[vtkusers] Colour map complex scalars

[David Gobbi]

Oops, I misread what you were attempting... you aren't mapping
real/imag independently, you're using them to compute a new scalar
value.  So you can ignore my previous response.

You could turn your real/imag scalar values into 2D texture
coordinates, and then use vtkTexture to make a 2D texture that will
map them to colors i.e. a 2D texture color table.  The texture would
have to be re-generated at each animation step... not a small task,
since it would be a 256x256 texture, but easily done at >60fps.  Any
other options that I can think of (apart from shader programs) would
require re-loading the mesh at each step.

 - David


On Thu, Apr 7, 2011 at 2:31 PM, David Gobbi <david.gobbi at gmail.com> wrote:
> Hi John,
>
> I've also been thinking about mapping complex scalars, because I know
> it is an issue that I will run into sooner rather than later.  However
> it isn't just complex data that I'll have to worry about, but
> multi-channel data in general.
>
> My thought was to add multi-channel mapping directly to
> vtkScalarsToColors (the base class of vtkLookupTable), by making it
> possible to add "child" tables that will be used if the input has
> multiple components.  In practice, it would look something like this:
>
> table->SetVectorModeToIndependent(); // map each component independently
> table->SetComponentTable(0, table_for_real);
> table->SetComponentTable(1, table_for_imag);
>
> Given a single-channel input, the base table would be used, but given
> a multi-channel input, the "child" tables would be used.
>
> Putting these features into the vtkScalarsToColors base class would
> also effectively force the mappers/painters to play along and provide
> similar multi-channel color mapping features.  For example, scalars
> with N independent channels can be color-mapped by an NxM texture
> where M is greater than or equal to the number of colors per table.
>
> For images I've used an in-house class called vtkImageComposite for
> accomplish basically the same thing (it is image-specific) but, as you
> say, it would be nice to have multi-channel capability built directly
> into vtkLookupTable.
>
>  - David
>
>
> On Thu, Apr 7, 2011 at 1:54 PM, John Platt <jcplatt at dsl.pipex.com> wrote:
>>
>> Hi,
>>
>> I would like to colour map complex scalars. This could be the magnitude or the real part after multiplying the scalars by exp(i theta) for a specified angle theta.
>>
>> I have written a class vtkLookupTableComplex derived from vtkLookupTable. MapScalarsThroughTable2() does the conversion of the real and imaginary components to a single value which is then mapped through the table.
>>
>> This appears to work except when InterpolateScalarsBeforeMapping is set on the mapper. In this case, the scalars are mapped by vtkScalarsToColorsPainter::MapScalarsToTexture().
>>
>> I am beginning to think a vtkDataSetAlgorithm based solution would be easier but I need the performance for animating the scalars by varying the angle theta from 0 - 360.
>>
>> Should I give up or is there another approach?
>>
>> Thanks for your time.
>>
>> John.
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