On Jan. 6th, I got a notice from CG Cookie saying there was a new animation course coming. This got me wondering about the infamous knee-pop problem in character animation.
I signed up as a citizen a couple of years ago and ran into the knee-pop problem, but no one could outline a technique to get past it, not the instructor and not any of the other students. I may have asked on this forum as well. I don't remember. The best thing I was told was to keep digging and it would eventually just happen. It didn't.
This knee-pop problem was insurmountable for me. I couldn't figure it out, got discouraged, dropped my citizenship, and put both Blender and animation aside.
I've been following Blender development since the release of 2.8, but haven't seen any new features that would seem to address this issue, so I thought I'd come back on CG Cookie to ask...
Thanks.
Hey Ron,
I remember this question. I'm sorry that was so frustrating for you. I'll spare you the long explanation of why you are experiencing knee pops because Wayne did a pretty good job in the last topic.
Unfortunately, there is nothing new in the current builds of Blender, or even Maya or any other animation software that will solve this problem. There are some rigs out there with what's called a "soft IK" that prevents the knee from snapping around but that feature can some times pose its own problems. It really comes down to perfecting your body mechanics. Make sure the hips are doing what they need to do, then confirm your feet are place where they should be and when that is all said and done track the spacing of your knees. Do what you need to do to make sure the knee is constantly moving along its path and not freezing for a frame.
Another user was having the same trouble you were so I created a very impromptu video walk thru on the techniques I use to fix his shot. They seemed to help him get over that hurdle. If I can find them would you like me to send them to you?
Yes, please do send them. Thanks, Phil.
Where is the topic you're referring to where Wayne explained why the knee-pops were happening?
This is the topic I was thinking of. https://cgcookie.com/questions/9656-how-to-deal-with-knee-pop-in-a-walk-cycle
It looks like I may have gotten rid of the videos, I'm asking the person I made them for if he still has a copy.
If I can't find them, I would be happy to do the same thing for a shot of yours if you would like. Just send the Blend file and I'll record the demo.
Hi Ron,
The new animation course is on the way, however, it is a revamp of the "Fundamentals of Animation". Which is the beginner stuff.
So the new course doesn't cover character animation or knee popping.
Like phoenix4690 mentioned, popping is a problem with the IK system. It has a mathematical cause where the acceleration of the joint approaches infinity. (see a tan curve - as in a sin and cos curve. The tangent wave has a discontinuity where it shoots off to infinity).
You can combat it by using a soft IK set up, where it dampens the speed of the knee joint so it never actually reaches the goal. But this has a its own set of different issues and it only solves half the problem (bent to straight, not straight to bent).
Sadly, the only real solution is improve your body mechanics and then tweak the knee with the bend controller as a very last resort.
I posit that you worked hard and improved your skills to a certain level where you could actually see the issue (most people can't) and didn't quite get to the point of resolution. I guess this is one of the down sides of improving your skills, the better you get, the more you see where you need to improve.
But don't let a knee pop stop you from animating!
Hi Phil,
I had a look, but I can't find any of the exercise files I was working on back then. They were just my attempt to do the Rivet walk cycle, the final exercise in Wayne's Animation Bootcamp series, so not my models or scenes, anyway.
Since it's been so long, I'll likely have to start the entire course from scratch again, it would be a while before I got to that exercise again. That said, I'll thank you for your kind offer and contact you again if/when I'm in a position to send you a file.
Hi Wayne,
> The new animation course is on the way, however, it is a revamp of the "Fundamentals of Animation".
Has very much changed between 2.79 and 2.8x, enough to warrant redoing the course?
> Like phoenix4690 mentioned, popping is a problem with the IK system.
I read somewhere that they're revamping Blender's animation system. Perhaps they'll finally address this.
> You can combat it by using a soft IK set up
What's a soft ID set-up? Is it build into one of the currently-available rigging systems already?
And speaking of current rigging systems... I'm assuming you've used them all. Which (if any) best addresses this issue?
Another way to approach this knee-pop issue would be to study other people's work wherein it's been solved, right? With that in mind, I looked at the standard walk cycle on Mixamo and to my untrained eye, it looks like no knee-pop. Could someone verify this, please?
https://www.mixamo.com/#/?page=1&query=walk (I'm looking at the third one listed on the left, top row)
Has very much changed between 2.79 and 2.8x, enough to warrant redoing the course?
The UI has changed as well as the buttons. The process is the same, however, users get confused when you show old software that isn't quite the same or drastically different that the current version.
I read somewhere that they're revamping Blender's animation system. Perhaps they'll finally address this.
The things that they are working on this year are the new library override system (to replace the proxy system), increasing the speed of the view port playback and a basic version of an asset manager.
Knee popping is a mathematical problem. It occurs when the IK goes too fast from straight to bent or vice versa. You can't change this behavior unless you can change the way mathematics works.
What's a soft ID set-up? Is it build into one of the currently-available rigging systems already?
Soft IK is a modified IK set up (extra drivers), that doesn't let the chain ever get straight. This acts as a way to dampen the 'pop'. However, it has its own set of problems (the foot won't ever reach the control when stretched, and it doesn't solve the pops when going from straight to bent)
And speaking of current rigging systems... I'm assuming you've used them all. Which (if any) best addresses this issue?
Yes I've used normal IK, stretchy IK and Soft IK. The best thing that addresses the issue is the actual animator.
I prefer Stretchy IK (even for realistic animation rigs) and then I control the knee pops by controlling the body mechanics.
Those animations look like edited motion capture to me. What does that mean?
It's motion capture that is tidied up by a artist. (But I could be wrong)
With motion capture, you will find 'pops' (most of which are cleaned by an artist) however, when it's captured from a real person the body mechanics come for free - if that makes sense.
In the one that you pointed out can you see that the both knees are different? The Right knee overshoots and pops before the contact where as the Left knee does not.
This type of pop is obeying the body mechanics.
This happens on a lot of people's walks - depending on the style of walk. (walk cycles are really complicated, if you didn't already know haha)
Thanks for taking the time to keep answering my questions, Wayne. It's very kind of you.
Yes I've used normal IK, stretchy IK and Soft IK.
:) I see now how I wasn't very clear in my question. I didn't mean the styles of rigging systems, but rather... well, let me try again...
Of the current rigging systems (Auto-Rig, Rigify, BlenRig) which makes the knee-pop issue easiest to deal with? Or, in light of this comment:
I prefer Stretchy IK
Which of them as Stretchy IK? (Because I can't tell.)
In the one that you pointed out can you see that the both knees are different? The Right knee overshoots and pops before the contact where as the Left knee does not.
I'll look more closely at this, rendering out the frames and loading them into layers in Photoshop where I can use guidelines to mark where each knee is on any given frame.
This type of pop is obeying the body mechanics.
This happens on a lot of people's walks - depending on the style of walk. (walk cycles are really complicated, if you didn't already know haha)
It's a wonder any of us ever learn to walk! :)
Seriously, though, if these mo-cap examples aren't good for study, can you point me at a good example of a walk cycle so I can study it and figure this out?
Also, what if the camera angle is changed slightly to cover up the knee-pop? Make it less noticeable? Or maybe I'll just chicken out and make sure my characters' knees are always hidden behind shrubbery and such. ;)
One final thing...
I came across this video: explaining why knee pops happen and how to fix them in Maya. Because of the similarity in this Maya rig and those used in Blender, I'd assume this technique could be adapted for use in Blender. That just leaves the question of which rigging system (Auto-Rig, Rigify, BlenRig) would lend itself best to this approach. If this can be adapted, perhaps it might inspire a quick lesson on fixing knee pops here on CG Cookie?
Hey @waylow , I see that you mentioned that the new Fundamentals of Animation course is on the way. Do you know approximately how many weeks/months it will be until it drops?
Looking forward to watching it.
Thanks!
Hi Ron. You're welcome.
Here's some more answers for you.
IK/Stretchy IK:
A normal IK, is just an IK chain that doesn't stretch.
The Stretchy IK, is a normal IK chain when stretching enabled. (in the IK settings)
Soft IK is a stretchy IK with drivers that dampen the stretch so it never actually gets straight.
From memory, Rigify/BlendRig/and Auto Rig pro are stretchy IK (but there is probably a property to slide it on/off.
(but as I said it's useful to have it on all the time so I don't think many people actually turn it off)
Walk Cycles and Body Mechanics are friggin' hard man. Animation is hard. Well good animation is hard.
People think walk cycles are easy because it's easy to walk.
Same with animation really. People think "It's easy to watch so it must be easy to make".
Not true haha
Motion capture is perfectly fine to study. It's the same as studying video reference!
I've heard animators talk about doing clean up for mo cap and learning to 'see' the movement from the curves. (I can't remember the animation podcast I heard this on, but maybe the iAnimate podcast??)
And as far as cheating the camera - YES!!
When animating, you favour the camera, or animate to the camera (except when you are working in games - then it's the from all angles). So all you need to do is make it look good from the camera view.
In the walk cycle exercise - I specifically made it from 3 angles to get the students to pay attention to the body mechanics from all angles. This is harder, but it is better in the long run to understand more.
And it's perfectly fine to hide parts of character when it's not important for the story.
I'm often suggesting alternate shots on projects that make it easier/faster to animate and don't detract from the story.
Full body shots are the hardest, especially with multi characters. So only show what you need to show.
RE: the video from Brian Horgan
That set up is already in the VonnBot rigs and Baker rigs for that exact purpose.
Brian explains what I was talking about earlier. Get the animation as close to where it needs to be and then polish the pops with the "Knee_bend" controls. Key it at 0, then adjust on the frames you need and then key it back at zero at the end.
It's personal preference if you use the footroll or the ik foot, to do the rotation.
What Brian doesn't explain are the other possible causes of the knee pops.
It could be the foot roll, it could be the hieght of the torso, it could be that the foot is stepping too far, it could be that the hips are not rotated enough......etc
That could the cause - but the symptom is the knee going from straight to bend (or vice versa) too fast.
Hope that helps.
I just did a little experiment with the standard Mixamo walk cycle. I started by making extrusions in the middle of the knees and colouring them yellow so they're more visible. Then I rendered out the 24 frames from one contact pose with the right foot forward to the next that has the right foot forward.
I pulled these rendered frames into Photoshop and marked where the centre of the knee is for each frame and what I'm seeing is this:
From frame 5 to frame 8, when the body weight is on the right leg, the right knee almost comes to a complete standstill. It occurs to me that this is the critical part of the walk as far as knee pops go. Throughout the rest of the cycle, the knee is obviously moving forward, so that's not the hard part of doing this. It's just those four frames (5-8) where the progress of the knee has to be handled delicately.
Does this make sense?
