[GRASSLIST:1492] Re: GDAL with CIR-photos

[Frank Warmerdam]

L. Junek wrote:
> Hi,
>  I imported pseudocolor-Fotos with GDAL, they`re scanned as a *.jpg
>  (o.k., this is maybe a challenge).
>  As they always appear splittet into RGB-Channels, what about the
>  Magenta, which is so important to handle the information in the
> photos?
>  I assume this color-information is splitted through additive
>  colorcomposition into blue and red? So maybe the blue channel shows a
>  part of the magenta color? 
> Is there maybe a loss of information now using GDAL? 
> Is there any experience on that, or does somebody know how the
>  GDAL-library handles this?

Hi,

If the input JPEG file is in the YCbCr "color space" it will be automatically
translated to RGB by libjpeg (under GDAL).  You mention Magenta.  While I don't
know too much about YCbCr I don't think it is exactly the same as CMY (Cyan,
Magenta Yello) or CMYK color space which includes a magenta component.

For most purposes the RGB color representation should be as expressive as the
original color space, though there is undoubtedly some slight color degredation
occuring in the color space translation.

I would suggest looking up a "color FAQ" or something similar if you want to
get formulas to compute a magenta component from the RGB.

Best regards,
-- 
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