Friday, May 2, 2008

Animation Ramble - Drag

Last week I tackled a job in the garden that I had been putting of for some time. Down the side of my shed is a hibiscus, I don’t use the space down there for anything in particular so I don’t pay much attention to it, and over the last couple of years the plant had gotten quite big. It was time to give it a serious hair cut. Because it was down between the shed and the fence at the side of our property where there isn’t much light the hibiscus had sent up hundreds of long straight and thin branches. I would cut a few down, grab them from the thicker end, drag them out and flip them over my head into a pile.

As is the way with these kinds of things I fell into a kind of rhythm, and as is often my way with things I started to focus on the rhythm as a way of getting through a tough job. Its went something along the lines of, saw, hack, chop, chop, draaaaaaaaag, flip, cuchunk. It was this last beat that got my attention, the flip, a slight pause and then the cuchunk as the branches crashed into the pile. I realised that because the branches were long and thin they were bending in the air as I flipped them over my head and the tip of the branch covered in leaves was crashing down on the pile of cuttings some time after I had finished flipping the branch over. DRAG!

We got some guys in to mulch the cuttings so we could use them in our garden, but I kept one branch aside so that I could use it for a little bit of live action analysis.

So I want to look at two things, what drag is and the process you might use to animate it.

The basic rule of drag is that something (anything) won’t move unless it absolutely has to. Its one of those physics things, as a general rule things are perfectly happy to stay where they are, its only when an irresistible force is applied and the object is given no choice that it will move. If stuff is moving without any motivation (internal or external) then you're dealing with unreal movement, and as a general rule you want things to seam real so that the audience will engage with the world you’re creating. Things that provide their own internal force (say from a muscle or engine) tend not to drag, but things that are dependant on an external force to move and aren't connected to the source of the movement in a rigid way tend to drag.

Lets look at an example......
and lets break it down.I intentionally picked something that was heavy on the end for this first example. You can see that the heavy plug dose not move until all the slack is taken up in the cord. I know it may seem obvious now that I've pointed it out, but its something you need to be mindful of as you work. If you were animating this scene the heavy plug could not start moving until several frames after the hand, making things move at different times is often called offsetting the timing or overlap, and drag this is one of the most common reasons for doing this.
OK so back to my hibiscus branch, I tied a rag to the end so it is easier to see.Break it down......hammer time! ooh ooooh ooooooh ooooh ohhh ohhh ohhhhh ooooh oooh :)

At the start the only force is coming from gravity, its keeping the branch in its place on the ground.So now I introduce a second force, its coming mainly from the muscles in my torso, so that's our source. But remember our rule, the tip of the dragging object will try to stay where it is for as long as it can. It waits until the very last minute when the connection between it and the source is pulled tight before moving. The heavier the tip is the more pronounced this will be, you can see there is still some bend near the base of the branch, if I tied some bricks to the end of the branch I would have to pull until the branch was much straighter before they would move. Few, finally got the tip off the ground, isn't that a great arc. My body is almost all the way forward, the tip of the dragging object points back to its original position.
At this stage my body has stopped moving forward, the tip of the branch is only half way over my head. This "S" shape is common in the second half of an action with drag, when I was an inexperienced animator I found this position easy to get to, but then hard to animate out so that the object ended up straight or flat again without looking stiff, I'll elaborate on this a bit later.
Here my body is moving up, but the dragging object is still only two thirds of the way through the original movement, the timing of the source (me) and the dragging object (the branch) are offset. Having these connected things moving in different directions at the same time can look amazing if you nail it. When done well, its the sort of thing that can send a shiver down your spine as you watch it.
I've completed the original movement forward and have had time to straighten up before the tip of the branch finally arrives on the ground.So now that we all understand what drag is, lets have a look at how I animate it. Just recently I've posted some links to blogs where animators have described their work process, while they have been interesting they have been very long (even for a seasoned ranter like myself) and there is often so much detail that you loose sight of the big picture. So I thought for this part I would take a leaf out of Jason Ryan's book and swap over to video tutorial mode.
So now for some live action examples, these clips are from Youtube, but I have put them up on my account as quicktime files so you can pause the quicktime player and flick through the frames one at a time with the left and right arrow keys.. This first one I saw some time back on the Spline Doctors Blog, its amazing. Slow motion really helps with this stuff, who would have thought that there would be more bend in a baseball bat than in a golf club.
It can be parts of people that drag too.

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