#80 Output goes to stderr instead of stdout

[BILLY NOAH]

Seems all output going to stderr? Is this a bug or by design? Example:

$optipng -preserve -o2 example.png 2> output.txt
$cat output.txt
** Processing: example.png
60x60 pixels, 3x8 bits/pixel, RGB
Input IDAT size = 6599 bytes
Input file size = 6677 bytes

Trying:
  zc = 9  zm = 8  zs = 1  f = 5     IDAT size = 6599

example.png is already optimized.