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## Setting opacities

The Volume plot attributes window provides several controls that allow you to define the opacity portion of the volume transfer function. The opacity portion of the volume transfer function determines what can be seen in the volume-rendered image. Data values with a lower opacity allow more to be seen and give the plot a gel-like appearance, while data values with higher opacity appear more solid and occlude objects behind them. The controls for setting opacities are located in the center of the window in the Opacity area.

Figure 60

You can set opacity two ways. You can hand-draw an opacity map, or create it by designing curves that specify the opacity when they are added together. Both methods use the controls shown in figure.

The interaction mode determines how opacity is set. Clicking on the Freeform or Gaussian radio buttons selects the interaction mode. If the interaction mode switches from Gaussian to Freeform, the shape constructed by the Gaussian controls is copied to the Freeform control. Both controls pretend that the plot's data range is positioned horizontally such that the values on the left of the control correspond to the low data values while the values on the right of the control correspond to high data values. The vertical direction corresponds to the opacity for the given data value. Taller curves are more opaque while shorter curves are more transparent.

Figure 61

To design an opacity map using the Freeform control, position the mouse over it and click the left mouse button while moving the mouse. The shape traced by the mouse is entered into the Freeform control so you can draw the desired opacity curve. Immediately under the Freeform control, there are four buttons, shown in figure, which can be used to manipulate the curve. The first three buttons initialize a new curve. The black button makes all data values completely transparent. The ramp button creates a linear ramp of opacity that emphasizes high data values. The white button makes all data values completely opaque. The Smooth button smooths out small bumps in the opacity curve that occur when drawing the curve by hand.

Figure 62

The Gaussian control used during Gaussian interaction mode is complex but it provides precise control over the shape of a curve. The basic paradigm followed by the Gaussian control is that new curves are added and reshaped to yield the desired opacity curve. You add new curves by clicking and dragging in the control. Right clicking with the mouse on an existing curve removes the curve. Each curve has five control points which can change the curve's position and shape. The control points are shown in figure along with the shapes that a curve can assume. A control point changes color when it becomes active so there you know which control point is used. Curves start as a smooth Gaussian shape but they can change between the shapes shown in figure by moving the shape control point up and down or left and right. Opacity maps are typically created by adding several curves to the window and altering their shapes and sizes until the desired image is obtained in the visualization window. The Attenuation slider, the final control involved in creating an opacity map, controls the opacity of the entire opacity map defined by the Freeform or Gaussian controls. It provides a knob to scale all opacities without having to modify the opacity map.