Histogram equalization is a process of adjusting the image so that each intensity level contains an equal number of pixels. In many cases, this can improve the appearance of the image by balancing light and dark areas. Other times, it degrades the image's appearance by accentuating image defects. This is particularly true with JPEGs.
Tnimage uses a new histogram equalization algorithm that redistributes pixels into equal-size bins much more equitably than other algorithms. This produces a noticeably smoother result than other programs.
One problem that arises when equalizing color images is that if the red, green, and blue are equalized independently, as tnimage does, this can alter the color balance of the image. If you want to remove this feature, change the line
#define COLORS_EQUAL 0
in xmtnimage4.cc to
#define COLORS_EQUAL 1
and recompile tnimage. This will reduce the amount of equalization that is performed, and more closely mimic other software. Alternatively, you can readjust the color balance with the Contrast or Intensity controls in tnimage.