# 2. LinearÂ interpolation

## Video transcript

(lamp springing) (lamp clicking) - Okay, straight ahead animation works, but a lotta times it's hard to figure out where you're going. A lot of times it's helpful to start by working out what we call the Key Poses first. For a bouncing ball, that's gonna be where the ball is at it's highest point and where it hits the ground. I'll start by drawing frame one up high. Now let's draw frame nine, on the ground. Now I can draw the inbetweens using these key poses as a guide. I'll draw frame five in the middle. We call this a Breakdown. Now I can keep adding more frames, subdividing the space. Sometimes it helps to make a little chart to keep track of all the numbers. (pencil sketching rapidly) Great, now let's shoot our drawings and play 'em back. In fact we can cheat and make it bounce back by reusing the same drawings on the way back up. (ball bouncing) Hmm, the ball's moving, but it's not very realistic. Sometimes before you get started, it helps to shoot video reference, so you have some idea what you're doing. (camera clicking) Remember the ball needs to speed up as it falls to the ground. (ball bouncing) Our frame five is halfway through the animation in time, but the ball shouldn't be halfway to the ground at that point. (slow clicking) Instead I'm gonna draw a new frame five that's biased towards the first pose. (reverse slow clicking) Maybe I'll draw it a third of the way down. Animators have developed a visual language using these timing charts, to show how to space the drawings in between key poses. So frame five, remember, is a third of the way between one and nine. I'll draw frame three a third of the way between one and five, and so on. These charts help us keep track of the inbetween drawings. So we end up with the proper timing. (pencil sketching rapidly) When we play this back, the ball accelerates towards the ground. (ball bouncing) Now let's see how we use the same technique of pose-to-pose animation on the computer. We'll start by positioning the ball up in the air on frame one, just like before. And then we'll skip ahead to frame nine and put the ball on the ground. Down below the picture, the computer shows us this graph. It's kinda like the 2D timing chart, and it shows us how the computer is going to calculate the inbetween frames. The horizontal axis is time, and the vertical axis is how high up the ball is. Check this out. If we project the intersection of each frame onto the vertical axis, we end up with exactly the same timing chart that the 2D animators use. You can see that by default the computer connects our poses with a straight line in the graph. This is called Linear Interpolation. It results in even spacing of the poses. When we play it back, the ball will move at a constant rate. Now you give it a try. See if you can animate a convincing bouncing ball using linear interpolation. And here's a hint, you can add extra key frames in between to get the spacing that you want.