3.8   Saving and re-using a stretch

When you want to compare images visually it is important to use the same stretch for all the images you are working with. If different stretches are used, you may easily fall into the trap of seeing differences that are not actually there. The need for consistency applies equally to the redisplay stretches and manual stretch documents.

This section looks at how you can enhance the display to reveal features of interest, using a stretches that are the same for all the images being compared.

The images you will use are monthly AATSR data from the Indian Ocean at 10 arc-minutes resolution from four different seasons, saved as real values (32-bit floating-point data).

1. Start by opening the images for a preview of the frequency distribution. Don't worry about the Redisplay stretch at the moment, but set the null value to -1, and leave the rest to Bilko's defaults.
2. Take a histogram of each image (do NOT apply stretches) and arrange them so you can see all four histograms at the same time. (You can achieve this by minimizing the images and selecting Tile from the Windows menu).

Question 1
a)	Why is it advisable to use a linear redisplay stretch to load these images?
b)	On the basis of these histograms, what would your minimum and maximum be for the linear redisplay stretch?

Designing a curved stretch

Redisplay the image with the linear stretch given by your answer to question 2b. You will see that this improves the contrast slightly for several of the images. However, you can further improve the display by additional stretching using a manual stretch document.

You cannot use the automatic stretches because they are calculated from the frequency distribution of the input values, which is differs from image to image. However, as you know that a Gaussian and Equalize stretches often gives good results, you could perhaps use a manual stretch document with knees arranged so that it broadly resembles the shape of either of these two stretch options.

To give you an idea of what such a stretch document would look like, use the stretch menu to apply a Gaussian stretch to each image, and open the corresponding stretch document ( [CTRL+S] > [V] ). Repeat for the Equalize stretch.

Question 2
What are the common features of these stretches?

Bilko allows a maximum of 10 knees in a manual stretch. Between these the stretch function is linear. One way of finding suitable positions for the 10 knees would be to carry out an equalize stretch with only 11 (10+1) output levels. You could then create a similar manual stretch by placing a knee in the positions corresponding to each step in this stretch document.

Question 3
Based on this stepped Equalize stretch for the December image, where would you place the knees in the manual stretch for these images? Note: At present there is a bug in Bilko that means the null values are included in the stretch calculations. For the moment, use the equalize stretch document supplied here to find the position of the knees.

Open a manual stretch document and place the knees as indicated by your answer to question 3. Apply this to the December image.

Saving the stretch

To apply this new stretch to all the images you first need to save it as a stretch file:

1. Select save from the menu bar (or click the save icon ). When the Save As diaolog opens, the file type is automatically set to Stretch (*.str).
2. Call the stretch ats_sst_io.str (this tells you it is for AATSR SST images of the Indian Ocean.
3. Close the stretch document (necessary for the next step of applying the stretch to the other images).

Applying a saved stretch

Now that the stretch is saved and closed you are ready to apply it to the four images.

1. Activate the March image
2. From the file menu select Open ( or [CTRL+O] ).
3. In the Open dialog change the file type to Stretch (*.str); this allows you to see all stretch documents in the folder.
4. Select the stretch document you just saved (ats_sst_io.str), and watch how the stretch is automaticaly applied to the March image. (The stretch doucment does not open, but is applied directly.)
5. Repeat 1-4 for the other images.