Index
Single Raster Analysis Tools VIII FUZZY	A-Z	Map Pairs Analysis Tools I Comparing images

How to work with Palettes and Symbol Files
- a recreative chapter?

• Palettes
  You might have asked yourselves a lot of questions about colors and IDRISI:
  
  • How does IDRISI match colors with cellvalues?
  • What for does the software give us the choice between several already fixed palettes? (see that kind of torn DISPLAY LAUNCHER figure with the palette options in its middle part)
  • Which palette should one use with which types of data?
  • Why 16- versus 256-colors palettes?
  • When do I need own palettes and how do I create them?

  Let us unroll the whole one by one. Matching the colors has to do a lot with that technical staff called 'data types'. IDRISI builds colors upon the RGB-colorscheme (Red Green Blue) in the way that we have the image in one hand and a table of 256 color-indices in the other. These indices point to RGB-combinations, where each of that base colors ranges from 0 - 255. Imagine the number of possible combinations! 256 x 256 x 256 = 16777216.
  

  But remember that at this time IDRISI allows to use a 256 colors-subset out of the 16.7 millions. How that 256 colors will appear on your screen in turn depends on the graphics card that's in the computer you use (and - of course to a smaller amount - on your screens quality and settings). So-called '8-bit cards' (known also as the 'color depth') are cappable to display 256 colors, as each of the 8 bits can be in 2 states - on or off, 1 or 0 - so 2⁸ makes the magic 256. You should not run IDRISI for WINDOWS with a 4-bit color depth card (only 16 colors). Computer users often receive their systems 'fully set up', but now comes the moment of truth where you really should know about the resolution (how many pixels can be displayed on the screen) and the color depth. All modern graphics cards allow to be adjusted to a variety of different resolution/color depth-combinations under MS WINDOWS (3.x / 95 / NT / WIN-OS/2). The decision which to use, depends also upon the size of your monitor. Sorry to say, but serious working with graphical applications requires that basic knowledge about your hardware and its appropriate settings. Plug-and-Play? Hahaha!!

  Now, on we go! IDRISI looks at the data type of the image file: if it is byte (integers from 0 to 255) the matching works perfect, as each cellvalue can be interpreted as a colorindex number, e.g. a cellvalue of 10 will receive the RGB-color of the color no. 10, 143 will show the color of color index no. 143 and so on (see also Single Raster Analysis Tools I).
  Fine, but how does IDRISI with cellvalues such as -32? Or 45.789? Or 257? 54654.432? In the case of integers or reals IDRISI cannot perform that 1:1 matching of data values to color indices, so it takes the lowest and the highest value of the image and gives them the color no. 0 resp. 255. All the other values are autoscaled to fit in that range. This is indicated by automatically activating the autoscaling within the DISPLAY LAUNCHER module (again see the figure there to find that checkbox; with byte images we can force the IDRISI display system to do that autoscaling by clicking the checkbox to on).

  Unpuzzling the next miracles: why those many different palettes and when to use which one? Take a glance at the following examples:

  Idrisi 256	Qualitative 256	Grey 256
  Idrisi 16	Qualitative 16	Grey 16

  It is a DEM, a digital elevation model, where each cellvalue means a height at this location. How different it looks in each of the 6 palettes! Which one would you mostly prefer if the task was to roughly interpret the general height distribution?
  

  the Idrisi 256 palette

  looks rather beautiful (know that plasma clouds generating algorithms in programs for computing fractals?) ... but is maybe not the one to show us information at its best here

  the Qualitative 256 palette

  produces a really shrill output. These (the Qual256 and Qual16 palettes) have colors arranged in a manner that allows easy separation between nearby color indices. So a color is always followed by a completely different color in these types of palette tables. Use it for nominal data as an example.

  the Grey 256 palette

  again smooth clouds that may not be easy translated to a fine imagination

  the Idrisi 16 and the Qualitative 16 palettes

  do best demonstrate the two concepts behind: with the Idrisi 16 option the DEMs structures begin to crystallize, whereas the Qualitative 16 one does not really indicate a continuity.

  the Grey 16 palette

  makes most sense as the separation of 16 greys is more comfortable to the human eye than a differentiation between 256 nuances of greys or color shades. As we face IDRISI with real type height values, autoscaling is being activated. By using the Grey 16 (or Idrisi 16, Qualitative 16, or whatever palette with 16 colors), all our cellvalues are pressed in the range 0 to 16, what causes the images to show this stepped appearance.

  If we want to demonstrate seamless transitions in our data, the usage of Idrisi 256 or Grey 256 (or maybe the 16 values variants depending on the number of classes in the data) would be reasonable. Examples are DEMs, sound ranging maps or maps showing the spread of pollution.
  To show clearly distinct neighbouring areas holding disrecte data, the Qualitative 256 or 16 palette options looks best (e.g. vegetation maps, geological formation maps).

  Which palette option - the 256 or the 16 - to choose is a question of the number of different classes in your image. The palettes named and pictured above are just a subset of what's included in IDRISI. It knows two types of palette files:

  1. ASCII color tables: these are the *.pal-files those of you IDRISI for DOS wizards know quite well. They are simply 4-columns tables, where the first column indicates the color index number and the 3 remaining columns the red / green / blue values. These range between 0 and 63, thus allowing for 64 levels (reasons are former graphical hardware limits).
  2. binary palette files: you recognize them from their *.smp-extension. The various palette options, we were talking about are in fact nothing but single palette files that have been copied to your IDRISI for WINDOWS program directory. So don't kill anything from there! When you open a pick file list from the palette options field, IDRISI finds and displays those in the program- and those in the current data-directory.

  When do I have to create my own palettes and how can IDRISI help?
  In most of your projects you will have to alter the IDRISI palettes or create them new to fit your needs. The colors in the figure to the left for example show a new built palette file for a vegetation map. The digital colors should be somewhat similar to those used in the analog maps.

  IDRISI for WINDOWS includes a valuable little program, called Palette Workshop. I will not explain its functions in detail but just the purpose of it in short. You call it from within the IDRISI menu but it has its own menubar. Simply load the palette to be altered or start creating your own new one. Color entries may be changed very comfortable by moving RGB-sliders. The palette may be reversed as a whole. You can fix the allowable autoscaling range to a certain subset (the reverse function allways acts on these subset). The big Blend-button (litt'l bit down and right from the middle of the graphic) lets you interpolate the colors between a start and a end color. Of course your new colorful creations can be saved (in program directory, what makes them permanently available or just in the actual project directory; as *.smp or as *.pal file).
  (Mis)usage of colors would be a chapter for its own. They are a important part of your product and can direct or guide the viewer. Read more about in e. g. Mark MONMONIERs 'How to lie with maps' (Univ. of Chicago Press, Chicago 1991).

  It should be noted here, that IDRISI has its own Palette conversion utility PALIDRIS.

• Symbol Files
  IDRISI uses symbol files to attach colors, shades, pattern-fills, textstyles, etc. to vector layers. In the chapter about vector files we learnt that IDRISI knows 4 vector object types - points, lines, polygons and texts. Each one of these has its exclusive symbol file type. These filetypes can be easily recognized by their extensions.
  • *.sm0 .... point symbol files
  • *.sm1 ..... line -- "" --
  • *.sm2 .. polygon -- "" --
  • *.smt ..... text -- "" --

  As common with IDRISI, pick file lists for symbol file options always offer you the appropriate symbol type. Each type comes shipped with one default symbol file from the beginning.
  Equivalent to the Palette Workshop a Symbol Workshop allows you to change existing or create new ones. Again the functionality as described above for the Palette Workshop is included, but will be enhanced in future versions I believe.

	Index
Single Raster Analysis Tools VIII FUZZY	A-Z	Map Pairs Analysis Tools I Comparing images

last modified: | Comments to Eric J. LORUP