Re: The Colormap of the example polar_7

Just as a small addition to Mary's suggestion, the "10-16 levels" comes from the value given to the resource cnMaxLevelCount, which is by default set to 16. For the automatic level selection mode, the contour object looks for an evenly-spaced set of "nice" values that produce a number of levels equal to or less than the cnMaxLevelCount. So if you want more levels, but don't require that they be exactly the size of your color map then you can simply set cnMaxLevelCount to the most levels you can tolerate, given the current colormap size.
 -dave

On Mar 28, 2011, at 8:44 AM, Mary Haley wrote:

> Dear Lou,
>
> NCL does not pick vector or contour levels based on the number of colors in your color map.
>
> Rather, it looks at the range of your data and tries to pick "nice" levels giving you about 10-16 levels. It then goes through the colormap and assigns colors to each level, based on an even spacing through the color map (in this example, anyway).
>
> If you want the color bar to contain exactly the number of colors as the range of your color map, then you will need to know how many colors are in your color map, and set the number of levels equal to this number minus 1.
>
> For example, in the polar_7.ncl code, you would do something like this:
>
> . . .
> gsn_define_colormap(wks,"gui_default") ; choose colormap
> i = NhlNewColor(wks,0.7,0.7,0.7) ; add gray to colormap
> ;---Get number of colors in color map
> getvalues wks
> "wkColorMapLen" : cmaplen
> end getvalues
>
> ;---Don't want to use first two or last two colors
> nlevels = (cmaplen-4)-1 ; One fewer levels than colors
> levels = fspan(1,15,nlevels)
>
> . . .
> res@vcLevelSelectionMode = "ExplicitLevels"
> res@vcLevels = levels
> res@lbLabelAutoStride = True
> ;
> ; The default will be to start at color 2, and go one by one through the
> ; color map until you run out of colors, so we don't need these three
> ; resources.
> ; res@gsnSpreadColors = True ; use full colormap
> ; res@gsnSpreadColorStart = 2
> ; res@gsnSpreadColorEnd = -3
>
> Note that doing this way causes you to have vector levels with values like 1.737, 2.474, 3.211, etc.
>
> Also note that I hard-coded the vector levels, because I knew the approximate minimum/maximum of the data.
> It is better to calculate this information rather than hard-code it.
>
> --Mary
>
> On Mar 28, 2011, at 12:24 AM, 娄一凡 wrote:
>
>> The colormap of polar_7 is "gui_default“.
>> It contains 24 colors.The start of it is 2 and the end is -3.So the colormap displayed in the wks should contain 20 colors.
>> While the example contains only 16 colors,can you tell me the reason?
>> I'm a greenhand at ncl and want get more help from you.<330.gif>
>>
>> --
>> Lou Yifan
>> Tsinghua University
>> Department of Computer Science and Technology
>> 86-15710059560
>>
>>
>>
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Received on Mon Mar 28 11:55:56 2011