Re: R: Re: understand ImageType, ClassType and ColorspaceType

On Thu, 11 Apr 2013, [email protected] wrote:

> Hi,
> Thank you very much for your reply.
> Could you help me understand if my reasoning is correct?
> Can I have two possibilities:
> 1) If I have a PseudoClass image then I will have a single channel defining
> the index to the colormap. In this case the depth-per-sample is identical to
> the depth-per-pixel

Yes.  In the case of Q8 and Q16, the colormap indexes are the same 
size as the pixel color sample (8 or 16 bits).  For the case of Q32, 
the indexes are constrained to 16-bit but the colors in the colormap 
support 32-bit sample sizes.

> 2) If I have a DirectClass the depth-per-sample will be equal to a quantum
> size and the number of channels is 4 by default, even if the alpha channel can
> be ignored.

This is true.  However, there is also support for CMYKA images.  In 
that case the 'indexes' are used to store the opacity channel.  CMYKA 
is rarely used (and perhaps not perfectly supported).

Opacity is stored inverted (i.e. MaxRGB - Alpha) from normal alpha 
values.

Bob

> Thanks again.
> Paolo Pazziani
>
>
>
>> ----Messaggio originale----
>> Da: [email protected]
>> Data: 11/04/2013 16.03
>> A: "[email protected]"<[email protected]>
>> Cc: <[email protected]>
>> Ogg: Re: [GM-apis] understand  ImageType, ClassType and ColorspaceType
>>
>> On Thu, 11 Apr 2013, [email protected] 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
>> [email protected], http://www.simplesystems.org/users/bfriesen/
>> GraphicsMagick Maintainer,    http://www.GraphicsMagick.org/
>
>
>

-- 
Bob Friesenhahn
[email protected], http://www.simplesystems.org/users/bfriesen/
GraphicsMagick Maintainer,    http://www.GraphicsMagick.org/

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