[magick-developers] DICOM bug

[jcupitt at gmail.com]

Hi,.

I think I've found a bug in the DICOM reader. In the current 6.4.9-8,
coders/dcm.c has in sketch form:

  unsigned long
    index,
  ...
  index=ReadBlobLSBShort(image)

then at line 3487:

        if (signed_data == 1)
          index-=32767;

then later:

                index=1UL*ConstrainColormapIndex(image,index);

                pixel.red=1UL*image->colormap[index].red;
                pixel.green=1UL*image->colormap[index].green;
                pixel.blue=1UL*image->colormap[index].blue;

So if we are reading signed data we subtract 32767 from the colour map
index (an unsigned value). This produces a huge positive value for all
indexes less than 32767, which then gets clipped to 65535 by
ConstrainColormapIndex().

I suggest just removing the "if (signed_data == 1) index-=32767;"
lines. I'm not sure where this functionality should go. Perhaps after
looking up in the colormap?

John