Transfer Functions

Displaying a visual representation of numerical image data such as a CCD image requires 2 steps: applying a transfer function to slice the pixel values into a smaller range of 256 numerical indices and then applying a grayscale or pseudocolor palette to these numerical indices. The Transfer Function is the mathematical prescription for how the image pixel data are to be sliced into these 256 palette indices. RGB images having R, G, and B channels skip the transfer function step and directly apply a palette. For images containing numerical data, often called "luminance" or "intensity" images, the display process works in the following order:

The Transfer Function slices a range of image brightness into 256 levels.
The Palette maps the 256 brightness levels into a spectrum of 256 colors or gray shades.

The transfer function has two main purposes:

To accommodate images having a dynamic range too great to be fully displayed on a computer screen. The transfer function compresses or expands the range of brightness that is mapped into the display.
To allow viewing of image details that span only a small part of the total range of image brightness. The transfer function can be adjusted to display certain features at the expense of others.

In separating the transfer function from the palette, Mira is distinguished from many image processing applications that allow either transfer function adjustments or palette adjustments. Separating these enhancements into separate adjustments confers greater flexibility in revealing the properties of the image data. See Choosing Transfer Function Properties and the Related Topics listed below.

Changing the Transfer Function

The Transfer Function Pane and Transfer Function Properties dialog are used to change the transfer function settings and apply the changes. The Transfer Function Properties dialog sets parameters for the default transfer function used to display images. After an image is displayed, the Transfer Function Pane may be used to change the transfer function used by a specific Image Window. To open the Transfer Function Pane dialog, use the View > Transfer Function menu command or the button on the Image Tools toolbar. You can also change settings using the Transfer Function Popup Menus.

In addition, the Capture Cursor to TF command copies a specific bounding rectangle bsad from the Image Cursor for use by the default transfer function. This allows a specific image region to be used for computing the initial transfer function when images are opened.

Mira manages the default transfer function settings using a Profile control. You can load all the Properties at any time simply by loading the profile by name. One of the profiles is the default one used to open an image. After an image is open, you can change the profile or the settings of the profile for that particular image, an then use these settings to update the named profile or create a new one. Detailed descriptions of the transfer function parameters are given in Choosing Transfer Function Properties.