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List:       graphicsmagick-apis
Subject:    Re: [GM-apis] understand  ImageType, ClassType and ColorspaceType
From:       Bob Friesenhahn <bfriesen () simple ! dallas ! tx ! us>
Date:       2013-04-11 14:03:21
Message-ID: alpine.GSO.2.01.1304110846330.2267 () freddy ! simplesystems ! org
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On Thu, 11 Apr 2013, maracas83@libero.it wrote:

> 
> Hi, I would like to know what's the meaning of enumerators ImageType, ClassType and \
> ColorspaceType. 
> In this regard, I read the documentation of the page \
> http://www.graphicsmagick.org/api/types.html 
> What combinations of these three enumerators can be found?  In particular in some \
> tests I have noticed that different types of images (Grayscale, black and white, \
> RGB, BGRA) provides always the same ColorSpaceType that is sRGBColorspace.
> 
> Furthermore after reading images with 32 bit per pixel when the number of colors is \
> low ImageType=PaletteType, while in the case of high number of color \
> ImaType=TrueColorType 
> There is a deterministic way to know the original and the current type? There is a \
> more comprehensive documentation?

The confusing thing about these properties is that they report 
GraphicsMagick's opinion about the image as it is loaded into memory 
and does not really have anything to do with the input file format. 
It has more to do with the subformat automatically selected when the 
image is saved than the input file subformat.

For example, the input file might be in a 'RGB' storage format, but 
the pixels are actually only gray levels.  In this case, the ImageType 
would be reported as GrayscaleType.

If the image is represented in PseudoClass (color-mapped) format, then 
it will be reported as PaletteType or PaletteMatteType.  However, a 
simple enumerated field (Image 'storage_class') can be changed so that 
such images may be instantaneously switched to TrueColorType or 
TrueColorMatteType.  GraphicsMagick is able to support both 
color-mapped and direct representation of pixels at once, but only one 
representation will be used at once.  If any image processing has been 
done on an image which was originally color-mapped, it is likely to be 
converted to DirectClass representation since it is most appropriate 
for image processing.

ColorSpaceType is most likely to be reported as RGBColorspace rather 
than sRGBColorspace since the sRGBColorspace is peculiar to Kodak 
PhotoCD format.  If the input format has only one sample per pixel 
then it may be reported as GRAYColorspace.

GraphicsMagick supports a huge number of input formats.  When a file 
is read, the data is normalized into the internal representation. 
Almost all knowledge of the input format is then lost.

Bob
-- 
Bob Friesenhahn
bfriesen@simple.dallas.tx.us, http://www.simplesystems.org/users/bfriesen/
GraphicsMagick Maintainer,    http://www.GraphicsMagick.org/



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