Questions I have with optical flow/slow motion in final cut pro x

Hi all

I am new to final cut pro x, just purchased it last week. And I was trying to achieve the slow motion effect with the optical flow function in final cut pro x. But then I noticed something: Lets say I have a person walking across the scene in moderate speed. And I want to apply optical flow to this clip. After it finishes rendering, I get this ripple effect on that person's leg. Same issue to the clip with only my hand waving in front of the camera in moderate to fast speed. Then i will get the ripple effect on my hand even more. But then I did another test of my friend doing the club dance---the slow one, then I applied optical flow on my club dancing friend and the slow mo turned out to be perfect(no ripple effect). So my question is that do I have to have certain speed limit for the moving objects in order to make the slow mo work? (no ripple effect).

By the way, my camera is broken so i was doing all those test shots with my iPhone 4s.

Thanks in advance


EDITTED: I forgot to mention I usually put the slow down speed around 40--25%. And the lower the speed gets, the more ripple effect it shows on the screen. I think it is caused by the speed of the moving objects. Still, am I correct to think like that or there are other reasons?

Hi Sam,

There are many other reasons. The golden rule is: good slow motion is done during the shooting, not in the editing room.

Optical Flow analyzes your clip and creates intermediary frames to create the slow motion. The better your video looks the better optical flow will be able to create these frames. So what should you do to prepare your video for good optical flow? I will give you some examples.

1. You absolutely should not use optical flow on native long-GOP footage (H.264, AVCD, MPEG-2 etc...). Optical flow needs clear individual frames to create its dupe frames from. Long-GOP video only has a one or a few clear frames per second and this will make the optical flow analysis difficult. So if you have shot your video in a very compressed inter-frame format you should convert the footage to an all i-frame codec such as ProRes 422 or higher. Of course it is always much, much better to shoot in an all i-frame format using a high-quality camera from the start. The better the quality of your original video, the better optical flow will perform.

2. One thing that can be disastrous for optical flow is motion blur. Motion blur has the same effect as inter-frame video: the frames are not clear and this confuses optical flow. A good way to minimize motion blur is to shoot at a high shutter speed.

3. Most modern cameras allow you to shoot at higher frame rates. This is especially useful for slow motion.
Shoot at a high frame rate, drop the clip in a 25fps or 23.98fps timeline and use Automatic Speed to create a true natural slow motion effect out of the box. Then you can use optical flow to further slow down the footage, it will be much cleaner.

4. The more movement in your background the worse the slow-motion on your moving subject will look. A skater passing before a static camera on a tripod will produce lovely slow-motion. The same skater shot handheld while panning or zooming will look shitty.

5. Optical flow loves contrast. The more your moving subject stands out against the background (good lighting, subject in focus) the better your slow-motion will look.

- Ronny

ronny courtens wrote: Hi Sam,

There are many other reasons. The golden rule is: good slow motion is done during the shooting, not in the editing room.

Optical Flow analyzes your clip and creates intermediary frames to create the slow motion. The better your video looks the better optical flow will be able to create these frames. So what should you do to prepare your video for good optical flow? I will give you some examples.

1. You absolutely should not use optical flow on native long-GOP footage (H.264, AVCD, MPEG-2 etc...). Optical flow needs clear individual frames to create its dupe frames from. Long-GOP video only has a one or a few clear frames per second and this will make the optical flow analysis difficult. So if you have shot your video in a very compressed inter-frame format you should convert the footage to an all i-frame codec such as ProRes 422 or higher. Of course it is always much, much better to shoot in an all i-frame format using a high-quality camera from the start. The better the quality of your original video, the better optical flow will perform.

2. One thing that can be disastrous for optical flow is motion blur. Motion blur has the same effect as inter-frame video: the frames are not clear and this confuses optical flow. A good way to minimize motion blur is to shoot at a high shutter speed.

3. Most modern cameras allow you to shoot at higher frame rates. This is especially useful for slow motion.
Shoot at a high frame rate, drop the clip in a 25fps or 23.98fps timeline and use Automatic Speed to create a true natural slow motion effect out of the box. Then you can use optical flow to further slow down the footage, it will be much cleaner.

4. The more movement in your background the worse the slow-motion on your moving subject will look. A skater passing before a static camera on a tripod will produce lovely slow-motion. The same skater shot handheld while panning or zooming will look shitty.

5. Optical flow loves contrast. The more your moving subject stands out against the background (good lighting, subject in focus) the better your slow-motion will look.

- Ronny

Thanks for such a detailed input. I am new to editing and I can't say I understand everything you said:) But you gave me some basic understandings about slow mo and filming in general . Thanks again