Our animated box is another example. We have a key at 40 and a key at 80 in the same position, but the box is moving all over the place. Again, the smooth tangent of the Bezier controller is responsible.

Ease In and Ease Out

Ease in and ease out are more terms inherited from traditional cel animators. Animators at Disney discovered that they often wanted to emphasize a key pose by making the frames before it gradually decelerate the rate of change before the keyframe, and the frames after it gradually accelerate the change after. The classic example of this is the peak of a bouncing ball cycle, illustrated in Figure 7.4.

FIGURE 7.4  Ease In/Out of peak keyframe of a bouncing ball

Tangent Types

The translation of the ease in/out feature of animation into MAX terms is the tangent type in and out of a key. The tangent referred to is the tangent of the function curve of that parameter. In the case of our unwanted window movement, the solution would be to use a linear tangent leading into the first keyframe of the opening.

Clicking and holding the Tangent Type button in the Key Info dialog box produces a flyout of the tangent options. There is a relation between controllers and the types of tangents they use. The tangent types shown in Table 7.2 and Figure 7.5 are the ones available for a Bezier controller. A Linear controller would use only the Linear tangent type; an On/Off controller would use only the Step tangent type. The TCB controller, covered in Chapter 6, gives you a different way of looking at the same animation curve: by altering the tension, continuity, and bias settings, you can see the changes in the distribution of frames around the key.

FIGURE 7.5  Tangent types (drop-downs of the In and Out lists)

Shaping Function Curves with Tangent Types

As mentioned before, the word “tangent” refers to the function curve of the track. We were introduced to function curves in Chapter 6 and have discussed them several times. Function curves are a graphic representation of the animated data and are tremendously powerful once you understand how to read them. Let’s look at how the different tangent types shape function curves and what that means to the final animation results.

Fixing the Box Animation with Linear Tangents

First let’s get back to our box animation, as promised, and fix that stop.

1.  Open up your anim_box.max file.

2.  Select the box, open up its Track View, and open its hierarchy to reveal the position track.

	3. Click the Function Curves button. Your function curves should look something like Figure 7.6. The Z position is the flat blue line, and the X and Y positions are the curving red and green lines.

FIGURE 7.6  Function curves of the animated box’s Position track

	NOTE The line colors of the function curves are a visual mnemonic: XYZ = RGB; X = red, Y = green, Z = blue. RGB is the additive color space of all computer graphics, film, and TV.

4.  Right-click the third key (at about frame 40) on the red line to bring up the X position key info. Notice that the tangents in and out are the Smooth type.

5.  From the Out drop-down list, select the Linear tangent type.

6.  Click the Next Key button to get the key info for the fourth key.

7.  Select the Linear tangent as the incoming tangent of this key. (Do it this way instead of clicking the Copy Tangents button, because Copy Tangents in this case can result in slight changes in value.) The function curve between frames 40 and 80 should now be flat, as in Figure 7.7.

8.  Play the animation. The box now stands still between frames 40 and 80.

9.  Edit Ø Hold. We want to play around with other tangent types, to see what they do to the function curve, and then come back to this.

FIGURE 7.7  Linear tangents between frames 40 and 80

Step Tangents

Let’s see what Step tangents do to the curve.

1.  Right-click one of the keys at frame 40 again, and this time select the Step tangent in and out. You can change one to Step tangent and copy it to the other using the Copy Tangents buttons. Your function curves will now look like Figure 7.8.

FIGURE 7.8  Effect of step tangents on the function curve

2.  Play the animation now. Watch how the box moves between frames 20 and 80. It jumps from one position to another. With the Step tangent, there is no interpolation between the keys.

Slow In/Out and Fast In/Out Tangents

Slow in/out and fast in/out affect whether you get gradual or abrupt changes around a key. Let’s look at the effect.

1.  Edit Ø Fetch, just to clear the Step tangents out completely.

2.  Open the box’s Track View to the position track.

3.  Right-click the third key and change just the incoming tangent to the Slow In type.

4.  Play the animation several times to watch the motion carefully before the stop. Notice that the box decelerates before stopping, resulting in more natural-looking motion.

5.  Right-click the key again and change the incoming tangent to the Fast In type.

6.  Play the animation and, again, watch the motion carefully. This time the box actually accelerates into the stop, as if it hit an invisible wall or snagged on an invisible hook.

© 2000, Frol (selection, edition, publication)