2006.09.15 17:37 "Re: [Tiff] is there alpha component present in Grayscale or Palettecolor image", by Bob Friesenhahn
I'm trying to handle tiff images with alpha component, so i want to know that any tiff image which is Grayscale or Palette color can have alpha component.
It is possible for greyscale, just use the ExtraSamples tag as defined in section 18 of the TIFF spec. It is not possible for palette images; the ColorMap tag uses a fixed format with exactly three values per entry.
I would not go so far as to say it is not possible to have an alpha channel with a Palette-based image. It does seem to be quite possible, albeit rather inconvenient/senseless if associated alpha is used. I don't recall if I have ever written such an image and I have certainly never seen one in the real world. It is correct that the color Palette itself can not contain the alpha information.
GraphicsMagick Maintainer, http://www.GraphicsMagick.org/