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Hi - Yes, what Ryo says is exactly correct. Ferret determines the set of levels that will be used, and then linearly interpolates between the colors given in the palette. After Ferret has made a color plot, you can see some of the information about those levels by checking the value of a set of symbols. For instance, yes? USE etopo60Note that even if you specify a number of levels, Ferret uses that as a starting point for the algorithm that chooses "nice" levels, which break at round numbers. Often you'll want to use what Ferret has done and then use the /LEVELS qualifier to specify your own set of levels. Another resource for information about color palettes and LEVELS is in this FAQ, including information about the different kinds of palettes, How can I choose a good color palette for my plot? Ansley Ryo Furue wrote: Hi Peter, | I would like to know the algorythm of palette making or something | else that helps to be able to use the same palette and scale in | other applications? If I use "N" levels during plotting and i want | to use for example inverse_bluescale, how will i know the RGB values | (in percent or in values from 0...255) for the interlevels (2., | 3. ,..., N-1.) ? | | inverse_bluescale: | 0 95 95 95 !1. level | 100 0 0 95 !N. level I recommend that you start from Chapter 6, Section 5.2, of the Ferret User's Guide: http://ferret.pmel.noaa.gov/Ferret/documentation/users-guide/customizing-plots/COLOR/ I think that Ferret linearly interpolates the RGB values: you can regard a set of RGB values as a 3D vector (r,g,b) and the interpolation is to find a point on the line segment connecting the two end points, (r1,g1,b1) and (r2,g2,b2). So, for example, a 20% value should correspond to (95, 95, 95) + (0-95, 0-95, 95-95) * 0.2 = (76, 76, 95) By the way, if you want specify a correspondence between a level and a color, you might want to use the "RGB_Mapping By_level" option. (See the manual.) Hope this helps, Ryo |