3.   Stretches: Improving display contrast

This chapter of the tutorial introduces another Bilko document: Stretches. In Bilko stretches in are distinguished by the file extension .str. They allow you to customise the way data values read from an image file are mapped to the brightness values in the 8-bit image on the computer screen, in such a way that it becomes easier to see objects and patterns of interest.

Aim and objectives

The chapter shows how to use stretches to optimise the way images are displayed. By the end of the lesson you should be able to

• interpret histogram data to identify the range of pixel values containing the spatial structures you want to enhance,
• open a new Stretch file and understand what it represents,
• modify a stretch and apply it to an image,
• understand when to apply/not apply stretches to histograms, transects and new images,
• use the automatic stretches in the stretch menu,
• understand the effects of Logarithmic, Gaussian and Equalize stretches,
• understand the use of stretches when redisplaying 16 or 32-bit image data,
• extract and transfer a stretch from one image to another, and
• save a stretch for later use

Chapter content

1. Creating and modifying a manual stretch - preparing for, opening and adjusting a stretch document.
2. Setting the stretch options - when to apply a stretch to other image operations
3. A stretch that enhances land and sea - applying a sawthooth stretch
4. Using stretches to interpret data - working with stretches and histograms to investigate spatial patterns
5. Saving the display - creating an image with the stretch permanently applied
6. Using the automatic stretches - Autolinear, Gaussian and Equalise stretches
7. Redisplay stretch options - displaying 16-bit, 32-bit and floating-point data
8. Saving and re-using a stretch - using the same stretch on several images
9. Summary and conclusions - what you should have learnt in this section

Images used in this chapter

The images for this lesson (below) are usually stored in the folder called tutorial_data Throughout the lesson you can use the links on the right sidebar to get information about them.

• mer_20040201_algal1.dat
  Level 2 image of algal pigment concentration (chlorophyll) from the Benguela region of Southern Africa recorded by MERIS on Envisat.

• ATS_TOA_~_20040201~.N1
  Level 1 scene from the Benguela region on the west coast of southern Africa recorded by AATSR on Envisat on 1st Feb.2004.

• ASA_GM1_~20041130~.N1
  SAR image of the B-15A iceberg on 30 November 2004 heading for a collision with the Drygalski Ice Tongue near the US McMurdo Research Station in Antarctica, recorded by ASAR on Envisat.

• ATS_DSST10_IO_03~.dat
  Monthly (level 3) sea surface temperature from the Indian Ocean recorded by AATSR in March, June, September and December 2003, and saved at 10 arcsecond spatial resolution.