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Understanding Value and Speed Graphs

In the following sections, you’ll use the Graph Editor to (what else?) edit a graph of a property. But first, let’s take a moment to examine the two types of graphs you’ll encounter: the value graph and speed graph. As you proceed with the graph-editing tasks in the following sections, note how the shape of a value graph or speed graph corresponds to the animation (Figures 9.39 and 9.40, and Table 9.1).

Figure 9.39 A value graph shows changes in a property’s value (in this case, Rotation). Examine how each graph shape corresponds with certain types of temporal interpolation.

Figure 9.40 A speed graph shows changes in a property’s speed (here, Position).

Table 9.1 Recognizing Temporal Interpolation

Temporal interpolation

In the value graph

In the speed/velocity graph

No speed change

Horizontal line

Horizontal line

Constant speed

Straight line with any slope

Horizontal line

Sudden speed change

Sharp corner

Disconnected line/ease handles

Acceleration

Curve with steep slope

Upward-sloping curve

Deceleration

Curve with shallow slope

Downward-sloping curve

Holding

Horizontal line, unconnected

Horizontal line, where current speed = 0

Value graph

A value graph measures a property’s value vertically and its time horizontally. The units in which values are expressed depend on the type of property: Rotation is measured in rotations and degrees, Opacity in percentages, and so on. The slope of the line between keyframes represents the rate of change in units per second. Straight lines indicate a constant rate; curved lines indicate a changing rate, or acceleration.

A value graph is particularly easy to grasp when viewing properties such as Opacity and Audio Levels, because these properties correspond well with the “up and down” or “high and low” nature of the graph.

Note that some properties consist of more than one value, or dimension. For example, a Position property includes values for both an X and Y coordinate. Hence, a position value graph includes two color-coded lines: one representing the X coordinate value and the other representing the Y coordinate value.

Speed graph

A speed graph measures rates of change in a property’s values. The units measured by a speed graph depend on the property type: degrees of rotation/sec, percentage opacity/sec, and so on. Regardless of the specific property, the rate of change (units/sec) is measured vertically, and time (sec) is measured horizontally in both graphs. Therefore, the slope of the line represents acceleration (units/sec/sec). Compared to interpreting a value graph, interpreting a speed graph isn’t as straightforward. For example, a property’s value may be increasing or decreasing—but if its rate of change is constant, it results in a horizontal line in the speed graph.

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