3.1   Creating and modifying a manual stretch

In the previous chapter you finished by using a histogram to improve the Redisplay of 16-bit data. In this section you will be using a histogram of 8-bit data to prepare for adjustments to a manual stretch document, which you open and modify to improve the contrast of the image display, and later to help you interpret the data.

1. Open the image mer_rr_20040201_082314_algal1.dat
2. Right-click to open Redisplay set the null-value to 0, and note how this changes the display of the ocean in the image.
3. Create a histogram document derived from selecting the whole image ( [CTRL+A] ), making sure the Apply stretches box in the New dialog is unchecked.

Question 1
What is the range of pixel values associated with most of the pixels in the image?

What is contrast stretching?

The range data values in the histogram you just examined represents only about half of the possible values in the full range of 0-255. A display that uses the actual data values as the display values will make very limited use of the dark grey tones corresponding to data values below 50, and virtually no use of the pale grey to white tones corresponding to values of 181 or more. The result would be a low contrast image where the sea is shown mainly of mid-grey tones.

The purpose of contrast stretching is to spread the important range of pixel values more effectively through the available range of grey tones in the display. This is done by imposing a change whenever the computer takes a value from the image document to display it. The change is based on a look-up table which is defined in a Stretch Document.

Opening a stretch

To open a stretch document for this image, first make sure it is active by clicking on it. Then you can open a Stretch in one of several ways:

From the menu bar:
  - open the Stretch menu and select Manual from the drop down.
Toolbar :
  - Click on , and select STRETCH, leaving the Apply stretches box unchecked.
Keyboard shortcuts:
  - either type [ALT + S] together, followed by [[M],
  - or type [CTRL + N] together, followed by [[S].

The stretch document appears on the screen as a graph with a line drawn between [0,0] and [255,255]. This depicts the look-up table which determines the new value with which a pixel should be displayed. The abscissa (x-axis) corresponds to the value in the original Image Document (the input) and the ordinate (y-axis) is the new value to be assigned when the image is displayed (the output).

Modifying the stretch

From the examination of the histogram we choose 50 to 181 as the range of input values which should be spread over the full scale of the output values. The simplest modification that can be made to the stretch is to keep it linear over that range. Inputs values above 181 will be output as 255, input values below 55 will be output as 0.

To achieve this, the end points of the stretch should not be touched. As you can see there is already one knee (or tie-point) - a small square at a bend in the line currently found at the [x,y] position of [30,0]. Another knee is needed, and can be created by placing the mouse pointer anywhere on the diagonal line of the stretch and double-clicking the left mouse-button.