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List:       imagemagick-developer
Subject:    Re: [magick-developers] Questions about ImageMagick!
From:       yarrow () studio ! imagemagick ! org
Date:       2004-08-20 13:45:16
Message-ID: 200408201345.i7KDjGEx003780 () studio ! imagemagick ! org
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When an image is instantiated it defaults to a PixelPacket per pixel location
of red, green, blue, and opacity.  Opacity is considered undefined until you
set image->matte to MagickTrue.  The K channel of CMYK or colormapped indexes
are undefined until you set image->colorspace to CMYK or set 
image->storage_class to PseudoClass respectively.  They remain undefined until
your first call to GetImagePixels() or SetImagePixels().  Once defined you
access the K channel, for example, like this:

        <assume image is CMYK>
        cmyk_image->colorspace=CMYKColorspace;
        for (y=0; y < (long) image->rows; y++)
        {
          p=AcquireImagePixels(image,0,y,image->columns,1,exception);
          q=SetImagePixels(cmyk_image,0,i++,cmyk_image->columns,1);
          if ((p == (const PixelPacket *) NULL) || (q == (PixelPacket *) NULL))
            break;
          indexes=GetIndexes(image);
          cmyk_indexes=GetIndexes(cmyk_image);
          for (x=0; x < (long) image->columns; x++)
          {
            q->red=p->red;
            q->green=p->green;
            q->blue=p->blue;
            if (image->matte != MagickFalse)
              q->opacity=p->opacity;
            if (image->colorspace == CMYKColorspace)
              cmyk_indexes[x]=indexes[x];
            p++;
            q++;
          }

Even though you specify p->red, it represents the CYAN component for CMYK
images.
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