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Head over heels: Upper body movement in gameplay
Head over heels: Upper body movement in gameplay
September 21, 2012 | By Simon Unger

September 21, 2012 | By Simon Unger
More: Console/PC, Art

In this reprinted #altdevblogaday in-depth piece, IO Interactive lead animator Simon Unger demonstrates how developers can sometimes forget to take realistic head movements into account when animating characters.

For every gameplay feature that makes it in, ten were cut so that it could survive. In the epic battle (or collaboration, depending on your team/company) between designers, programmers, and animators to see these features get shipped, some core fundamentals often fall by the wayside. In games animation, it is very common to approach quality of movement in the wrong order; starting from the ground up.

Foot planting, phase matching, complicated IK solutions, procedural layering; they all have their place, but we often sacrifice much to keep them working. In film, animators long ago realized that audiences observe characters in a very specific priority order. We start at the eyes, then the head, then the silhouette. Animators work very hard to give the upper body a sense of weight, obey the laws of force, and move in appealing arcs.

When it comes to in-game locomotion, we rarely get to see the eyes clearly as we're mostly behind our character, so the next stop down the chain of importance is the head. Unfortunately, when we place such a high focus on maintaining solid foot-planting, we create a fulcrum point at the ground and our characters often pivot in extreme ways to compensate. This is especially noticeable in bigger direction changes and when aligning characters to interact with each other (melee, high fives, piggyback rides, etc.).

In reality, the human body pivots around its centre of mass, the mid to lower torso. Through a series of complex mechanisms, humans have evolved the ability to maintain stability in three major areas of the body; the torso (the vestibulospinal reflex), the eyes (the vestibulo-occular reflex), and the head (the vestibulocollic reflex).

The fundamental purpose of these mechanisms is to maintain equilibrium as well as visual focus on predators and prey. It's difficult to run away from a sabre-tooth tiger or catch a shark with your bare hands if your vision is blurry and you keep falling over.

You can see these mechanisms at work in this video. Pay close attention to the head movement of the girl in the green and yellow.

Her head hardly moves for the entire race! (Side note: this is also fantastic squash and stretch reference, but that's another article)

Let's look at another reference video. Again, pay attention to the stillness of the head and centre of gravity on each of the athletes. Draw a mental path that the head is traveling along in space. Even in extreme direction changes, such as the guy falling on the skateboard, the head is held relatively still compared to the other extremities.

As you can see, especially in the first two clips of the football players doing cone drills, the legs and arms are being used to redirect the inertia of the centre of mass and the head is doing its best to remain as still as possible. On the sharper turns, the entire body almost appears to rotate around the head's position in space.

As a former animation director was fond of telling us, the only animation principle you need to remember is "Force". You cannot move an object in any direction without force in the opposite direction. The amount of force being applied is how weight is demonstrated.

To make a sharp, 90-degree right-hand turn at a running speed, the character needs to decelerate and redirect his mass by applying force towards the front and left sides of his body. We can see this in action when we break down one of the turns in the video.

The green line represents the path the head is taking throughout the turn. You can see the first application of force on the #1 foot-plant. The right foot is out in front and to the right of the centre of mass to redirect the body.

On the #2 foot-plant, the right foot is now behind and to the right of the centre of mass to redirect to the left as well as accelerate the body forward. Notice how throughout the entire turn the body appears to pivot underneath the head to complete the turn.

Taking this reference and knowledge, let's review some in-game footage from a few random clips I found online.

Hopefully the issues in the head movement on screen are apparent to you now. These types of issues aren't exclusive to games either. Often in feature films, especially ones with a mix of live action and CG, the CG character "feel" wrong to the viewer but they just can't pinpoint the reason why. There's a good chance that the character is exhibiting some of the issues described above.

Here are a few clips showing fluid head and core movement. Notice how even though some of the movement is almost super-human, it still remains quite believable when this principle is applied.

So, the next time you sit down to create a movement system, try to keep the above in mind. As we move towards more elaborate motion requirements on the next-gen consoles, stepping back and taking stock of the fundamentals will be even more important.

Start with your higher-frequency animations. If you have some kind of metrics system built into your game, data-mining for which animations get played and seen the most should give you an excellent starting point. Usually this means your main character's core locomotion. Starts, stops, and turns. Try to avoid getting caught in the common trap of spending a lot of time worrying about an area that will be seen less than 20 percent of the time. Focus on the 80 percent.

Unfortunately, there is no "magic bullet" to solve all of this currently. Head tracking, IK, animation layers, procedural systems… all need to be authored with this in mind and working together to maintain a sense of weight and force. Often the cause of these issues is multiple animations systems working against each other. Always be asking if each new component is supporting the greater goal: responsive, believable character movement

I would love to hear in the comments about how other teams have approached this and what your challenges and successes were.

[This piece was reprinted from #AltDevBlogADay, a shared blog initiative started by @mike_acton devoted to giving game developers of all disciplines a place to motivate each other to write regularly about their personal game development passions.]

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Ali Afshari
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This was very cool and useful for my work. I didn't notice anything "off" about the animation in Max Payne 3 and Uncharted 3, but after comparing it to footage of real people, it is certainly noticeable.

Duong Nguyen
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It looks like all the next gen animation engines are context sensitive to the world. Look at the Last of U, latest Splinter Cell and Assassin's Creed. They all project probes into the world and do some stochastic modeling for the potential animations, queuing them up and aligning them to the current animation (just my guess).

I think the "next gen" animation engines will go far beyond just simple context blending and move into the realm of real acting. Expressive blending, using emotive weights to actions to convey not just the physical but also emotive state of the actors.

Maria Jayne
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The combination of eye focus stability and head movement is never represented in games, you show good examples of the head but even if the head is moving, if your eyes are focused on something your vision will not become disorientated unless the body receives significant impact during movement.

Something of the predators instinct within our eyes, evolutionary speaking, it's why they face forwards, prey always has eyes mounted on the sides of their head to be aware of danger, predators always have eyes facing forwards to judge distance and accuracy even at speed.

Likewise your awareness of your surroundings in your peripheral vision in the real world allows you to see "without looking" where as the aspect ratio in games is often narrow and restrictive, forcing you to look around in moment of movement that should be unnecessary.

I think it's a problem that won't feel right until we move common technology into VR headsets as opposed to 2D monitors. Since the headsets can also track with gimbals and eye lines without compromising player view or hand controls to create the effect.

Dan Jones
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Thanks for this post, sir. I must admit, this was a pretty big blind spot for me. In my mind's eye, I already know my last batch of player animations practically had the heads bobbing around all over the place, relative to how still it should have been. I am both ashamed AND inspired, because that's one pitfall I'll do my best to avoid from now on.

Ron Dippold
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I really felt this in Max Payne 3 - even though it has a fantastic physics/ragdoll engine, Max just moves wrong. The head should always lead - you can see this in good soccer games, though not sure how much of that is mocap or how much is an animator who's actually studied this. Head turns, upper body follows, the rest follows. Instead Max's entire body just pivots instantly on the spine.

I had never quite realized the level head was so level, though. The vaulters really drive that home!

zoe ananda
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I actually think trained runners are a bad example for head movement. At some point people who ran track and their trainers/coaches figured out that all the up/down movement while running is a waste of energy. This is for the most part only something that trained runners will do, and it takes quite a while for most people to be able to keep their head this level. It requires quite a bit of practice and skill to only use energy for forward momentum, and it's not exactly typical of how the vast majority of humans run at all.

Remember, the reason they are so top level is that they pay attention to tiny miniscule detail like this. It's likely that they spent years working on keeping their head this level while running. Some people do it naturally more so then others, but almost never this much. Start firing guns around these people though, in a super stressful situation, and that mental focus might start to degrade. Just try to run like that yourself, on video, and check the results. It's very likely that it would take years of work to get to that point, if you even could.

Personally I spent years training in distance running actually trying to get to this level and got no where close. We are literally talking about a half decade to 10 years + to get to this point, for the people that even have the physical capacity to do so. Most people would train for a half decade and still have more vertical head movement then these athletes.

If your character has a back story of being a professional track running athlete then by all means. But if your character is some 15 year old with no athletic training thrust into a stressful war zone, I think making them run like a well seasoned trained athlete in a relatively calm and safe environment would be foolish. In the same way that making a gangster character from the hood shoot like a Olympic level marksmen the first time they pick up a gun would seem foolish.

Also, if anyone has ever seen the difference between old Greek/Roman marble statues in real life verse photos of them... something becomes quite apparent that doesn't translate to images. Many of these statues use the center of gravity to suggest motion, for instance tilting the chest forward ever so slightly in the way someone would when they start to walk. It gives them this eery feeling, like they are just about to come alive, as if your brain just takes it the rest of the way. It's really convincing when you catch them out of the corner of your eye as well.