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I'm not sure whether I'm correctly understanding you. Let's consider a 
hypothetical engineering performance-versus-requirements scatter plot or 
contour plot of a performance metric that takes positive values. I'd 
like to map values to colors so that anything below 50 gets solid red, 
values from 50 to 70 get a color between red and orange (linear 
interpolation), values from 70 to 90 get a color between orange and 
yellow, values from 90 to 105 get a color between yellow and green, and 
anything greater than or equal to 105 gets solid green. Is this 
considered "smooth variation"? If so, how would I implement something 
like this?
Thanks!
Phillip
Eric Firing wrote:
> Phillip M. Feldman wrote:
>> Eric and Reinier-
>>
>> It seems to me that continuous (piecewise-linear) colormaps could 
>> work in much the same fashion. One would specify n boundary colors 
>> and n thresholds (for continuous colormaps, I believe that the number 
>> of thresholds and colors must be the same), and for any value between 
>> two thresholds, the colors associated with the bounding thresholds 
>> would be automatically interpolated. What do you think?
>
> How does this differ from LinearSegmentedColormap.from_list()? I 
> guess what you are getting at is the quantization problem I mentioned 
> in connection with discrete colormaps. But it is not a problem when 
> the colors are linearly interpolated--that is, smoothly varying from 
> one end of the map to the other. It is only a problem when there are 
> jumps.
>
> Eric
>
>>
>> Phillip
>>
>> Eric Firing wrote:
>>> What does allow you to specify the transitions exactly (to within 
>>> the limits of double precision) is this:
>>>
>>> cmap = ListedColormap(['r','g','b'])
>>> norm = BoundaryNorm([1.5+1.0/3, 1.5+2.0/3], cmap.N)
>>
>
>

Phillip M. Feldman wrote:
> Eric and Reinier-
> 
> It seems to me that continuous (piecewise-linear) colormaps could work 
> in much the same fashion. One would specify n boundary colors and n 
> thresholds (for continuous colormaps, I believe that the number of 
> thresholds and colors must be the same), and for any value between two 
> thresholds, the colors associated with the bounding thresholds would be 
> automatically interpolated. What do you think?
How does this differ from LinearSegmentedColormap.from_list()? I guess 
what you are getting at is the quantization problem I mentioned in 
connection with discrete colormaps. But it is not a problem when the 
colors are linearly interpolated--that is, smoothly varying from one end 
of the map to the other. It is only a problem when there are jumps.
Eric
> 
> Phillip
> 
> Eric Firing wrote:
>> What does allow you to specify the transitions exactly (to within the 
>> limits of double precision) is this:
>>
>> cmap = ListedColormap(['r','g','b'])
>> norm = BoundaryNorm([1.5+1.0/3, 1.5+2.0/3], cmap.N)
> 

Phillip M. Feldman wrote:
> 
> When I look at the online documentaiton for from_list, here's what I 
> see: "Make a linear segmented colormap with /name/ from a sequence of 
> /colors/ which evenly transitions from colors[0] at val=1 to colors[-1] 
> at val=1. N is the number of rgb quantization levels." There must be a 
> mistake here, because val=l at both ends. Also, is there web 
> documentation for Reinier's new version?
I'm not sure I know what you mean by web documentation, but in any case, 
I think all there is at present is the docstring. And yes, that 
docstring needs work. I'll try to correct and clarify it.
Eric