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Shape-Tweening Strategies

Shape tweening is a technique for interpolating amorphous changes that can't be accomplished with instance transformations such as rotation, scale, and skew. Fill, outline, gradient, and alpha are all shape attributes that can be shape-tweened.

Flash applies a shape tween by using what it considers to be the most efficient, direct route. This method sometimes has unpredictable results, creating overlapping shapes or seemingly random holes that appear and merge (Figure 1.33). These undesirable effects usually are the result of keyframes containing shapes that are too complex to tween at the same time.

Figure 1.33Figure 1.33 An attempt to shape-tween flash to shape all at once in a single layer. Notice the breakups between the s and the p, and the hole that appears between the h and the e.

As is the case with motion tweening, simplifying a complicated shape tween into more basic parts and separating those parts in layers results in a more successful interpolation. Shape hints give you a way to tell Flash what point on the first shape corresponds to what point on the second shape. Sometimes, adding intermediate keyframes will help a complicated tween by providing a transition state and making the tween go through many more-manageable stages.

Using shape hints

Shape hints force Flash to map particular points on the first shape to corresponding points on the second shape. By placing multiple shape hints, you can control more precisely the way your shapes will tween.

To add a shape hint:

  1. Select the first keyframe of the shape tween, and choose Modify > Shape > Add Shape Hint (Cmd-Shift-H for Mac, Ctrl-Shift-H for Windows) (Figure 1.34). A red-circled letter appears in the middle of your shape (Figure 1.35).

    Figure 1.34Figure 1.34 Select the first keyframe of the shape tween, and choose Modify > Shape > Add Shape Hint.

    Figure 1.35Figure 1.35 The first shape hint appears in the center of the Stage in the first keyframe.

  2. Move the first shape hint to a point on your shape. Make sure that the Snap to Object modifier for the Arrow tool is turned on to snap your selections to vertices and edges.

  3. Select the last keyframe of the shape tween, and move the matching circled letter to a corresponding point on the end shape. This shape hint turns green, and the first shape hint turns yellow, signifying that both have been moved into place correctly (Figure 1.36).

    Figure 1.36Figure 1.36 The first shape hint in the first keyframe (left) and its matching pair in the last keyframe (right).

  4. Continue adding shape hints, up to a maximum of 26, to refine the shape tween (Figure 1.37).

    Figure 1.37Figure 1.37 Changing from a T to an I with shape hints (left) and without shape hints (right).

TIP

  • Place shape hints in order either clockwise or counterclockwise. Flash will more easily understand a sequential placement than one that jumps around.

  • Shape hints need to be placed on an edge or a corner of the shape. If you place a shape hint in the fill or outside the shape, the shape hints will remain red, and Flash will ignore them.

  • To view your animation without the shape hints, choose View > Show Shape Hints (Cmd-Option-H for Mac, Ctrl-Alt-H for Windows). Flash deselects the Show Shape Hints option, and the shape hints are hidden.

  • If you move your entire shape tween by using Edit Multiple Frames, you will have to reposition all your shape hints. Unfortunately, you cannot move all the shape hints at the same time.

Adding intermediate keyframes can help a complicated tween by providing a transition state that creates smaller, more manageable changes. Think about this process in terms of motion tweening. Imagine that you want to create the motion of a ball starting from the top left of the Stage, moving to the top right, then to the bottom left, and finally to the bottom right (Figure 1.38). You wouldn't create just two keyframes—one with the ball at the top-left corner of the Stage and one with the ball in the bottom-right corner—and expect Flash to tween the zigzag motion. You would need to establish the intermediate keyframes so that Flash could create the motion in stages. The same is true of shape tweening. You can better handle a dramatic change between two shapes by using intermediate keyframes.

Figure 1.38Figure 1.38 A complicated motion tween requires several intermediate keyframes.

To create an intermediate keyframe:

  1. Study how an existing shape tween fails to produce satisfactory results when tweening the letter Z to the letter S (Figure 1.39).

    Figure 1.39Figure 1.39 Changing a Z to an S all at once causes the shape to flip and cross over itself.

  2. Insert a keyframe at an intermediate point within the tween.

  3. In the newly created keyframe, edit the shape that provides a kind of stepping stone for the final shape (Figure 1.40). The shape tween has smaller changes to go through, with smoother results (Figure 1.41).

    Figure 1.40Figure 1.40 An intermediate shape.

    Figure 1.41Figure 1.41 The Z makes an easy transition to the intermediate shape (middle), from which the S can tween smoothly.

Sometimes, providing an intermediate keyframe isn't enough, and you need shape hints to refine the tween even more. Here are three ways you can add shape hints to a shape tween that uses an intermediate keyframe.

To use shape hints across multiple keyframes:

  1. Select the intermediate keyframe, and add shape hints as though it were the first keyframe.

    Keep track of which shape hint belongs to which tween by noting their respective colors. Yellow is the shape hint for the beginning keyframe, and green is the shape hint for the end keyframe (Figure 1.42).

    Figure 1.42Figure 1.42 This intermediate keyframe contains two sets of shape hints. Some are the ending shape hints for the first tween; others are the beginning shape hints for the second tween.

  2. Insert a new keyframe adjacent to the second keyframe, and begin adding shape hints. A new keyframe allows you to add shape hints without the confusion that overlapping shape hints from the preceding tween may cause (Figure 1.43).

    Figure 1.43Figure 1.43 A shape hint in a new keyframe.

  3. Create a new layer that duplicates the intermediate and last keyframes of the shape tween, and begin adding shape tweens on this new layer. By duplicating the intermediate keyframe, you keep shape hints on separate layers, which also prevents the shape hints from overlapping (Figure 1.44).

    Figure 1.44Figure 1.44 Layer 2 keeps the beginning shape hints for the tween from frames 7 to 15 separate from the end shape hints for the tween from frames 1 to 7.

Using layers to simplify shape changes

Shape tweening lets you create very complex shape tweens on a single layer, but doing so can produce unpredictable results. Use layers to separate complex shapes and create multiple but simpler shape tweens.

When a shape tween is applied to change the letter F to the letter D, for example, the hole in the last shape appears at the edges of the first shape (Figure 1.45). Separating the hole in the D and treating it as a white shape allows you to control when and how it will appear. Insert a new layer, and create a second tween for the hole. The compound tween gives you better, more refined results (Figure 1.46).

Figure 1.45Figure 1.45 A hole appears at the outline of the first shape when a shape tween is applied to change an F to a D.

Figure 1.46Figure 1.46 The hole and the solid shapes are separated on two layers.

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