Lossless Optimization of PNG Image File Sizes

pngout and optipng are command-line utilities for lossless compression (optimization) of PNG image files. Here is a simple example, based on a black and white line plot, r-mle-z.png (source):

% pngout r-mle-z.png
 In:   11150 bytes        r-mle-z.png /c3 /f0 /d8
Out:    8476 bytes        r-mle-z.png /c0 /f5 /d8
Chg:   -2674 bytes ( 76% of original)

In this case, pngout was able to reduce the size by 24% while optipng achieved a reduction of 18% on the same original file:

% optipng -o 7 r-mle-z.png 
** Processing: r-mle-z.png
500x250 pixels, 8 bits/pixel, 256 colors in palette
Reducing image to 8 bits/pixel, grayscale
Input IDAT size = 10301 bytes
Input file size = 11150 bytes

Trying:
  zc = 9  zm = 9  zs = 0  f = 0         IDAT size = 9535
  zc = 9  zm = 9  zs = 1  f = 0         IDAT size = 9325
  zc = 9  zm = 9  zs = 0  f = 2         IDAT size = 9155
  zc = 9  zm = 8  zs = 0  f = 2         IDAT size = 9149
  zc = 9  zm = 9  zs = 1  f = 2         IDAT size = 9079
  zc = 9  zm = 8  zs = 1  f = 2         IDAT size = 9076
  zc = 9  zm = 9  zs = 1  f = 5         IDAT size = 9036
  zc = 9  zm = 8  zs = 1  f = 5         IDAT size = 9032

Selecting parameters:
  zc = 9  zm = 8  zs = 1  f = 5         IDAT size = 9032

Output IDAT size = 9032 bytes (1269 bytes decrease)
Output file size = 9089 bytes (2061 bytes = 18.48% decrease)

From the optipng output above, you can see that it is solving an integer programming problem to select the parameters zc, zm, zs, and f that minimize the file size (IDAT size).

The table above shows the file sizes, in bytes, resulting from each tool. As you can see, the relative performance of these two tools is file-specific. For a different file, a color screenshot named k.png (source), pngout reduced the size by 26% while optipng reduced it by 37%. So, it’s useful to keep both of these tools at hand.

optipng is available in MacPorts (sudo port install optipng) while to install pngopt you need to download a binary and put it in your path (e.g., in /usr/local/bin). Also be sure to see the tutorial recommend by the author of pngout.