Message-ID: 008401bf251e$26ced900$5101a8c0@ivo
To: <emari@medrad3.nac.ac.za>, <tiff@olympiakos.com>
Subject: Re: grayscale tiff
From: "Ivo Penzar" <ivo.penzar@infolink-software.com>
Date: Tue, 2 Nov 1999 11:35:46 +0100
References: <Pine.LNX.4.10.9911011432570.1196-100000@medrad3.nac.ac.za>
Sender: owner-tiff@olympiakos.com


> hi
>
> I have a ccd camera with a software package that saves TIFF images as
> grayscale images, but they have 16 Bits Per Sample.  (The TIFF6 standard
> only allows 4 or 8 bits per sample - so most readers can not read the
> images, or convert them to 8 bit images, but then the image contrast is
> lost). What is the best solution for this problem?  (without losing the
> information contained in an image)


TIFF 6.0 (and at least 5.0, too) allows even 13 bits per sample grayscale
images, with 9 extra samples per pixel (not used bits). OK, it is not
required for baseline TIFF readers to know how to handle such images...

You can check MinSampleValue and MaxSampleValue (if not present, you can
calculate them per each image, or you can try some of your images to see how
actually that driver/package stores the data) and then you can apply more
adequate remapping of 16 bpp grayscale data to 8 bpp. You can even apply the
gamma correction (nonlinear mapping) to enhance the contrast. For example,
your grazscale samples might all lay within the range of 0-1^12 or so. Most
readers would probably divide them by 256 (shift by 8 bits), instead of only
64 (4 bits), and thus they would really loose the contrast (the image would
appear as rasterized).

With the technique as described above (appropriate of 16 bpp grayscale
samples to 8 bpp), I can't believe that you would visibly loose any
information. After all, (most) graphic cards and CRTs use up to 8 bits per
each RGB componnent, and thus they can not show more than 256 levels of
gray.

Ivo



