2006.09.15 17:46 "RE: [Tiff] is there alpha component present in Grayscale orPalettecolor image", by Ed Grissom

Gerben -

For palette images, is an alpha channel explicitly forbidden by the spec

?

What I am thinking is an extra data plane, not an extra column in the color table.

Transparency and especially translucency are not always tied to the color of a pixel (which would be the effect of the extra column in the colortable), rather, it is often tied to position in the image (i.e. vignettes around the edges)

In that case, the advice for palette images would be the same as your advice for the greyscale images.

--
ed grissom
ed.grissom@intergraph.com

-----Original Message-----

From: tiff-bounces@lists.maptools.org [mailto:tiff-bounces@lists.maptools.org] On Behalf Of Gerben Vos

Subject: Re: [Tiff] is there alpha component present in Grayscale orPalettecolor image

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.