[vtkusers] Complementary color LUTs

[Jothybasu Selvaraj]

Fantastic David!

With the latest modification on table2 you sent, it  works well.

I agree, it will be great if VTK can add a class to do this where users can
choose a set of colors they need to fuse.


Thanks a lot!


Cheers

Jothy

On Thu, Apr 21, 2016 at 12:45 AM, David Gobbi <david.gobbi at gmail.com> wrote:

> Looks like I was wrong about the second table.  It should use this:
>
>     table2 ->SetValueRange(0.0,1.0);
>     table2 ->SetAlphaRange(0.5,0.5);
>
> Ideally, it would be nice if VTK provided a way to simply add the
> two colors together, but the best you can do is use the alpha to
> make VTK multiply the images by 0.5 before adding them.  As a
> result, you end up with grey instead of white.
>
>  - David
>
> On Wed, Apr 20, 2016 at 1:15 AM, David Gobbi <david.gobbi at gmail.com>
> wrote:
>
>> Hi Jothy,
>>
>> I see this mistake a lot... people try to blend images by setting the
>> alpha of both images to 0.5.  That's wrong.  If you want to blend images,
>> then the first image should be opaque, and the second image should have an
>> alpha of 0.5.
>>
>> C = C1*(1 - alpha2) + C2*alpha2
>>
>> The second mistake is that, if you want to blend complimentary colors,
>> then you don't want the Value of the second image to be the ramp.  Instead,
>> you want the Alpha of the second image to be the ramp.
>>
>>     // Create a GREEN lookup table
>>     vtkSmartPointer<vtkLookupTable> table =
>>             vtkSmartPointer<vtkLookupTable>::New();
>>     table->SetHueRange(0.33333,0.33333);
>>     table->SetSaturationRange(1.0,1.0);
>>     table->SetValueRange(0.0,1.0);
>>     table->SetAlphaRange(1.0,1.0); // make the bottom image opaque
>>     table->SetRampToLinear();
>>     table->Build();
>>
>>     // Create a MAGENTA lookup table
>>     vtkSmartPointer<vtkLookupTable>table2 =
>>             vtkSmartPointer<vtkLookupTable>::New();
>>     table2 ->SetHueRange(0.8333,0.8333);
>>     table2 ->SetSaturationRange(1.0,1.0);
>>     table2 ->SetValueRange(1.0,1.0); // use pure magenta, don't mix with
>> black
>>     table2 ->SetAlphaRange(0.0,1.0); // use an alpha ramp to blend the
>> images
>>     table2 ->SetRampToLinear();
>>     table2 ->Build();
>>
>> - David
>>
>>
>> On Tue, Apr 19, 2016 at 11:43 PM, Jothybasu Selvaraj <jothybasu at gmail.com
>> > wrote:
>>
>>> Hi All
>>>
>>> I am trying to overlay 2 registered images using complementary LUTs
>>> (magenta & green), so that the images will appear white where they match
>>> perfectly. However, I couldn't get the desired results. The image appears
>>> magenta where they match.
>>>
>>> Can anyone find any mistakes in this?
>>>
>>> Below are the LUTs I use:
>>>
>>> // Create a GREEN lookup table
>>>
>>>     vtkSmartPointer<vtkLookupTable> table =
>>>
>>>             vtkSmartPointer<vtkLookupTable>::New();
>>>
>>>     table->SetHueRange(0.33333,0.33333);
>>>
>>>     table->SetSaturationRange(1.0,1.0);
>>>
>>>     table->SetValueRange(0.0,1.0);
>>>
>>>     table->SetAlpha(0.5);
>>>
>>>     table->SetRampToLinear();
>>>
>>>     table->Build();
>>>
>>>
>>> // Create a MAGENTA lookup table
>>>
>>>     vtkSmartPointer<vtkLookupTable>table2 =
>>>
>>>             vtkSmartPointer<vtkLookupTable>::New();
>>>
>>>     table2 ->SetHueRange(0.8333,0.8333);
>>>
>>>     table2 ->SetSaturationRange(1.0,1.0);
>>>
>>>     table2 ->SetValueRange(0.0,1.0);
>>>
>>>     table2 ->SetAlpha(0.5);
>>>
>>>     table2 ->SetRampToLinear();
>>>
>>>     table2 ->Build();
>>>
>>>
>>> Any help is highly appreciated!
>>>
>>>
>>>
>>> Cheers
>>>
>>>
>>> Jothy
>>>
>>>
>>>
>
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