2.7   Histograms and Redisplay

In this section you will look at how histograms can help you display 16-bit data in an optimal way, by setting minimum and maximum values for the Stretch Settings in the Redisplay dialog.

Note: To carry out this work the following should be open in the Bilko program window:
  - the ATS_TOA_1COLRA20040201~.N1 file structure window,
  - the btemp_nadir_1100 band from this image, and
 - the histogram from the ocean/land selection saved in section 2.6.

How a redisplay stretch works

The range of values used for the Redisplay stretch are mapped to 255 grey scale values in the image display in the following way:

• Pixels with with values equal to or less than the minimum value for the stretch become black in the image display.
• Pixels with values equal to or greater than the maximum become white.
• All values in the range between the minimum and maximum are displayed as shades of grey. The shade given to a particular pixel depends on where its value lies along a line between the minimum and maximum stretch values.

Sometimes setting the null value and accepting Bilko's default stretch is sufficient to display the data in an acceptable way. However, often you can improve the display by choosing your own minimum and maximum values. A histogram can help you choose an appropriate range for your own Redisplay stretch.

Activity / Question 1
Activate the histogram of the land/ocean selection from the previous section and use the cursor to find the digital value corresponding to minimum and maximum values of the frequency distribution. What are they? (Round the minimum value down to the nearest 100 and the maximum up to the nearest 100).

Changing the stretch range

You will now use your answer from question 1 to change the stretch settings in the Redisplay dialog.

1. Activate the btemp_nadir_1100 image, and open the Redisplay dialog by right-clicking on the image and selecting Redisplay, from the pop-up menu.
2. Leave the null-value set to -5.
3. In the Stretch Settings, enter the rounded minimum and maximum values from your answer to question 1.
4. Leave the Use option as Linear stretch and accept the new settings.

Revealing ocean details