Editing PNG (section)

==== Tool list ====
* [[pngcrush]] is the oldest of the popular PNG optimizers. It allows for multiple trials on filter selection and compression arguments, and finally chooses the smallest one. This working model is used in almost every png optimizer.
* advpng and the similar advdef utility in the AdvanceCOMP package recompress the PNG IDAT. Different DEFLATE implementations are applied depending on the selected compression level, trading between speed and file size: zlib at level 1, libdeflate at level 2, [[7-zip]]'s [[LZMA]] DEFLATE at level 3, and [[zopfli]] at level 4.
* [[pngout]] was made with the author's own deflater (same to the author's zip utility, kzip), while keeping all facilities of color reduction / filtering. However, pngout doesn't allow for using several trials on filters in a single run. It's suggested to use its commercial GUI version, pngoutwin, or used with a [[#Wrapper tools|wrapper]] to automate the trials or to recompress using its own deflater while keep the filter line by line.<ref name="pngoutreusefilter" group="note">Use pngout -f6 to reuse previous filter set</ref>
* [[zopfli]]png was also made with its own deflater, zopfli. It has all the optimizing features pngcrush has (including automating trials) while providing a very good, but slow deflater.

A simple comparison of their features is listed below.
{| class="wikitable"
|-
! Optimizer !! Chunk removal !! Color reduction !! Filtering !! Filter reuse<ref group="note">The tools offering such feature could act as a pure re-deflater to PNG files.</ref> !! Multiple trials on filters in a single run !! Deflater<ref group="note">[[zlib]], the reference deflate implementation, compression is suboptimal even at the maximum level. See [[Zopfli]], [[7-zip#Others|zip format in 7-zip]] and [[pngout]].</ref>
|-
| advpng|| Yes|| No<ref group="note">Not only does advpng not support color reduction, it also fails on images with a reduced colorspace.</ref>|| 0|| No|| N/A<ref group="note">Advpng can only apply filter 0 globally, thus it's neither yes or no, but N/A.</ref>|| (multiple)
|-
| advdef|| No|| No|| Reuses previous filter set|| Always|| N/A|| (multiple)
|-
| [[pngcrush]]|| Yes|| Yes|| 0–4 or adaptive|| No|| Yes|| zlib
|-
| [[pngout]]|| Yes|| Yes|| 0–4 or adaptive|| Yes<ref name="pngoutreusefilter" group="note"/>|| No|| kzip
|-
| [[zopfli]]png|| Yes|| Yes|| 0–4 or adaptive with 2 different algorithms, or with a brute way|| Yes|| Yes|| zopfli
|}

Before zopflipng was available, a good way in practice to perform a png optimization is to use a combination of 2 tools in sequence for optimal compression: one which optimizes filters (and removes ancillary chunks), and one which optimizes DEFLATE. Although pngout offers both, only one type of filter can be specified in a single run, therefore it can be used with a [[#Wrapper tools|wrapper tool]] or in combination with [[pngcrush]],<ref name="pngoutreusefilter" group="note"/> acting as a re-deflater, like advdef.