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I am well aware of the Exposure setting, however that appears to just change the luminosity of the image (doesn't make sense to have the option even there if that is all it does, any other insights?).

The effect I am trying to achieve is like the classic night time city shot with the traffic blurred along the route. Is the only way to achieve this to animate light sources going very fast and use motion blur? This seems like it could be a very inefficient method, and might yield bad results.

Also note that the render times could be quite immense (8+ hours each) so rendering as few frames as possible is necessary and will help decide which method I use.

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You can use motion blur, and you don't even have to animate the objects going absurdly fast or anything. Just set the shutter speed really slow (just like on a real camera):

enter image description here

However, there is one rather major problem; by default cycles only uses one step of interpolation between the positions of the object at the beginning and end of the exposure. This is fine in most cases, but for long exposures this is likely won't do at all:

Animation:

enter image description here

20 frame exposure:

enter image description here

To convince cycles to interpolate between start and end locations with more steps, we have to use Deformation motion blur. This allows us to set the number of interpolation steps per object in Properties > Object > Motion Blur.

20 frame exposure with 5 interpolation steps:

enter image description here


Setting up deformation motion blur

To do this everything must be in a single object (motion must come from the mesh itself deforming). A simple way to accomplish this when you already have animated objects:

  1. Duplicate the objects you want to be blurred
  2. Join them into one object
  3. Hook each individual part of the joined object to it's corresponding original object (Select the original object, then the joined object, enter edit mode (↹ Tab), select the part with L, and create a hook with ⎈ CtrlH> Hook to selected object)
  4. Hide the original objects in the render so they don't interfere

Once everything is joined into a single object, cycles will use the steps setting in Object > Motion Blur for determining the number of interpolation steps. Select the joined object and bump this up, e.g. to something like 5:

enter image description here

Result:

enter image description here

Note that deformation motion blur is quite a bit slower than normal motion blur. Ideally there would be a interpolation steps option for object level motion blur too, but unfortunately this is still on the todo list.

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  • $\begingroup$ This looks real promising! $\endgroup$ – VRM Nov 18 '15 at 23:38
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I would suggest render an animation of the scene (in this case a textured disk depicting the sky) where each frame represents a simulation of one film's recording step.

enter image description here

and overlay all the rendered frames with an Add blending mode.

enter image description here

Above you can see an image made with the first 100 frames of the animation (with every layer's opacity lowered to 10%).


I composite the layers in an external image editor because it was faster to set the Add blending mode, but with paticence and/or the helps of some trick I would be able to do that in the Blender's compositor too.

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  • $\begingroup$ Related: blender.stackexchange.com/q/32718/599 $\endgroup$ – gandalf3 Nov 18 '15 at 23:11
  • $\begingroup$ Note that with a scene this will be harder to combine, and also at quite possibly 8 hours per frame, this is probably not an option. Good idea though. $\endgroup$ – VRM Nov 18 '15 at 23:15
  • $\begingroup$ Thanks! May I ask why you think they would be harder to combine? You just have to work with the "final frames", the ones before the encoding. Rendertimes can be a problem, but maybe you can render for the effect only the parts in motion, and depending on the trail size you want, perhaps you just need 5-6 additional frames. If you are planning to make an animation, consider that you are going to render most of all anyway. $\endgroup$ – Carlo Nov 18 '15 at 23:32
  • $\begingroup$ BTW BI has a built in implementation of this method of motion blur $\endgroup$ – gandalf3 Nov 18 '15 at 23:40
  • $\begingroup$ Thanks for both links @gandalf3. I didn't think bended trails like this were achievable with motion blur. $\endgroup$ – Carlo Nov 19 '15 at 0:13

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