If you change settings for an Image Set, the changes may be applied to either the active image or to the entire set, depending on the state of the (Process) button. For RGB images, the adjustments of the transfer function are handled by the Palette Properties dialog. The current transfer function parameters are shown below the histogram graph. In top-down order are the Sample Region, Range, and Stretch settings. These properties are described in detail in Transfer Functions. See Choosing Transfer Function Properties for tips on setting the controls.

Transfer Function properties can be specified using controls on the pane, by using equivalent properties in the View > Transfer Function submenus, or by using menus commands opened from the drop menu on the button on the Image Tools Toolbar.

Tip:

Changing parameters while running an image animation (fast forward or fast reverse) is not advised. Mira fills the controls with the image-specific values each time a new image is displayed, and that will overwrite the parameters you are trying to change. To change parameters, stop the animation, set the desired parameters, and then restart the animation.

Tip:

Set the transfer function properties before changing the selections in the drop boxes. Then the dropbox selection uses the specific parameters.

Sample Selector (top)

The Sample selector specifies the region where the image intensity is sampled for calculating the transfer function. The options are as follows:

Entire Image: Samples the entire image.

Cursor: Samples intensity inside the Image Cursor.

Cursor Live: Samples intensity inside the Image Cursor and updates as the cursor moves. This continually updates as the cursor is moved or resized on the image.

If you close the transfer function pane with Cursor Live selected, Mira switches to Cursor mode on close.

Range Selector

(middle)

The Range parameter specifies how the limits (end points) of image intensity are determined for mapping the intermediate values to grayscale. All of these methods involve computing the image value histogram:

Autoscale: Determines the optimal range of the image data. This method uses the entire image to compute the histogram. The Autoscale method uses the two properties Contrast and Offset under the Autoscale Contrast caption.

Min/Max: Uses the full range of the image histogram.

99.9%: Uses 99.9% of the histogram range.

99.5%: Uses 99.5% of the histogram range.

99%: Uses 99% of the histogram range.

98%: Uses 98% of the histogram range.

95%: Uses 95% of the histogram range.

Percentile: You specify the percentile histogram limits.

Specify Z: You specify the image values independent of the image histogram

Stretch Selector

(bottom)

The Stretch provides controls the way the transfer function maps image values to grayscale between the limits. The options are as follows:

Linear: Uses no Bias with equal Grayscale spacing between the transfer function's limiting values.

Log: Extends the transfer function to greater contrast at lower intensity values.

Gamma: Extends the transfer function to higher or lower intensity based on the gamma setting. A gamma less than 1 biases toward higher contrast in lower intensity, and vice versa.

Autoscale Contrast

(Contrast)

If Autoscale is selected, Mira determines a nice scaling for mapping image values to gray levels. This method samples the entire image and maps its values to gray levels using two parameters under the Autoscale Contrast caption.

After changing either of these properties, the Autoscale transfer function is applied by pressing the Enter key:

Contrast is left-most property, which is the actual contrast factor. This is in the range 5 to 100. A lower number gives lower contrast.

Offset is the right-most property, which shifts the histogram median up or down in grayscale. This is in the range -100 to 100. A negative value shifts the median to a darker gray level.

Recommendation: Use 25 and -25 as good starting points.

Min Z

Specifies the minimum Z (intensity) value. This is active only when the Specify Z stretch is selected.

Max Z

Specifies the maximum Z (intensity) value. This is active only when the Specify Z stretch is selected.

Min %

Specifies the minimum percentile intensity. This is active only when the Percentile stretch is selected.

Max %

Specifies the maximum percentile intensity. This is active only when the Percentile stretch is selected.

Gamma Slider

The slider sets the gamma value when the Gamma stretch is selected.

 Boost

Check this box to selectively boost the contrast by enhancing contrast where image values crowded into a narrow range. In astronomical images, this crowding is usually near the background level, and the method usually makes the background very contrasty. See Using the Boost Transfer Function Option.

[Save]

Opens the Transfer Function Properties dialog for saving the current settings as a profile for future use.

Note

The Transfer Function Pane does not work with RGB images. For RGB images, use the Palette Pane or Palette Properties dialog.