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By: [ Admin ] Asked from India

3d motion blur and retimed plates

If a plate is sped up 250%, and 3d elements will be added, should the shutter in the 3d software be reduced by a factor of 2.5? Or is it best to apply some extra 2d motion blur to the plate and render with the usual shutter settings - or apply extra motion blur to everything in comp?

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2 answers

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hugh_gid [ Editor ]

I would recommend the following:

  1. Track the unretimed plate.
  2. Retime the plate (for use as the comp backplate)
  3. Retime your track, using the same curve as the plate retime
  4. Clean up retime artefacts on your retimed plate
  5. Comp your (already retimed) CG onto this.

You're right about the motion blur - if your plate is at 200% speed, then your shutter angle will be 50% (divide by 2)

I would say that, if you do desperately want to add the motion blur back on (and generally, people don't want to) then you might be best off doing this in comp on the whole thing. Or, if it looks good, when you do the retime, if you use something like Furnace, get it to do the motion blur at the same time, and render out your CG with the shutter angle that you want.

NN comments
julian
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i just assumed one would want to push the motion blur back to the level it would be if the shot was not retimed.

julian
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I mean, the same as surrounding un-retimed shots from the same camera

hugh_gid
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In theory, yes, but this can prove quite tricky, especially if there is a lot of fast movement. It really depends on how much time you want to spend on it (or money the clients have to spend on it!). In an ideal world, yes – you would absolutely want to get it back to the same level, but this often ends up having to be faked…

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ian_68

Hi Julies

The renders/tracking etc should be done in the normal way with the pre-retime plate. Once they are composited together you do the retime of the final files. That way they will match for sure.

hope this helps, cheers Ian

NN comments
hugh_gid
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I have to say, I’d disagree… Cleaning up retime artefacts in comp can be a real pain. Even more so if you’ve got to clean up retime artefacts on CG that could be rendered already-retimed in the first place.

julian
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– what are retime artifacts?

hugh_gid
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When you get, maybe, double edges.

If you’re just doing a round number retime (200%, 300%, etc) then the retime will, in its simplest form, just be using every other, or every 3rd frame of the original. However, with a 250% retime, half of the new frames will actually be half way between two of the original frames. So it’ll have to figure out what this should look like. That will often leave you with double edges, or tearing, depending on the kind of movement and quality of the retiming tool that you’re using.

julik
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This approach DOES NOT work, unless you feel like painting for a week.

ian_68
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I see your point Hugh. If the retime has large amounts of artifacts then having a clean CG render will help. If the artifacts can’t be removed in the plate, then you would want them on the CG, other wise the CG will not blend with the plate.

cheers, Ian

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