[Maposmatic-dev] [task #9917] Apply optipng (or pngcrush) on generated PNG files

[Jeroen van Rijn]

Update of task #9917 (project maposmatic):

        Percent Complete:                      0% => 10%                    

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Follow-up Comment #1:

I've done some preliminary testing on a limited set of maps, doing the
following:

Listed times are on a 5337x3316 pixels, 3x8 bits/pixel map of Haarlem.
Recorded on a Phenom II quad core @ 3.2Ghz with 8Gb RAM.  Keep in mind that
optipng doesn't use multithreading and uses only a single core.

OptiPNG 0.6.2.1: Advanced PNG optimizer. (version that comes with Ubuntu
Karmic).

# exhaustive search
time optipng -zc1-9 -zm1-9 -zs0-3 -f0-5 <png>
  15 minutes, 23 seconds (real)
# 240 trials - too long to run on the server 
time optipng -o7 <png>
  4 minutes, 5 seconds (real)

# 16 trials - doable
time optipng -o3 <png>
  39 seconds (real)

# about 8 or so trials - much better
time optipng -zm 9 -zc 9 <png>
  22 seconds (real)

-o7 came up with a setting that reduced the size the most:
Selecting parameters:
  zc = 9  zm = 9  zs = 0  f = 0         IDAT size = 2485922
Output file size = 2485997 bytes (429704 bytes = 14.74% decrease)

I noticed that -o3 covers this range as well, and so does the explicit
selection of -zm 9 and -zc 9. They just do so much more quickly.

Conclusions at this point:

* optipng -o3 is a good tradeoff between time to run and size reduction,
particularly because it also found the best parameters.

Todo:
* Create a harness to create two dozen maps of various parts in the world and
their indexes;
** Have the harness time optipng at exhaustive, -o7, -o3, and zc/zm and use
-log;
** See if the best parameters selected by the exhaustive search for each of
the 24 maps are covered by the other, faster searches as well;
** Select the parameters with the best size/time tradeoff;
** Run the entire test set 3 times to have more accurate timing

* Build a system optimised binary and see if it runs significantly faster
than the stock Karmic supplied .deb

* Allow maposmaticd to call optipng with the parameters we find and decide on
(read from the configuration file)
** If it turns out maps compress better with 'x' and indexes better with 'y'
and this has no time tradeoff, add a configuration value for both separately.

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