rotating. I calculated the rotation speed for both rotors using average speeds I found on the net for various choppers.

I have linked to 3 layers from frame 185. All are 32bit exr's.

The 2nd picture should show water for the entire frame but for some reason when using velocity vectors it cuts off half the image to black for the first 5 or so frames unit the camera moves away from the ocean plane. I've had this happen before so I'm not sure if this is a bug or if it is how I'm exporting or using the velocity channel.

https://dl.dropboxusercontent.com/u/...%20testing.zip

[Joelaff]

I had no problems blurring it with RSMB in Fusion (see attached). I don't know how fast the camera is moving so I took a guess at the amount.

What result do you get when you use these same files you posted (that same frame)?

Check your alpha (both on the vectors and the image) on the frame you blurred. Also, check that the vectors and RGB image are the same dimensions and pixel aspect ratio. I see some areas that are not blurred on yours (at the back). What do the motion vectors look like there? They are fine on the frame you sent.

Also, and this is likely the issue: your images are saved as EXRs, but they are in sRGB (at least gamma 2.2 space, not linear, gamma 1.0 space). You likely need to adjust the interpret footage setting for your motion vectors and go to the color management tab and turn OFF Interpret as Linear light (the popup). AE is likely interpreting your motion vectors as linear, when yours are gamma 2.2. This will change the direction and amount of blur.

Note that your motion blur on your helicopter in your comp looks odd (white halo) because the image needs to be premultiplied (in other words import it as a straight alpha in AE). Alternately you could render it premultiplied against black. Note that this image is also not in linear space. For maximum quality you want to set your color mapping to either Color Mapping only (no gamma), or None. Then do the color mapping in the post.

[Joelaff]

If you post the other images shot I can take a look. Perhaps a clipping plane issue? I see you have clipping planes on in the camera (it was in the meta data in the EXRs you posted).

[stevesideas]

Thanks for taking a look at this.

The speed shown in the velocity pass was about 48 at that frame.

I get the same results. I realised that the EXR was in sRGB so I interpreted them. The result I got is shown above in the first post.

I tried rendering against black for the helis but ended up having a halo round the body of the chopper even though I had selected straight in interpret footage. The problem with this method also is the fact you lose detail in the alpha. If you turn off the alpha for the helis you can see more detail in the rear rotor. As soon as you blend the helis using straight the detail disappears.

Everything is using the same dimensions (1920x1080) with the same pixel ratio. I've been using elements from exactly the same scene for all passes.

You should be able to see from the images I've sent you that all images are rendered out to the same pixel dimension etc

I have my color mapping set to color mapping only (no gamma).

Assuming the velocity channel is being calculated correctly for the background shot, why would the helicopters motion blur be so different ? I really do not get this.

[stevesideas]

If you post the other images shot I can take a look. Perhaps a clipping plane issue? I see you have clipping planes on in the camera (it was in the meta data in the EXRs you posted).

I have clipping planes off at render time so this shouldn't affect anything. The only reason I use clipping planes was to calc z-depth using clipping boundaries but this is only taken from the values they are set at. You don't need the clipping plane on.

[Joelaff]

Perhaps I misinterpreted what your issue was with the original image. I thought it was the areas in the background that were not blurred properly.

So with frame 185 you get results like what I posted? What is it you are objecting to, then? The way the distant helicopter is sharper in the render? Can't you just turn down the amount of the blur in RSMB until it matches? Motion vectors this way are generally not precise (I.e. .5 amount is not always going to match a 180 deg shutter perfectly). You end up having to eyeball the amount to taste.

Regarding the alpha issue, everything should work fine if you render out to linear space with a premultiplied alpha. How are you generating the gamma 2.2 exr image now? You have linear workflow off in Max? Maybe after effects in misinterpreting your alpha channel in the wrong space because it thinks it is linear in the EXR. (Don't know for sure; we use Fusion, hardly touch AE). If AE thinks your alpha is linear, and then doesn't fix it when you set the color management to off because this should normally only affect the RGB this could be the issue.

Try rendering the RGBA to another format (16bit per channel png or something) and see if it works. Or render out to linear EXR files. A premultiplied alpha against black should work fine (assuming it is unfulfilled internally in floating point).

I think the non-linear EXRs are confusing AE. You might try toggling Linearize Working space in AE.

[Joelaff]

In the water closeup shot does the RGB image have the big black area? Or does that only happen when you apply RSMB? What does the motion vector file look like on that one?

[Joelaff]

Is it possible you have camera motion blur disabled in the render so only object motion blur is on?