[vtkusers] Re: Build One Lookup Table For Two Different Scalar Groups

[Amy Squillacote]

Hi Janny,

The easiest way to accomplish what you want is with a 
vtkColorTransferFunction. It is a subclass of vtkScalarsToColors, so you 
can pass it to your mapper the same way you would with a vtkLookupTable. 
The code in C++ would look like the following.

vtkColorTransferFunction *colors = vtkColorTransferFunction::New();
colors->AddRGBPoint(50, 1, 0, 0); // 50 - scalar value; (1, 0, 0) - red 
color
colors->AddRGBPoint(51, 0, 0, 1);
colors->AddRGBPoint(52, 0, 0, 0); // > 50, so use black

By default, in the vtkColorTransferFunction, scalar values that fall 
outside the scalar values you specify are mapped to the color of the 
closest endpoint. So in the above example, values less than or equal to 
50 are mapped to red, and values greater than or equal to 52 are mapped 
to black. (If you have values between 51 and 52 that should be mapped to 
black, change the scalar value in the last AddRGBPoint to get the 
coloring the way you want it. The scalar values are of type double.)

- Amy

Janny Dong wrote:
> The effect I want to get is that some polygons' colors change 
> according to their scalar values, while others only have one color. 
> How can I do that?
>
> Janny
>
> On 9/24/07, * Janny Dong* <janny.dong at gmail.com 
> <mailto:janny.dong at gmail.com>> wrote:
>
>     Hello,
>
>     I have some 2D polygons and corresponding scalars.One group of
>     scalars is between 50 and 51, the other is 100. I'd like to build
>     a color lookup table for those two groups of scalar data. For
>     scalars between 50 and 51, I want the colors to be linearly
>     interpolated between red and blue. I want 100 to be mapped as
>     black. I don't want any interpolation between those two groups. I
>     know how to build lookup tables separately for these two different
>     cases. Is it possible to make such a lookup table?
>
>     I have not got any answer to my past few questions. Please help me
>     with this one.
>
>     Thanks,
>     Janny
>
>
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-- 
Amy Squillacote
Kitware, Inc.
28 Corporate Drive
Clifton Park, NY 12065
Phone: (518) 371-3971 x106