3.2 Setting the stretch options

Stretch Options When to apply stretches Manual stretch or redisplay?

TUTORIAL

3. Stretches

Images used:

Description of mer_20040201 _algal1.dat

Download mer_20040201 _algal1.dat
(83 K)

Description of ATS_TOA_1C_ 20040201~.N1

Download ATS_TOA_1C_ 20040201~.N1
(7.7 MB)

Useful information:

Numerical data formats

Null values and missing data

In the previous section you created a manual stretch for the image mer_rr_20040201_082314_algal1.dat based on a histogram of the image. If your purpose in applying a stretch was simply to improve the display, you may want all other manipulation of the image to deal with the original data values. For example, oceanographers would prefer that a histogram of part of this image should refer to the actual data values whether or not a stretch is applied.

The Stretch Options dialog

To ensure that this happens, activate the image and select Stretch > Options from the menu bar ( [ALT+S] > [O] ), and when the Stretch Options dialog opens, make sure the box marked Apply stretches to charts, clipboard etc. is unchecked (see figure (5K)).

Note: As you can see the Stretch Options dialog allows you to change a number of other settings. You will encounter these later in this chapter, here we are just concerned with the option to apply stretches to charts, clipboard etc.

Activity / Question 1
a)	Try creating a new histogram document from a subsection of the image (for example [x,y]=[0,315] DX=360, DY=390). Note how the check box to the bottom left of the New dialog is unchecked. Leave it this way and open the new histogram. How does the new histogram compare to the histogram of the full image, which you created in section 3.1?
b)	Now open the Stretch Options dialog again change the stretch options by checking the Apply Stretches box, and create another histogram from the same subsection. (Note how the checkbox at the bottom of the New dialog is now checked. Why do you think gaps appear in the main peaks of the new histogram?
c)	The image information on the sidebar tells you how the 8-bit data values relate to real pigment concentrations. Use this information to calculate the range of pigment concentrations represented by the narrow double peak at the low chlorophyll end of the frequency distribution. Which of the two histograms would you choose for this? Explain your choice.

When to apply stretches

So when should you apply stretches?
Although the safest option is usually to leave the box unchecked, there are times when it is useful to apply a stretch to image operations. One instance when it is important to apply stretches is when you are saving an image for use in reports etc. Such an image should have the optimal contrast for revealing the information you want to convey. This has usually been achieved by applying a stretch - and saving the stretch with the image is therefore essential. You will learn how to save displays in a later section of this chapter.

Another instance is the use of histograms to prepare for a manual stretch of image data that are not 8-bit integer values. The btemp_nadir_1100 band of the image ATS_TOA_1COLRA20040201~.N1, which you worked with in section 2.7, provides an example of this. Even when you do not set the Redisplay stretch options yourself, Bilko will use the default settings to calculate how the large number of values in 16-bit, 32-bit or floating point data should be mapped to the 256 possible values of the 8-bit display. The Stretch document used for manual stretches in Bilko takes as its input these 8-bit display values. The histogram you use to prepare for further stretchin of the data must therefore be based on the display values loaded during redisplay.

Activate (or open) this image and right-click to open the Redisplay diaolog.
Make sure the null value is set to -5 and the Stretch Settings specify a linear stretch with a range of 28400-31900.
When the image opens create a new histogram of the whole image, making sure the Apply Stretches check-box is checked.
Right-click on the new histogram and adjust the y-axis by checking the Ignore Zero box (You may remember from chapter 2 that the peak now mapped to zero in the display represents cloud pixels.)

Activity / Question 2 Place your cursor at the lowest point between the ocean and land peaks representing land (the low, wide peak at the high temperature end).
a)	What is the display value (x-value) of this 'valley' between the ocean and land peaks?
b)	Open a stretch document and modify it so that all land pixels become white, and the full range of grey tones are used to display the ocean. Where would you place the knee(s) in order to achieve this?
c)	How is this stretch represented on the colour bar at the bottom of the image window?

Manual stretch or redisplay?

Superficially this image looks the same as the one you created in section 2.7 using the Redisplay dialog to map land temperatures to 255 (white), but there are subtle differences. To reveal these you can create a copy of the image, and use Redisplay instead of a Stretch document to spread the full-range of grey tones across the temperature range of ocean pixels.

Select the whole image ( [CTRL+A]) ,
open a New IMAGE document, making sure Apply Stretches is uncheked.
Right-click on the new image window to open the Redisplay dialog, and change the maximum of the linear stretch range to 29350. The two images should now look virtually identical.

Activity / Question 3 Make sure the Apply Stretches checkbox is ticked, and create a new histogram for each of these two images.
a)	What are the main differences between them?
b)	How would you explain this?

The answer to question 3 implies that when displaying data of more than 8 bits, it is preferable to use a Redisplay stretch to display the range of data values of interest. This is particularly important when the numerical range of interest is small compared to the total range of the data.

Answers:
(Resizable
pop-ups)

Answer 1

Answer 2

Answer 3

Back up to:
Q1 Q2

You will find that this is often the case. Cloud and land pixels increase the range of data values dramatically, but the range represented by the ocean remains small. A good way to deal with this is to load the data with the initial default settings, and use a histograms to identify the range represented by the ocean. Once you have done that you can use Redisplay with stretch settings based on the ocean range. You can then use a manual stretch to help you interpret spatial patterns in in the ocean.

The next section introduces the saw-tooth stretch, a clever way of revealing thermal structue both in the land and the ocean. Before moving on, tidy up your desktop by closing all histograms and stretch documents. (There is no need to save them, as new ones are quickly created when you need them.) Leave the following windows open:

The image mer_rr_20040201_082314_algal1.dat (minimized)
The file structure window of ATS_TOA_1COLRA20040201~.N1 (minimized)
Two copies of the btemp_nadir_1100 band of the ATS image.

Next: A stretch that enhances land and sea