[Insight-users] Managing conversion of pixel types during file read

[Zachary Pincus]

Hmm, after looking at DefaultConvertPixelTraits and ConvertPixelBuffer, 
I am not convinced that it is possible to create a input-type savvy 
ConvertPixelTraits.

While the documentation for DefaultConvertPixelTraits states:

> This implementaion, does a simple assignment operator, so if you are 
> going from from a higher bit representation to a lower bit one (int to 
> char), you may want to specialize and add some sort of transfer 
> function.

there does not seem to be any way to do this. For example, in the 
ConvertGrayToGray method of ConvertPixelBuffer, this gets called:

OutputConvertTraits
       ::SetNthComponent(0, *outputData++,
                         static_cast<OutputComponentType>
                         (*inputData));

Note the static_cast. So if you wanted to write a ConvertPixelTraits 
class that was savvy about, say, converting ints to chars (as per the 
documentation), you're out of luck because this conversion was already 
done in the static_cast in ConvertPixelBuffer. (I think.)

So it seems that you would really have to write a new version of 
ConvertPixelBuffer that was smart about transforming pixel types 
between each other. OK, this is a bit more of a pain than just writing 
a new ConvertPixelTraits, but whatever. However, even this is not 
possible, because it only seems possible to provide an ImageFileReader 
with a new version of the ConvertPixelTraits, not of the 
ConvertPixelBuffer class.

This seems to be due to the fact that this matter *should* be 
accomplishable through ConvertPixelTraits, but at least to my eye, it 
is not.

Am I missing something fundamental here? Or is it not currently 
possible to add a specialized pixel transfer function to an 
ImageFileReader?

Thanks,

Zach


On Apr 14, 2004, at 4:03 PM, Zachary Pincus wrote:

> Hello,
>
> My plan is to convert images I read in (initially in heterogeneous 
> pixel types) to floating-point pixels. I intend to rescale each image 
> such that the dynamic range of the image type (e.g. 8 bits, 16 bits) 
> is compressed to the floating-point range [0, 1].
>
> That is, I don't mean to "normalize" each image I read in so that 
> [image.min_value, image.max_value] goes to [0, 1], but to convert 
> [ImageType.min_possible_value, ImageType.max_possible_value] to [0, 
> 1]. (If you follow my abused notation!)
>
> Is there any simple way to do this in ITK now? I could write a new 
> version of DefaultConvertPixelTraits for use with the FileReaders, but 
> (a) I don't quite know what all needs to go into this, and (b) if 
> there's an easy way already, I should use that.
>
> Thanks,
> Zach Pincus
>
>
> Department of Biochemistry and Program in Biomedical Informatics
> Stanford University School of Medicine
>
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