1OPTIPNG(1) General Commands Manual OPTIPNG(1)
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6 OptiPNG - Optimize Portable Network Graphics files
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10 optipng [-? | -h | -help]
11 optipng [options...] files...
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15 The OptiPNG program shall attempt to optimize PNG files, i.e. reduce
16 their size to a minimum, without losing semantic information. In addi‐
17 tion, this program shall perform a suite of auxiliary functions like
18 integrity checks, metadata recovery and pixmap-to-PNG conversion.
19 The optimization attempts are not guaranteed to succeed. Valid PNG
21 files that cannot be optimized by this program are normally left in‐
22 tact; their size will not grow. The user may request to override this
23 default behavior.
24
27 The input files are raster image files encoded either in PNG format
28 (the native format), or in an external format. The currently supported
29 external formats are GIF, BMP, PNM and TIFF.
30 OptiPNG processes each image file given in the command line as follows:
32 - If the image is in PNG format:
34 Attempts to optimize the given file in-place. If optimization is
36 successful, or if the option -force is enabled, replaces the
37 original file with its optimized version. The original file is
38 backed up if the option -keep is enabled.
39 - If the image is in an external format:
41 Creates an optimized PNG version of the given file. The output
43 file name is composed from the original file name and the .png
44 extension.
45 Existing files are not overwritten, unless the option -clobber is en‐
47 abled.
48
51 General options
52 -?, -h, -help
53 Show a complete summary of options.
54 -backup, -keep
56 Keep a backup of the modified files.
57 -clobber
59 Overwrite the existing output and backup files.
60 Under this option, if the option -backup is not enabled, the old
61 backups of the overwritten files are deleted.
62 -dir directory
64 Write the output files to directory.
65 -fix Enable error recovery. This option has no effect on valid input
67 files.
68 The program will spend a reasonable amount of effort to recover
69 as much data as possible, without increasing the output file
70 size, but the success cannot be generally guaranteed. The pro‐
71 gram may even increase the file size, e.g., by reconstructing
72 missing critical data. Under this option, integrity shall take
73 precedence over file size.
74 When this option is not used, the invalid input files are left
75 unprocessed.
76 -force Enforce writing of a new output file.
78 This option overrides the program's decision not to write such
79 file, e.g. when the PNG input is digitally signed (using dSIG),
80 or when the PNG output becomes larger than the PNG input.
81 -log file
83 Log messages to file. For safety reasons, file must have the
84 extension .log.
85 This option is deprecated and will be removed eventually. Use
86 shell redirection.
87 -out file
89 Write output file to file. The command line must contain ex‐
90 actly one input file.
91 -preserve
93 Preserve file attributes (time stamps, file access rights, etc.)
94 where applicable.
95 -quiet, -silent
97 Run in quiet mode.
98 The messages are still written to the log file if the option
99 -log is enabled.
100 -simulate
102 Run in simulation mode: perform the trials, but do not create
103 output files.
104 -v Enable the options -verbose and -version.
106 -verbose
108 Run in verbose mode.
109 -version
111 Show copyright, version and build info.
112 -- Stop option switch parsing.
114 PNG encoding and optimization options
116 -o level
117 Select the optimization level.
118 The optimization level 0 enables a set of optimization opera‐
119 tions that require minimal effort. There will be no changes to
120 image attributes like bit depth or color type, and no recompres‐
121 sion of existing IDAT datastreams.
122 The optimization level 1 enables a single IDAT compression
123 trial. The trial chosen is what OptiPNG thinks it's probably the
124 most effective.
125 The optimization levels 2 and higher enable multiple IDAT com‐
126 pression trials; the higher the level, the more trials.
127 The behavior and the default value of this option may change
128 across different program versions. Use the option -h to see the
129 details pertaining to your specific version.
130 -f filters
132 Select the PNG delta filters.
133 The filters argument is specified as a rangeset (e.g. -f0-5),
134 and the default filters value depends on the optimization level
135 set by the option -o.
136 The filter values 0, 1, 2, 3 and 4 indicate static filtering,
137 and correspond to the standard PNG filter codes (None, Left, Up,
138 Average and Paeth, respectively). The filter value 5 indicates
139 adaptive filtering, whose effect is defined by the libpng(3) li‐
140 brary used by OptiPNG.
141 -full Produce a full report on IDAT. This option might slow down the
143 trials.
144 -i type
146 Select the interlace type (0-1).
147 If the interlace type 0 is selected, the output image shall be
148 non-interlaced (i.e. progressive-scanned). If the interlace type
149 1 is selected, the output image shall be interlaced using the
150 Adam7 method.
151 By default, the output shall have the same interlace type as the
152 input.
153 -nb Do not apply bit depth reduction.
155 -nc Do not apply color type reduction.
157 -np Do not apply palette reduction.
159 -nx Do not apply any lossless image reduction: enable the options
161 -nb, -nc and -np.
162 -nz Do not recode IDAT datastreams.
164 The IDAT optimization operations that do not require recoding
165 (e.g. IDAT chunk concatenation) are still performed.
166 This option has effect on PNG input files only.
167 -zc levels
169 Select the zlib compression levels used in IDAT compression.
170 The levels argument is specified as a rangeset (e.g. -zc6-9),
171 and the default levels value depends on the optimization level
172 set by the option -o.
173 The effect of this option is defined by the zlib(3) library used
174 by OptiPNG.
175 -zm levels
177 Select the zlib memory levels used in IDAT compression.
178 The levels argument is specified as a rangeset (e.g. -zm8-9),
179 and the default levels value depends on the optimization level
180 set by the option -o.
181 The effect of this option is defined by the zlib(3) library used
182 by OptiPNG.
183 -zs strategies
185 Select the zlib compression strategies used in IDAT compression.
186 The strategies argument is specified as a rangeset (e.g.
187 -zs0-3), and the default strategies value depends on the opti‐
188 mization level set by the option -o.
189 The effect of this option is defined by the zlib(3) library used
190 by OptiPNG.
191 -zw size
193 Select the zlib window size (32k,16k,8k,4k,2k,1k,512,256) used
194 in IDAT compression.
195 The size argument can be specified either in bytes (e.g. 16384)
196 or kilobytes (e.g. 16k). The default size value is set to the
197 lowest window size that yields an IDAT output as big as if
198 yielded by the value 32768.
199 The effect of this option is defined by the zlib(3) library used
200 by OptiPNG.
201 Editing options
203 -snip Cut one image out of multi-image, animation or video files.
204 Depending on the input format, this may be either the first or
205 the most relevant (e.g. the largest) image.
206 -strip objects
208 Strip metadata objects from a PNG file.
209 PNG metadata is the information stored in any ancillary chunk
210 except tRNS. (tRNS represents the alpha channel, which, even if
211 ignored in rendering, is still a proper image channel in the
212 RGBA color space.)
213 The only option currently supported is -strip all.
214 Notes
216 Options may come in any order (except for --), before, after, or alter‐
217 nating with file names. Option names are case-insensitive and may be
218 abbreviated to their shortest unique prefix.
219 Some options may have arguments that follow the option name, separated
221 by whitespace or the equal sign ('='). If the option argument is a num‐
222 ber or a rangeset, the separator may be omitted. For example:
223 -out newfile.png <=> -out=newfile.png
225 -o3 <=> -o 3 <=> -o=3
226 -f0,3-5 <=> -f 0,3-5 <=> -f=0,3-5
227 Rangeset arguments are cumulative; e.g.
229 -f0 -f3-5 <=> -f0,3-5
231 -zs0 -zs1 -zs2-3 <=> -zs0,1,2,3 <=> -zs0-3
232
235 The PNG optimization algorithm consists of the following steps:
236 1. Reduce the bit depth, the color type and the color palette of the
238 image. This step may reduce the size of the uncompressed image,
239 which, indirectly, may reduce the size of the compressed image
240 (i.e. the size of the output PNG file).
241 2. Run a suite of compression methods and strategies and select the
243 compression parameters that yield the smallest output file.
244 3. Store all IDAT contents into a single chunk, eliminating the over‐
246 head incurred by repeated IDAT headers and CRCs.
247 4. Set the zlib window size inside IDAT to a mininum that does not af‐
249 fect the compression ratio, reducing the memory requirements of PNG
250 decoders.
251 Not all of the above steps need to be executed. The behavior depends on
253 the actual input files and user options.
254 Step 1 may be customized via the no-reduce options -nb, -nc, -np and
256 -nx. Step 2 may be customized via the -o option, and may be fine-tuned
257 via the options -zc, -zm, -zs and -zw. Step 3 is always executed. Step
258 4 is executed only if a new IDAT is being created, and may be fine-
259 tuned via the option -zw.
260 Extremely exhaustive searches are not generally expected to yield sig‐
262 nificant improvements in compression ratio, and are recommended to ad‐
263 vanced users only.
264
267 optipng file.png # default speed
268 optipng -o5 file.png # slow
269 optipng -o7 file.png # very slow
270
273 Lossless image reductions are not completely implemented. (This does
274 not affect the integrity of the output files.) Here are the missing
275 pieces:
276 - The color palette reductions are implemented only partially.
278 - The bit depth reductions below 8, for grayscale images, are
279 not implemented yet.
280 Encoding of images whose total IDAT size exceeds 2GB is not supported.
282 TIFF support is limited to uncompressed, PNG-compatible (grayscale, RGB
284 and RGBA) images.
285 Metadata is not imported from the external image formats.
287 There is no support for pipes, streams, extended file attributes or ac‐
289 cess control lists.
290
293 png(5), libpng(3), zlib(3), pngcrush(1), pngrewrite(1).
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297 The files produced by OptiPNG are compliant with PNG-2003:
298 Glenn Randers-Pehrson et al. Portable Network Graphics (PNG) Specifi‐
299 cation, Second Edition.
300 W3C Recommendation 10 November 2003; ISO/IEC IS 15948:2003 (E).
301 http://www.w3.org/TR/PNG/
302
305 OptiPNG is written and maintained by Cosmin Truta.
306 This manual page was originally written by Nelson A. de Oliveira for
308 the Debian Project. It was later updated by Cosmin Truta, and is now
309 part of the OptiPNG distribution.
310OptiPNG version 0.7.8 2023-Nov-03 OPTIPNG(1)