Thursday, 8 September 2011

Animating Dogs: Breaking into a Gallop from Standstill

This evening I was looking through my reference footage of Rufus (a friend's dog who I borrowed for a weekend back at the start of the summer) when I stumbled across an interesting gait (or combination of gaits) which Rufus uses when he breaks into a gallop from complete standstill (lying down in this case).  I was actually searching for front view footage of a gallop to study how the legs are abducted under the body, but this accidental discovery has helped explain why a shot I animated last week came out looking slightly unnatural.  How serendipitous! (As an aside, I put that word in for the enjoyment of the non-native English speakers reading this blog as its a phrase which allegedly has few direct translations to any other languages, it's also how Dave Prosser describes his BAFTA nominated short, Matter Fisher!)

Have a look at the image below made up of a few random frame grabs from one of my reference shots.  Notice how the back legs make a bounding motion (as they do in the gallop, but in this case they are completely twinned).  Even if there's no reason to avoid twinning, I find that when I'm animating, I now do it out of habit, which isn't necessarily a good thing.  In fact avoiding twinning at all costs is probably just as much of an animation sin as twinning negligently is!  The rule really should be 'twin appropriately'.

In the image below the two back legs are exactly in sync with each other.  My guess is that this is so that the drive is as powerful as possible, and so that the best use can be made of the folding and unfolding motion of the spine and the hips.  The spine, the rear hips, and the two back legs all work in time, and together they act like a piston expanding and contracting.  Basic physics tells us that for a big acceleration we need a big force.  Anyone who's been in a tug-of-war (or a rowing boat, or pushed a piano up a set of stairs) will tell you that to get the biggest force you need everyone to pull together at once.  This is what the rear legs, hips and spine are doing - lots of working together, lots of twinning, no time offset!

click on the image to enlarge

Right, so if a dog can get the greatest force using both rear legs, the spine and the hips all in sync, why doesn't it do this all the time, in every gallop.  I'm not a biologist but I think the simple answer is that it's too tiring!  Having all the parts of the body working in one huge push together just isn't sustainable... remember the tug-of-war and pushing the piano up the stairs analogies?  A large force gives a large acceleration which gets the dog up to speed, but once the dog is at full speed it doesn't need to accelerate any more, only to maintain speed, so it switches into a smoother more economical motion which involves offsetting the timing of the legs slightly... avoiding twinning.  If this still doesn't make sense imagine your ankles are tied together.  You can definitely jump higher if you jump using both legs at the same time (twinned), rather than jumping with your right leg a few frames before your left leg (un-twinned or time offset), but now try bounding with your legs tied together... pretty tiring right?  You'd probably last longer if you used alternate legs one after the other (more commonly known as running).  For more than a few bounds, having those back legs in sync doesn't make sense, so the dog breaks out into a gallop, but only after it's got maximum acceleration by using both legs together (like a 'bunch' or 'bullet' sprint start).

The strangest part is yet to come though.  Have a look at that image sequence again.  Notice the front legs aren't bounding at all, but rather they're doing a very very fast walk.  At first glance it seems odd that to go as fast as it possibly can, the dog uses a gait which is usually used for very slow speeds!  Check out the posing on the front legs - those are pretty similar positions to the contacts and cross-overs you find on a regular walk, but in this case a stride measured from contact to contact only takes 4 frames, more than twice as fast as a regular dog walk cycle!

Switching back into my amateur biologist mode again (I've read Sir James Gray's Animal Locomotion and How Animals Move cover to cover and can recommend them both for anyone interested!) I believe this use of the walk gait on the front legs only is to stabilise the dog's chest.  While the back legs are generating a huge amount of power to move the dog forwards, something needs to be done to keep the dog's body up in the air.  Normally both pairs of legs do two jobs:
  1. move the body forwards
  2. keep the body up off the ground
So if the back legs are angling their force horizontally (ie. only moving the dog forwards), the front legs must be keeping the dog up in the air, otherwise Rufus would be face-planting the flower beds (!) rather than getting up to speed.  Take a look at how these forces work...

In a gallop the legs spend more time in the air than on the ground.  In a slow walk the reverse is true - the legs spend much more time on the ground than in the air.  However the amount of time the legs spend on the ground or in the air isn't necessarily related to the speed of the gait, but rather to the amount of vertical support needed.  Think of the slowest walk - a horse in a field eating grass has four legs on the ground and intermittently to take a step it will pick up one leg for a short while, move it forward a small amount and put it down again.  In a very slow walk there are never less than three legs on the ground, and the leg which is in the air, isn't up off the ground for very long.  A horse doesn't move very fast when its eating grass, and it doesn't leave its feet hanging round in the air because that's tiring, and legs dangling in the air don't help to support the horse's body weight.  The strange 'fast walk' that the dog's front legs do in the sequence above is to support the weight of the dog's chest.  Notice how in all the shots above there is at least one front foot on the ground and sometimes both front feet are down together!

Look again at the forces diagram above.  Note how the arrow which makes greater forward progress is lower to the ground.  Think of this as being more streamlined.  The more effort the dog is putting into accelerating, the less effort goes into bouncing up and down.  Let's test this theory:  is the opposite true?  If a dog is slowing down it needs to get rid of horizontal energy.  One way it can do this is to dissipate it as vertical energy.  Read the below image strip from left to right - Rufus catches the tennis ball then as he suddenly decelerates he explodes upwards, changing that fast forward motion into up and down vertical motion.  Then gravity and cushioned bounces (Rufus' shock absorbing legs) helps us get rid of the up and down energy in the same way a bouncing ball bounces less and less on each bounce.

click on the image to enlarge

Looking at these images has just made me realise something.  Have a look at the image below.  Who's accelerating and who's decelerating.  Who has a horizontal motion, and who has a vertical motion:

sprinters accelerating and decelerating

Or how about this one:

looks familiar?

So in summary here's the main positions roughed out.  Remember that the chest is generally more stable than the hips (which tend to jog around a bit at the back... think of how "Slinky's" hips move in Toy Story, always left behind and catching up again).

some rough sketched poses

If you wanted to study the clip the first set of images came from, here it is below on youtube.  You might want to use saveyoutube or similar to download the clip to play frame by frame in QuickTime.

The combination of a bounding gait (ie. a twinned or symmetrical gallop) on the rear legs, with an un-twinned gallop on the front legs is technically known as half-bound.  Here's a cat doing something between a half-bound and a gallop from the Muybridge Animals in Motion books.  I've annotated the "1-2" landing of the front legs and the simultaneous contact for the rear legs:

And as always, please fire off in the comments if you don't get something or think I've got something wrong. It's all open to discussion!

See Also:
How to Key a Dog's Walk
Slow Four Legged Walks

1 comment:

  1. the non-native speakers say thanks!
    this blog is becoming a reference book or what?!