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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 TRANSITIONAL//EN"
"http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<meta name="Author" content="Cosmin Truţa">
<title>OptiPNG - The complete list of features</title>
</head>
<body bgcolor="white" text="black">
<h2>OptiPNG - The complete list of features</h2>
<p>
The main purpose of OptiPNG is to <i>optimize</i> PNG files, i.e.
to reduce their size to a minimum, without losing any information.
In order to achieve this goal, OptiPNG performs the following
tasks:
</p>
<ul>
<li>
It losslessly reduces the bit depth, the color type and the
color palette of the image. This step reduces the size of the
uncompressed image, which, indirectly, reduces the size of the
compressed image (i.e. the size of the PNG file).
</li>
<li>
It runs a suite of compression methods and strategies and
selects the parameters that yield the smallest output file.
</li>
<li>
It concatenates all the IDAT data in a single chunk, minimizing
the overhead incurred by chunk headers and CRCs.
</li>
</ul>
<p>
In addition, OptiPNG has the following capabilities:
</p>
<ul>
<li>
It offers a suite of optimization level presets.
Given the number of trials that the user is willing to run,
these presets will select the range of compression parameters
that are most likely to yield the smallest output.
</li>
<li>
It offers a high degree of freedom to advanced users who
can choose among many kinds of compression parameters.
The optimization level presets and the custom compression
parameters may be used in any combination.
</li>
<li>
It allows advanced users to disable certain image reductions.
</li>
<li>
It corrects integrity errors found in the input files.
Bad CRCs, invalid data in ancillary chunks, incomplete PNG
files, or extraneous data in IDAT, are examples of such
integrity errors.
</li>
<li>
It recognizes several external file formats:
<ul>
<li>
<b>BMP</b>
<br>
The reading capabilities are limited to uncompressed BMP
files. This is not a big problem, since few programs produce
compresses BMPs. On the other hand, it is easy to uncompress
a BMP file, by opening it in a BMP processing program (such
as MS Paint), and re-saving it uncompressed.
</li>
<li>
<b>GIF</b>
<br>
All static (i.e. non-animated) GIF images are supported.
OptiPNG preserves the transparency information, but it
does not preserve the textual comments embedded in GIFs.
Animated GIFs cannot be converted to PNG. Instead, only
the first frame is converted, and a warning is issued.
</li>
<li>
<b>PNM (PBM, PGM, PPM)</b>
<br>
All these Unix-based formats are fully supported by OptiPNG.
However, losslessness is only guaranteed when the maximum
sample value is one of the following: 1, 3, 7, 15, 255, 65535.
(This limitation applies to any PNM-to-PNG format converter,
not only to OptiPNG.)
</li>
<li>
<b>TIFF</b>
<br>
The read support is currently limited to uncompressed TIFF
images that are encoded in a PNG-compatible color space
(grayscale, RGB or RGBA). Enhanced TIFF support is under
development.
</li>
</ul>
</ul>
<hr>
<address>
<font size="-1">
Copyright © 2003-2006 Cosmin Truţa. Permission to distribute freely.
<br>
Appeared: 3 Apr 2003.
<br>
Last updated: 21 Jul 2006.
</font>
</address>
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