Permanent Stretching

The best use of permanent stretching is for bringing both bright and faint details into view at the same time.  Galaxies are the perfect example of objects with a large dynamic range (both bright and dark regions), but other examples exist such as the Orion Nebula.  We begin again with the Whirlpool Galaxy image from the last section, displayed with the MaxIm DL histogram defaults.

Above:  Unprocessed image.  Compare to results of stretching below.

MaxIm DL contains a number of powerful functions for stretching an image to enhance detail.  These include the Stretch Functions, Histogram Specification, and Transfer Functions.

Stretch Functions

The Stretch Function contains a variety of ways to manipulate the image data.  There are three sections in the Stretch window (shown below) where adjustments can be made.  Each section is described below.

Permanent Stretch Type

• Linear Only - This is primarily used if stretching a 16-bit image into an 8-bit image so that it may be read or displayed by other programs (for example if converting to a JPEG image for web publishing or to email it to a friend).

• Log - This stretch mode compresses the data within the image making both bright and dim regions visible.

• Gamma - This mode gives you more control than Log mode by allowing you to selectively enhance bright or dim details.

Input Range

• Max Pixel - This setting includes all of the pixel values in the image from 0 to the brightest pixel in the image.

• Screen Stretch - This is used with Linear Only mode to create 8-bit images which appear as seen on the monitor.

• Manual Settings - This allows you to set pixel values similar to the Screen Stretch function.

Output Range

• 8-Bit - Compresses data into 256 levels (useful for transferring to other programs like Photoshop or for web publication).

• 12-Bit - Sets the minimum pixel value to 0 and the maximum to 4096 in the output image (any higher values are thrown out).

• 16-Bit - Sets the minimum pixel value to 0 and the maximum to 65,535 in the output image (any higher values are thrown out).

• Unlimited - This setting prevents any values from being truncated and is best for use with Log or Gamma modes.

Log Scaling

To bring out more detail in the Whirlpool Galaxy image above several methods can be used.  One of the best scaling functions is Log Scaling.

Open the Stretch window by selecting Process > Stretch from the menu or by clicking the Stretch button on the toolbar.

Above:  Stretch button on the toolbar is on the left.

This will open the Stretch window showing a preview of the image.  With a large image it may be necessary to zoom out the preview to see a useful amount of the image.

In the Stretch window, select Log under Permanent Stretch Type and Max Pixel under Input Range.  For Output Range select Unlimited.

Above:  Stretch window set for Log scaling.

Click OK.  The screen stretch will need to be adjusted for the resulting image and it should be seen that more detail has emerged from the bright regions of the picture.

Above:  Log scaled image of M51.

Next, Using Filters to Enhance Detail...

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