[Insight-users] How to cast from RGB to short?

[Gaëtan Lehmann]

Le 31 août 09 à 17:23, Michael Xanadu a écrit :

> Hi Gaëtan,
>
> thank you for your support. Actually, I thought that there's a  
> difference in the number of segments between using <long> or  
> colormapper for the output of watershed filter. It seemed to me that  
> I receive more segments by using a RGB output. I found out that the  
> problem lies in the viewer I use to watch the images. It had a wrong  
> color level and showed the image to bright, so that a lot of  
> segments disapeared.
> The clue of watershed is, that it delivers no single segment I'm  
> looking for, but all segments it can find. To extract a single  
> segment, I want to use Connected Threshold (with seed) afterwards.  
> That was the reason I asked for help.
>

For a single object, I think it's easier (and more efficient) to  
simply use a BinaryThresholdImageFilter, and to set both  
UpperThreshold and LowerThresold to the pixel value of the object of  
interest. The output is a simple binary image with only your object.

Gaëtan


> Regards, Michael
>
>
>
> 2009/8/31 Gaëtan Lehmann <gaetan.lehmann at jouy.inra.fr>
>
> Le 31 août 09 à 15:33, Michael Xanadu a écrit :
>
>
> Hi,
>
> I've got a watershed filtered image processed by a colormapper. So  
> it's an image of type:
>
>    typedef itk::RGBPixel<short>   RGBPixelType;
>    typedef itk::Image<RGBPixelType, 2>    RGBImageType;
>
> Now I want to filter that image with a connected threshold filter,  
> because I need it this way.
> But ConnectedThresholdImageFilter can't process such RGB images.
> I tryed to convert the image into a short type by using  
> CastImageFilter, too.
> But I always get:
>
>    error C2440: 'static_cast' : cannot convert from  
> 'itk::RGBPixel<TComponent>' to 'short'
>
> So how can I cast from RGB to short? Or how to filter a RGB image  
> with a connected threshold filter?
>
>
> You can extract one of the channels with  
> VectorIndexSelectionCastImageFilter or go to an intensity image with  
> RGBToLuminanceImageFilter, but I doubt that's what you want. The  
> color image is only a representation of the watershed output easier  
> to read for humans, but not for computer. If you want to work on the  
> output of the watershed, it's better to not use a color mapper, but  
> directly on the output image of the watershed - the image should be  
> of type itk::Image< unsigned long, 2 >.
>
> And even if you don't use a color mapper, it may not be a good idea  
> to process the output of the watershed filter (a labeled image) with  
> a filter (ConnectedThresholdImageFilter) made to work on a grayscale  
> images.
>
> Can you explain what you want to achieve?
>
> Regards,
>
> Gaëtan
>
> -- 
> Gaëtan Lehmann
> Biologie du Développement et de la Reproduction
> INRA de Jouy-en-Josas (France)
> tel: +33 1 34 65 29 66    fax: 01 34 65 29 09
> http://voxel.jouy.inra.fr  http://www.itk.org
> http://www.mandriva.org  http://www.bepo.fr
>
>

-- 
Gaëtan Lehmann
Biologie du Développement et de la Reproduction
INRA de Jouy-en-Josas (France)
tel: +33 1 34 65 29 66    fax: 01 34 65 29 09
http://voxel.jouy.inra.fr  http://www.itk.org
http://www.mandriva.org  http://www.bepo.fr

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