File: //usr/share/doc/optipng/optipng.man1.html
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN"
"http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en">
<head>
<meta http-equiv="Content-Type" content="application/xhtml+xml; charset=UTF-8" />
<meta name="generator" content="AsciiDoc 10.2.1" />
<title>OPTIPNG(1)</title>
<style type="text/css">
/* Shared CSS for AsciiDoc xhtml11 and html5 backends */
/* Default font. */
body {
font-family: Georgia,serif;
}
/* Title font. */
h1, h2, h3, h4, h5, h6,
div.title, caption.title,
thead, p.table.header,
#toctitle,
#author, #revnumber, #revdate, #revremark,
#footer {
font-family: Arial,Helvetica,sans-serif;
}
body {
margin: 1em 5% 1em 5%;
}
a {
color: blue;
text-decoration: underline;
}
a:visited {
color: fuchsia;
}
em {
font-style: italic;
color: navy;
}
strong {
font-weight: bold;
color: #083194;
}
h1, h2, h3, h4, h5, h6 {
color: #527bbd;
margin-top: 1.2em;
margin-bottom: 0.5em;
line-height: 1.3;
}
h1, h2, h3 {
border-bottom: 2px solid silver;
}
h2 {
padding-top: 0.5em;
}
h3 {
float: left;
}
h3 + * {
clear: left;
}
h5 {
font-size: 1.0em;
}
div.sectionbody {
margin-left: 0;
}
hr {
border: 1px solid silver;
}
p {
margin-top: 0.5em;
margin-bottom: 0.5em;
}
ul, ol, li > p {
margin-top: 0;
}
ul > li { color: #aaa; }
ul > li > * { color: black; }
.monospaced, code, pre {
font-family: "Courier New", Courier, monospace;
font-size: inherit;
color: navy;
padding: 0;
margin: 0;
}
pre {
white-space: pre-wrap;
}
#author {
color: #527bbd;
font-weight: bold;
font-size: 1.1em;
}
#email {
}
#revnumber, #revdate, #revremark {
}
#footer {
font-size: small;
border-top: 2px solid silver;
padding-top: 0.5em;
margin-top: 4.0em;
}
#footer-text {
float: left;
padding-bottom: 0.5em;
}
#footer-badges {
float: right;
padding-bottom: 0.5em;
}
#preamble {
margin-top: 1.5em;
margin-bottom: 1.5em;
}
div.imageblock, div.exampleblock, div.verseblock,
div.quoteblock, div.literalblock, div.listingblock, div.sidebarblock,
div.admonitionblock {
margin-top: 1.0em;
margin-bottom: 1.5em;
}
div.admonitionblock {
margin-top: 2.0em;
margin-bottom: 2.0em;
margin-right: 10%;
color: #606060;
}
div.content { /* Block element content. */
padding: 0;
}
/* Block element titles. */
div.title, caption.title {
color: #527bbd;
font-weight: bold;
text-align: left;
margin-top: 1.0em;
margin-bottom: 0.5em;
}
div.title + * {
margin-top: 0;
}
td div.title:first-child {
margin-top: 0.0em;
}
div.content div.title:first-child {
margin-top: 0.0em;
}
div.content + div.title {
margin-top: 0.0em;
}
div.sidebarblock > div.content {
background: #ffffee;
border: 1px solid #dddddd;
border-left: 4px solid #f0f0f0;
padding: 0.5em;
}
div.listingblock > div.content {
border: 1px solid #dddddd;
border-left: 5px solid #f0f0f0;
background: #f8f8f8;
padding: 0.5em;
}
div.quoteblock, div.verseblock {
padding-left: 1.0em;
margin-left: 1.0em;
margin-right: 10%;
border-left: 5px solid #f0f0f0;
color: #888;
}
div.quoteblock > div.attribution {
padding-top: 0.5em;
text-align: right;
}
div.verseblock > pre.content {
font-family: inherit;
font-size: inherit;
}
div.verseblock > div.attribution {
padding-top: 0.75em;
text-align: left;
}
/* DEPRECATED: Pre version 8.2.7 verse style literal block. */
div.verseblock + div.attribution {
text-align: left;
}
div.admonitionblock .icon {
vertical-align: top;
font-size: 1.1em;
font-weight: bold;
text-decoration: underline;
color: #527bbd;
padding-right: 0.5em;
}
div.admonitionblock td.content {
padding-left: 0.5em;
border-left: 3px solid #dddddd;
}
div.exampleblock > div.content {
border-left: 3px solid #dddddd;
padding-left: 0.5em;
}
div.imageblock div.content { padding-left: 0; }
span.image img { border-style: none; vertical-align: text-bottom; }
a.image:visited { color: white; }
dl {
margin-top: 0.8em;
margin-bottom: 0.8em;
}
dt {
margin-top: 0.5em;
margin-bottom: 0;
font-style: normal;
color: navy;
}
dd > *:first-child {
margin-top: 0.1em;
}
ul, ol {
list-style-position: outside;
}
ol.arabic {
list-style-type: decimal;
}
ol.loweralpha {
list-style-type: lower-alpha;
}
ol.upperalpha {
list-style-type: upper-alpha;
}
ol.lowerroman {
list-style-type: lower-roman;
}
ol.upperroman {
list-style-type: upper-roman;
}
div.compact ul, div.compact ol,
div.compact p, div.compact p,
div.compact div, div.compact div {
margin-top: 0.1em;
margin-bottom: 0.1em;
}
tfoot {
font-weight: bold;
}
td > div.verse {
white-space: pre;
}
div.hdlist {
margin-top: 0.8em;
margin-bottom: 0.8em;
}
div.hdlist tr {
padding-bottom: 15px;
}
dt.hdlist1.strong, td.hdlist1.strong {
font-weight: bold;
}
td.hdlist1 {
vertical-align: top;
font-style: normal;
padding-right: 0.8em;
color: navy;
}
td.hdlist2 {
vertical-align: top;
}
div.hdlist.compact tr {
margin: 0;
padding-bottom: 0;
}
.comment {
background: yellow;
}
.footnote, .footnoteref {
font-size: 0.8em;
}
span.footnote, span.footnoteref {
vertical-align: super;
}
#footnotes {
margin: 20px 0 20px 0;
padding: 7px 0 0 0;
}
#footnotes div.footnote {
margin: 0 0 5px 0;
}
#footnotes hr {
border: none;
border-top: 1px solid silver;
height: 1px;
text-align: left;
margin-left: 0;
width: 20%;
min-width: 100px;
}
div.colist td {
padding-right: 0.5em;
padding-bottom: 0.3em;
vertical-align: top;
}
div.colist td img {
margin-top: 0.3em;
}
@media print {
#footer-badges { display: none; }
}
#toc {
margin-bottom: 2.5em;
}
#toctitle {
color: #527bbd;
font-size: 1.1em;
font-weight: bold;
margin-top: 1.0em;
margin-bottom: 0.1em;
}
div.toclevel0, div.toclevel1, div.toclevel2, div.toclevel3, div.toclevel4 {
margin-top: 0;
margin-bottom: 0;
}
div.toclevel2 {
margin-left: 2em;
font-size: 0.9em;
}
div.toclevel3 {
margin-left: 4em;
font-size: 0.9em;
}
div.toclevel4 {
margin-left: 6em;
font-size: 0.9em;
}
span.aqua { color: aqua; }
span.black { color: black; }
span.blue { color: blue; }
span.fuchsia { color: fuchsia; }
span.gray { color: gray; }
span.green { color: green; }
span.lime { color: lime; }
span.maroon { color: maroon; }
span.navy { color: navy; }
span.olive { color: olive; }
span.purple { color: purple; }
span.red { color: red; }
span.silver { color: silver; }
span.teal { color: teal; }
span.white { color: white; }
span.yellow { color: yellow; }
span.aqua-background { background: aqua; }
span.black-background { background: black; }
span.blue-background { background: blue; }
span.fuchsia-background { background: fuchsia; }
span.gray-background { background: gray; }
span.green-background { background: green; }
span.lime-background { background: lime; }
span.maroon-background { background: maroon; }
span.navy-background { background: navy; }
span.olive-background { background: olive; }
span.purple-background { background: purple; }
span.red-background { background: red; }
span.silver-background { background: silver; }
span.teal-background { background: teal; }
span.white-background { background: white; }
span.yellow-background { background: yellow; }
span.big { font-size: 2em; }
span.small { font-size: 0.6em; }
span.underline { text-decoration: underline; }
span.overline { text-decoration: overline; }
span.line-through { text-decoration: line-through; }
div.unbreakable { page-break-inside: avoid; }
/*
* xhtml11 specific
*
* */
div.tableblock {
margin-top: 1.0em;
margin-bottom: 1.5em;
}
div.tableblock > table {
border: 3px solid #527bbd;
}
thead, p.table.header {
font-weight: bold;
color: #527bbd;
}
p.table {
margin-top: 0;
}
/* Because the table frame attribute is overridden by CSS in most browsers. */
div.tableblock > table[frame="void"] {
border-style: none;
}
div.tableblock > table[frame="hsides"] {
border-left-style: none;
border-right-style: none;
}
div.tableblock > table[frame="vsides"] {
border-top-style: none;
border-bottom-style: none;
}
/*
* html5 specific
*
* */
table.tableblock {
margin-top: 1.0em;
margin-bottom: 1.5em;
}
thead, p.tableblock.header {
font-weight: bold;
color: #527bbd;
}
p.tableblock {
margin-top: 0;
}
table.tableblock {
border-width: 3px;
border-spacing: 0px;
border-style: solid;
border-color: #527bbd;
border-collapse: collapse;
}
th.tableblock, td.tableblock {
border-width: 1px;
padding: 4px;
border-style: solid;
border-color: #527bbd;
}
table.tableblock.frame-topbot {
border-left-style: hidden;
border-right-style: hidden;
}
table.tableblock.frame-sides {
border-top-style: hidden;
border-bottom-style: hidden;
}
table.tableblock.frame-none {
border-style: hidden;
}
th.tableblock.halign-left, td.tableblock.halign-left {
text-align: left;
}
th.tableblock.halign-center, td.tableblock.halign-center {
text-align: center;
}
th.tableblock.halign-right, td.tableblock.halign-right {
text-align: right;
}
th.tableblock.valign-top, td.tableblock.valign-top {
vertical-align: top;
}
th.tableblock.valign-middle, td.tableblock.valign-middle {
vertical-align: middle;
}
th.tableblock.valign-bottom, td.tableblock.valign-bottom {
vertical-align: bottom;
}
/*
* manpage specific
*
* */
body.manpage h1 {
padding-top: 0.5em;
padding-bottom: 0.5em;
border-top: 2px solid silver;
border-bottom: 2px solid silver;
}
body.manpage h2 {
border-style: none;
}
body.manpage div.sectionbody {
margin-left: 3em;
}
@media print {
body.manpage div#toc { display: none; }
}
</style>
</head>
<body class="manpage">
<div id="header">
<h1>
OPTIPNG(1) Manual Page
</h1>
<h2>NAME</h2>
<div class="sectionbody">
<p>optipng -
optimize Portable Network Graphics (PNG) files
</p>
</div>
</div>
<div id="content">
<div class="sect1">
<h2 id="_synopsis">SYNOPSIS</h2>
<div class="sectionbody">
<div class="paragraph"><p><strong>optipng</strong> [<<em>options…</em>>] [--] <<em>files…</em>></p></div>
</div>
</div>
<div class="sect1">
<h2 id="_description">DESCRIPTION</h2>
<div class="sectionbody">
<div class="paragraph"><p>The OptiPNG program shall attempt to <em>optimize</em> PNG files, i.e. to
reduce their size to a minimum, without losing semantic information.
In addition, this program shall perform a suite of auxiliary functions
like integrity checks, metadata recovery and pixmap-to-PNG conversion.</p></div>
<div class="paragraph"><p>The optimization attempts are not guaranteed to succeed.
Valid PNG files that cannot be optimized by this program are normally
left intact; their size will not grow.
The user may request to override this default behavior.</p></div>
</div>
</div>
<div class="sect1">
<h2 id="_files">FILES</h2>
<div class="sectionbody">
<div class="paragraph"><p>The input files are raster image files encoded either in PNG format
(the native format), or in an external format.
The currently supported external formats are GIF, BMP, PNM and TIFF.</p></div>
<div class="paragraph"><p>OptiPNG processes each image file given in the command line as
follows:</p></div>
<div class="ulist"><ul>
<li>
<p>
If the image is in PNG format:
</p>
<div class="paragraph"><p>It attempts to optimize the given file in-place.
If optimization is successful, or if the option <strong>-force</strong> is enabled,
the original file will be replaced by its optimized version.
In any case, the original file will be backed up if the option <strong>-keep</strong>
is enabled.</p></div>
</li>
<li>
<p>
If the image is in an external format:
</p>
<div class="paragraph"><p>It creates an optimized PNG version of the given file.
The output file name will be constructed using the original file name
and the <code>.png</code> extension.</p></div>
</li>
</ul></div>
<div class="paragraph"><p>Aside from the rules stated above, any existing file is <em>not</em>
overwritten, unless the option <strong>-clobber</strong> is enabled.</p></div>
</div>
</div>
<div class="sect1">
<h2 id="_options">OPTIONS</h2>
<div class="sectionbody">
<div class="sect2">
<h3 id="_general_options">General options</h3>
<div class="dlist"><dl>
<dt class="hdlist1">
<strong>-?</strong>, <strong>-h</strong>, <strong>-help</strong>
</dt>
<dd>
<p>
Show a complete summary of options.
</p>
</dd>
<dt class="hdlist1">
<strong>-backup</strong>, <strong>-keep</strong>
</dt>
<dd>
<p>
Keep a backup of the modified files.
</p>
</dd>
<dt class="hdlist1">
<strong>-clobber</strong>
</dt>
<dd>
<p>
Overwrite the existing output and backup files.
</p>
<div class="paragraph"><p>Under this option, if the option <strong>-backup</strong> is not enabled, the old
backups of the overwritten files will be deleted.</p></div>
</dd>
<dt class="hdlist1">
<strong>-dir</strong> <em>directory</em>
</dt>
<dd>
<p>
Write the output files to <em>directory</em>.
</p>
</dd>
<dt class="hdlist1">
<strong>-fix</strong>
</dt>
<dd>
<p>
Enable error recovery. This option has no effect on valid input
files.
</p>
<div class="paragraph"><p>The program will spend a reasonable amount of effort to recover as
much data as possible, without increasing the output file size, but
the success cannot be generally guaranteed.
The program may even increase the file size, e.g., by reconstructing
missing critical data.
Under this option, integrity shall take precedence over file size.</p></div>
<div class="paragraph"><p>When this option is not used, the invalid input files are left
unprocessed.</p></div>
</dd>
<dt class="hdlist1">
<strong>-force</strong>
</dt>
<dd>
<p>
Enforce writing of a new output file.
</p>
<div class="paragraph"><p>This option overrides the program’s decision not to write such file,
e.g. when the PNG input is digitally signed (using dSIG), or when the
PNG output becomes larger than the PNG input.</p></div>
</dd>
<dt class="hdlist1">
<strong>-out</strong> <em>file</em>
</dt>
<dd>
<p>
Write output file to <em>file</em>.
</p>
<div class="paragraph"><p>The command line must contain exactly one input file.</p></div>
</dd>
<dt class="hdlist1">
<strong>-preserve</strong>
</dt>
<dd>
<p>
Preserve file attributes (time stamps, file access rights, etc.)
where applicable.
</p>
</dd>
<dt class="hdlist1">
<strong>-quiet</strong>, <strong>-silent</strong>
</dt>
<dd>
<p>
Run in quiet mode.
</p>
</dd>
<dt class="hdlist1">
<strong>-simulate</strong>
</dt>
<dd>
<p>
Run in simulation mode: perform the trials, but do not create
output files.
</p>
</dd>
<dt class="hdlist1">
<strong>-v</strong>
</dt>
<dd>
<p>
Enable the options <strong>-verbose</strong> and <strong>-version</strong>.
</p>
</dd>
<dt class="hdlist1">
<strong>-verbose</strong>
</dt>
<dd>
<p>
Run in verbose mode.
</p>
</dd>
<dt class="hdlist1">
<strong>-version</strong>
</dt>
<dd>
<p>
Show the copyright, version and build info.
</p>
</dd>
<dt class="hdlist1">
<strong>--</strong>
</dt>
<dd>
<p>
Stop option switch parsing.
</p>
</dd>
</dl></div>
</div>
<div class="sect2">
<h3 id="_png_encoding_and_optimization_options">PNG encoding and optimization options</h3>
<div class="dlist"><dl>
<dt class="hdlist1">
<strong>-o</strong> <em>level</em>
</dt>
<dd>
<p>
Select the optimization level.
</p>
<div class="paragraph"><p>The optimization level 0 enables a set of optimization operations
that require minimal effort.
There will be no changes to image attributes like bit depth or color
type, and no recompression of existing IDAT datastreams.</p></div>
<div class="paragraph"><p>The optimization level 1 enables a single IDAT compression trial.
The trial chosen is what OptiPNG <em>thinks</em> it might be the most
effective.</p></div>
<div class="paragraph"><p>The optimization levels 2 and higher enable multiple IDAT compression
trials; the higher the level, the more trials.</p></div>
<div class="paragraph"><p>The behavior and the default value of this option may change across
different program versions.
Use the option <strong>-h</strong> to see the details pertaining to your specific
version.</p></div>
</dd>
<dt class="hdlist1">
<strong>-f</strong> <em>filters</em>
</dt>
<dd>
<p>
Select the PNG delta filters.
</p>
<div class="paragraph"><p>The <em>filters</em> argument is specified as a rangeset (e.g. <strong>-f0-5</strong>), and
the default <em>filters</em> value depends on the optimization level set by
the option <strong>-o</strong>.</p></div>
<div class="paragraph"><p>The filter values 0, 1, 2, 3 and 4 indicate static filtering, and
correspond to the standard PNG filter codes
(<em>None</em>, <em>Left</em>, <em>Up</em>, <em>Average</em> and <em>Paeth</em>, respectively).
The filter value 5 indicates adaptive filtering, whose effect is
defined by the <strong>libpng</strong>(3) library used by OptiPNG.</p></div>
</dd>
<dt class="hdlist1">
<strong>-full</strong>
</dt>
<dd>
<p>
Produce a full report on IDAT.
This option might slow down the trials.
</p>
</dd>
<dt class="hdlist1">
<strong>-i</strong> <em>type</em>
</dt>
<dd>
<p>
Select the interlace type (0-1).
</p>
<div class="paragraph"><p>If the interlace type 0 is selected, the output image shall be
non-interlaced (i.e. progressive-scanned).
If the interlace type 1 is selected, the output image shall be
interlaced using the <em>Adam7</em> method.</p></div>
<div class="paragraph"><p>By default, the output shall have the same interlace type as the
input.</p></div>
</dd>
<dt class="hdlist1">
<strong>-nb</strong>
</dt>
<dd>
<p>
Do not apply bit depth reduction.
</p>
</dd>
<dt class="hdlist1">
<strong>-nc</strong>
</dt>
<dd>
<p>
Do not apply color type reduction.
</p>
</dd>
<dt class="hdlist1">
<strong>-np</strong>
</dt>
<dd>
<p>
Do not apply palette reduction.
</p>
</dd>
<dt class="hdlist1">
<strong>-nx</strong>
</dt>
<dd>
<p>
Do not apply any lossless image reduction:
enable the options <strong>-nb</strong>, <strong>-nc</strong> and <strong>-np</strong>.
</p>
</dd>
<dt class="hdlist1">
<strong>-nz</strong>
</dt>
<dd>
<p>
Do not recode IDAT datastreams.
</p>
<div class="paragraph"><p>The IDAT optimization operations that do not require recoding
(e.g. IDAT chunk concatenation) are still performed.</p></div>
<div class="paragraph"><p>This option has effect on PNG input files only.</p></div>
</dd>
<dt class="hdlist1">
<strong>-zc</strong> <em>levels</em>
</dt>
<dd>
<p>
Select the zlib compression levels used in IDAT compression.
</p>
<div class="paragraph"><p>The <em>levels</em> argument is specified as a rangeset (e.g. <strong>-zc6-9</strong>),
and the default <em>levels</em> value depends on the optimization level set
by the option <strong>-o</strong>.</p></div>
<div class="paragraph"><p>The effect of this option is defined by the <strong>zlib</strong>(3) library used by
OptiPNG.</p></div>
</dd>
<dt class="hdlist1">
<strong>-zm</strong> <em>levels</em>
</dt>
<dd>
<p>
Select the zlib memory levels used in IDAT compression.
</p>
<div class="paragraph"><p>The <em>levels</em> argument is specified as a rangeset (e.g. <strong>-zm8-9</strong>), and
the default <em>levels</em> value depends on the optimization level set by
the option <strong>-o</strong>.</p></div>
<div class="paragraph"><p>The effect of this option is defined by the <strong>zlib</strong>(3) library used by
OptiPNG.</p></div>
</dd>
<dt class="hdlist1">
<strong>-zs</strong> <em>strategies</em>
</dt>
<dd>
<p>
Select the zlib compression strategies used in IDAT compression.
</p>
<div class="paragraph"><p>The <em>strategies</em> argument is specified as a rangeset (e.g. <strong>-zs0-3</strong>),
and the default <em>strategies</em> value depends on the optimization level
set by the option <strong>-o</strong>.</p></div>
<div class="paragraph"><p>The effect of this option is defined by the <strong>zlib</strong>(3) library used by
OptiPNG.</p></div>
</dd>
<dt class="hdlist1">
<strong>-zw</strong> <em>size</em>
</dt>
<dd>
<p>
Select the zlib window size (32k,16k,8k,4k,2k,1k,512,256) used in
IDAT compression.
</p>
<div class="paragraph"><p>The <em>size</em> argument can be specified either in bytes (e.g. 16384) or
kilobytes (e.g. 16k). The default <em>size</em> value is set to the lowest
window size that yields an IDAT output as big as if yielded by the
value 32768.</p></div>
<div class="paragraph"><p>The effect of this option is defined by the <strong>zlib</strong>(3) library used by
OptiPNG.</p></div>
</dd>
</dl></div>
</div>
<div class="sect2">
<h3 id="_editing_options">Editing options</h3>
<div class="dlist"><dl>
<dt class="hdlist1">
<strong>-snip</strong>
</dt>
<dd>
<p>
Cut one image out of multi-image, animation or video files.
</p>
<div class="paragraph"><p>Depending on the input format, this may be either the first or the
most relevant (e.g. the largest) image.</p></div>
</dd>
<dt class="hdlist1">
<strong>-strip</strong> <em>objects</em>
</dt>
<dd>
<p>
Strip metadata objects from a PNG file.
</p>
<div class="paragraph"><p>PNG metadata is the information stored in any ancillary chunk except
tRNS.
(tRNS represents the alpha channel, which, even if ignored in
rendering, is still a proper image channel in the RGBA color space.)</p></div>
<div class="paragraph"><p>The only option currently supported is <strong>-strip all</strong>.</p></div>
</dd>
</dl></div>
</div>
<div class="sect2">
<h3 id="_notes">Notes</h3>
<div class="paragraph"><p>Options may come in any order (except for <strong>--</strong>), before, after, or
alternating with file names.
Option names are case-insensitive and may be abbreviated to their
shortest unique prefix.</p></div>
<div class="paragraph"><p>Some options may have arguments that follow the option name,
separated by whitespace or the equal sign (<em><strong>=</strong></em>).
If the option argument is a number or a rangeset, the separator may
be omitted.
For example:</p></div>
<div class="literalblock">
<div class="content">
<pre><code>optipng -out newfile.png <=> optipng -out=newfile.png
optipng -o3 <=> optipng -o 3 <=> optipng -o=3
optipng -f0,3-5 <=> optipng -f 0,3-5 <=> optipng -f=0,3-5</code></pre>
</div></div>
<div class="paragraph"><p>Rangeset arguments are cumulative.
For example:</p></div>
<div class="literalblock">
<div class="content">
<pre><code>optipng -f0 -f3-5 <=> optipng -f0,3-5
optipng -zs0 -zs1 -zs2,3 <=> optipng -zs0-3</code></pre>
</div></div>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_extended_description">EXTENDED DESCRIPTION</h2>
<div class="sectionbody">
<div class="paragraph"><p>The PNG optimization algorithm consists of the following steps:</p></div>
<div class="olist arabic"><ol class="arabic">
<li>
<p>
Reduce the bit depth, the color type and the color palette of the
image.
This step may reduce the size of the uncompressed image, which,
indirectly, may reduce the size of the compressed image
(i.e. the size of the output PNG file).
</p>
</li>
<li>
<p>
Run a suite of compression methods and strategies and select the
compression parameters that yield the smallest output file.
</p>
</li>
<li>
<p>
Store all IDAT contents into a single chunk, eliminating the
overhead incurred by repeated IDAT headers and CRCs.
</p>
</li>
<li>
<p>
Set the zlib window size inside IDAT to a minimum that does not
affect the compression ratio, reducing the memory requirements of
PNG decoders.
</p>
</li>
</ol></div>
<div class="paragraph"><p>Not all of the above steps need to be executed.
The behavior depends on the actual input files and user options.</p></div>
<div class="paragraph"><p>Step 1 may be customized via the no-reduce options <strong>-nb</strong>, <strong>-nc</strong>,
<strong>-np</strong> and <strong>-nx</strong>.
Step 2 may be customized via the <strong>-o</strong> option, and may be fine-tuned
via the options <strong>-zc</strong>, <strong>-zm</strong>, <strong>-zs</strong> and <strong>-zw</strong>.
Step 3 is always executed. Step 4 is executed only if a new IDAT is
being created, and may be fine-tuned via the option <strong>-zw</strong>.</p></div>
<div class="paragraph"><p>Extremely exhaustive searches are not generally expected to yield
significant improvements in compression ratio, and are recommended to
advanced users only.</p></div>
</div>
</div>
<div class="sect1">
<h2 id="_examples">EXAMPLES</h2>
<div class="sectionbody">
<div class="literalblock">
<div class="content">
<pre><code>optipng file.png # default speed
optipng -o5 file.png # slow
optipng -o7 file.png # very slow</code></pre>
</div></div>
</div>
</div>
<div class="sect1">
<h2 id="_bugs">BUGS</h2>
<div class="sectionbody">
<div class="paragraph"><p>Lossless image reductions are not completely implemented.
(This does <em>not</em> affect the integrity of the output files.)
Here are the missing pieces:</p></div>
<div class="ulist"><ul>
<li>
<p>
The color palette reductions are implemented only partially.
</p>
</li>
<li>
<p>
The bit depth reductions below 8, for grayscale images, are not
implemented yet.
</p>
</li>
</ul></div>
<div class="paragraph"><p>Encoding of images whose total IDAT size exceeds 2GB is not currently
supported.</p></div>
<div class="paragraph"><p>TIFF support is limited to uncompressed, PNG-compatible
(grayscale, RGB and RGBA) images.</p></div>
<div class="paragraph"><p>Metadata is not imported from the external image formats.</p></div>
<div class="paragraph"><p>There is no support for pipes, streams, extended file attributes or
access control lists.</p></div>
</div>
</div>
<div class="sect1">
<h2 id="_see_also">SEE ALSO</h2>
<div class="sectionbody">
<div class="paragraph"><p><strong>png</strong>(5),
<strong>libpng</strong>(3),
<strong>zlib</strong>(3),
<strong>pngcrush</strong>(1),
<strong>pngrewrite</strong>(1).</p></div>
</div>
</div>
<div class="sect1">
<h2 id="_standards">STANDARDS</h2>
<div class="sectionbody">
<div class="paragraph"><p>The files produced by OptiPNG are compliant with the most recent PNG
standard ("PNG Second Edition"), as well as the most recent PNG draft
standard ("PNG Third Edition").</p></div>
<div class="ulist"><ul>
<li>
<p>
<em>Portable Network Graphics (PNG) Specification, Second Edition</em>.<br />
W3C Recommendation 10 November 2003; ISO/IEC IS 15948:2003 (E).<br />
<a href="https://www.w3.org/TR/2003/REC-PNG-20031110/">https://www.w3.org/TR/2003/REC-PNG-20031110/</a>
</p>
</li>
<li>
<p>
<em>Portable Network Graphics (PNG) Specification, Third Edition</em>.<br />
W3C Proposed Recommendation 15 May 2025.<br />
<a href="https://www.w3.org/TR/2025/PR-png-3-20250515/">https://www.w3.org/TR/2025/PR-png-3-20250515/</a>
</p>
</li>
</ul></div>
</div>
</div>
<div class="sect1">
<h2 id="_authors">AUTHORS</h2>
<div class="sectionbody">
<div class="paragraph"><p>OptiPNG is written and maintained by Cosmin Truta.</p></div>
<div class="paragraph"><p>This manual page was originally written by Nelson A. de Oliveira
for the Debian Project.
It was later updated by Cosmin Truta, and is now part of the OptiPNG
distribution.</p></div>
</div>
</div>
</div>
<div id="footnotes"><hr /></div>
<div id="footer">
<div id="footer-text">
Last updated
2025-05-20 22:39:10 EEST
</div>
</div>
</body>
</html>