4.   Palettes: using colour to enhance display

This chapter of the tutorial introduces another Bilko document: Palettes. In Bilko (as in many other image processing packages) palettes in are distinguished by the file extension .pal.

Palettes allow you to replace the 256 grey shades you have been working with 256 different colours. The human eye can distinguish a large number of colours and is highly sensitive to colour changes. Applying a colour palette therefore enables the eye to see more detail in the patterns than is possible with a grey-scale palette. A particular benefit of colour is that it enables the same data value to be identified in widely separated regions of the image.

Aim and objectives

This chapter shows how to use colour palettes in order to convey useful information. By the end of the lesson you should be able to

• apply a colour palette to an image,
• transfer a palette from one image to another,
• save a paletted image,
• understand how a palette relates to numerical values in an image,
• understand the relationship between palettes and stretches,
• modify an existing palette,
• design and apply a new custom palette,
• save a palette and apply the same palette to a series of images, and
• use both colour and grey-scale palettes appropriately.

Chapter content

1. Applying a colour palette - opening and a applying a colour palette to an image.
2. Transferring palettes - copying a palette from one image to another.
3. Saving paletted images - how to save the paletted colour display.
4. How palettes work - the relationship between palette colours and numerical display values.
5. Palettes and stretches - how stretches affect the colour display.
6. Modifying existing palettes - using the palette tool to adjust colour displays.
7. Designing a new palette - using the palette tool to create your own palette.
8. Colour or grey-scale? - advantages and disadvatages of using colour palettes.
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_1COLRA20040201~.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_1PNPDK20041130~.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.