By Xah Lee. Date: .


optipng manual

pngcrush vs optipng

pngcrush is an earlier tool.

following is from [A guide to PNG optimization By Cosmin Truta. At https://optipng.sourceforge.net/pngtech/optipng.html ]

pngcrush by Glenn Randers-Pehrson, available at http://pmt.sourceforge.net/pngcrush, is an open-source program that iterates over PNG filters and zlib (Deflate) parameters, compresses the image repeatedly using each parameter configuration, and chooses the configuration that yields the smallest compressed (IDAT) output. At the user's option, the program can explore few (below 10) or many (a brute-force traversal over more than 100) configurations. The method of selecting the parameters for "few" trials is particularly effective, and the use of a brute-force traversal is generally not recommended.

In addition, pngcrush offers a multitude of extra features, such as recovery of erroneous PNG files (e.g. files containing bad CRCs), and chunk-level editing of PNG meta-data.

OptiPNG by Cosmin Truţa, available at http://www.cs.toronto.edu/pngtech/optipng, is a newer open-source program, inspired from pngcrush, but designed to be more flexible and to run faster. Unlike pngcrush, OptiPNG performs the trials entirely in memory, and writes only the final output file on the disk. Moreover, it offers multiple optimization presets to the user, who can choose among a range of options from "very few trials" to "very many trials" (in contrast to the coarser "smart vs. brute" option offered by pngcrush).

It is important to mention that the achieved compression ratio is less and less likely to improve when higher-level presets (trigerring more trials) are being used. Even if the program is capable of searching automatically over more than 200 configurations (and the advanced users have access to more than 1000 configurations!), a preset that selects around 10 trials should be satisfactory for most users. Furthermore, a preset that selects between 30-40 trials should be satisfactory for all users, for it is very, very unlikely to be beaten significantly by any wider search. The rest of the trial configurations are offered rather as a curiosity (but they were used in the experimentation from which we concluded they are indeed useless!)

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