[oz42]

Hi,

I've done a couple of quick animations with Maxwell before but right now I'm doing an interior walk through (!). I'd like to add motion blur for realism but I am having a bit of a mental block as I've not used it before.

Initially I just turned on Motion Blur in the Global Render Option (I'm using Softimage, by the way) and the results looked terrible - just a huge motion blur smudge! I thought the camera was moving too fast but then realised that my shutter speed was set to 1 second! (I had the f-Stop at f22 and ISO at 100!). And I guess that, true to it's physical nature, the Maxwell camera rendered with this exposure and therefore caused huge blurring.

I dropped the shutter speed down to 1/25 second (I'm rendering in PAL), adjusted the f-Stop and ISO to suit and that reduced the blur a lot. Is this the correct way to do things? Do I need to set the camera shutter speed to the same as the frames per second?

I then also turned on the Rotary Shutter motion blur in the camera settings (initially leaving the Shutter Angle at 180 degrees) and this reduced the motion blur even more. Finally I reduced the camera rotary shutter angle to 15 degrees and this almost removed the motion blur.

In the manual (p 21), it give some equations but my figures don't seem to work out;
Shutter Angle = FPS * 360 / Maxwell Shutter (exposure) which in my case gives; Shutter Angle = 25[FPS] *360 / 25[th second] = 360 degrees!
and % of motion blur = Shutter Angle * 100 / 360 which in my case gives; % of motion blur = 360 * 100 / 360 = 100%!

Has anyone else played with motion blur and can give me some helpful advice please.

[Mihnea Balta]

In a real film camera, the longest exposure you can get is equal to 1/fps, because that just means keeping the shutter open all the time. If the camera takes a picture 25 times per second, there's no way to expose the film for more than 1/25 seconds, because then you wouldn't be able to take 25 pictures in the first place.

Maxwell doesn't have this limitation because it's software, but if you exceed the physical limit it will produce unrealistic images. The anomalies will be hard to spot, but if your exposure is longer than the frame time you can have a motion trail in frame N which extends beyond where the object actually is in frame N+1, which is impossible in reality. Also, due to the fact that Maxwell is currently limited to linear motion blur, objects with curved motion will have trails that are not aligned to their actual paths.

The rotary shutter is a device which allows you to control the exposure time as a fraction of the frame time. It's an occluder shaped like a circle sector which rotates in front of the film and it does a full rotation each frame. The angle of the open part of the circle determines how long the film is exposed during each frame. A shutter angle of 180 degrees means the exposure time will be half the frame time. 360 degrees means there's no shutter and the film gets exposed for the whole duration of the frame.

In Maxwell, the rotary shutter is a way to change motion blur without affecting your exposure. If, for example, you have a scene set up for a still like you did, with an exposure of 1 second and f/stop 22, when you enable rotary shutter, the plug-in will export the shutter time computed from the shutter angle and fps and then it will adjust the ISO to get the same exposure as you would with the original shutter time. Example:

Shutter time: 0.5s
ISO: 100
Shutter angle: 90
FPS: 25

The frame time is 1/25, or 0.04 seconds. The shutter angle is 90 degrees, so the film has to be exposed for a quarter of the frame, i.e. 0.01 seconds. That's 50 times lower than the original shutter time, so the ISO will be multiplied by 50. If you check the exported MXS in Studio, you will see the following values:

Shutter time: 0.01s
ISO: 5000

This results in the same exposure as shutter time 0.5s and ISO 100, but the motion trails will be significantly shorter.

[oz42]

Minhea,

fantastic explanation, thank you very much (I think you ought to put this in the next version of the manual!).

Just to make sure I understand;

1. For realistic animations always keep the shutter speed at or below the FPS of the animation
(i.e. use the shutter speed (and ISO) to adjust the exposure for a given f-Stop but don't exceed the FPS)

2. Once the exposure is set, use the camera's rotary shutter angle to alter the length of the motion blur trails.
An angle of 360 degrees gives trails the same length the shutter speed defines, lowering the angle reduces their length.
(changing the rotary shutter angle will not change the exposure as the ISO is automatically adjusted to suit)

Is this right?

[Mihnea Balta]

Yes, that's a good workflow. Technically you can set any shutter time if you're using the rotary shutter because the plug-in will ensure that the exported shutter time is always less or equal to the frame time. However, if you set a realistic shutter time in the first place (like 1/fps) it will be easier to reason about the quantities later.

[oz42]

Mihnea,

thanks for the confirmation. This definitely need to be distilled into the next version of the manual...