Re: 8bit tiff -- Change in Behavior / Bug : 8bit, 256color, PHOTOMETRIC_PALETTE == converted to 16-bit tiff

> The primary problem for my application is that the 8bit tiff gets
> converted to a 16bit image using the current version of ImageMagick.

Of course.  The colormap has a value of 65280 which does not fit in an
8-bit value without losing precision.

> The current versions of ImageJ and Gimp read this tiff as a 8bit image.

We recommend you file a bug report with these projects.

> I appears the code in coders/tiff.c [ near line 1000 ] is causing the
> pixels to b e converted from 8bit to 16bit pixels.  If ImageMagic code
> was modified so the pi xels were kept at 8bits (as with ImageJ and Gimp),
> then this would also avoid the
>  problem you outline below.

We certainly will if you can tell us how to fit the value 65280 in 8
bits without losing color information.  Note the accepted conversion
of 16-bit to 8 is 16-bit-value / 257.  See Google for an explanation on
why 257 rather than 256.  The TIFF library is the definitive standard for
the TIFF specification.  We cut-n-pasted code directly from the library for
converting the 16-bit colormap to 8 and we get

  if (checkcmap(in, 1<<bitspersample, red, green, blue) == 16) {
    int i;
  #define CVT(x)          (((x) * 255L) / ((1L<<16)-1))
    for (i = (1<<bitspersample)-1; i >= 0; i--) {
        red[i] = CVT(red[i]);
        green[i] = CVT(green[i]);
        blue[i] = CVT(blue[i]);
    }
  #undef CVT
    }

Notice how it returns 254 for the maximum color value.

> The image below that experiences this problem from an automated
> microscope that i s used by many biologists and scientists [ this
> image is of fluorescent cell nucl ei ].  Due to the quantitative
> nature of this work, it is important that pixel va lues are accurate.
> http://www.huntermikic.com/imagemagick/imagemagick_AS_09047_050428030001_O01f00d0
> .TIF

That is precisely why we return 16-bits instead of 8.  We are returning the
precise values that the image colormap reports (see below).  You can return
an 8-bit image with this command:

  convert image.tif -depth 8 new.tif

However, the maximum value will be 254.  You can skew the data slightly to
return the desired 255 maximum value with the -evaluate option.