Spectrum

Spectrum Control

The spectrum widget is used to define the color spectrum of materials, spectrum objects, and the independent spectrum for each image object. Color spectra are used to map a unit value derived from a mesh field or from a voxel value into a color with transparency, thus values from 0.0 to 1.0 are mapped to colors defined by a spectrum from left to right as displayed in the interface. Transparency is controlled by a separate function curve which defines the ''alpha'' for the mapped color, 0.0 for completely transparent to 1.0 for completely opaque.

The above example interface defines a rainbow spectrum with 5 colors: blue at 0.0, cyan at 0.25, green at 0.5, yellow at 0.75, and red at 1.0. The transparency curve defines any values at 0.0 (blue) to be completely invisible, those approaching 0.5 becoming more opaque then dropping off again around 0.75, then rising to fully opaque at 1.0 (red).

The interface allows a spectrum to be modified by placing and moving color control points to change the color mapping, and by placing and moving curve control points to change the transparency curve. These elements are outlined here:

• Spectrum Area: This is the visual representation of the color map. Double-clicking in this area will place a new transparency curve control point at the cursor.
• Transparency Curve: This represents the transparency of the mapped color along the spectrum. The higher the curve is in the spectrum the more opaque the mapped color.
• Control Point: Click a control point to select it then drag to move it and adjust the transparency curve.
• Color Bar: This shows where in the spectrum's range colors are explicitly defined. Double-clicking in a blank space on this bar will create a new Color Control Point whose color is chosen such that the spectrum is not affected.
• Color Control Point: These represent where colors in the spectrum are defined. Left-Click to select one, then drag left or right to move where the defined color is in the spectrum's range. Double-clicking a control point will bring up a platform-dependent color picking dialog box which can be used to choose a new color for that point.
• Delete Button (-): Left-clicking this button will remove the selected control point (either transparency or color) from the spectrum. The Delete or Backspace key has the same effect.
• !Linear/Cubic Button: Left-clicking this button toggles the spectrum curve between cubic and linear line definitions.