1Pngtopam User Manual(0) Pngtopam User Manual(0)
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6 pngtopam - convert a PNG image into a Netpbm image
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10 pngtopam [-verbose] [-alphapam | -alpha | -mix] [-background=color]
11 [-gamma=value] [-text=filename] [-time] [-byrow] [pngfile]
12 Minimum unique abbreviation of option is acceptable. You may use dou‐
14 ble hyphens instead of single hyphen to denote options. You may use
15 white space in place of the equals sign to separate an option name from
16 its value.
17
20 This program is part of Netpbm(1).
21 pngtopam reads a PNG image (Portable Network Graphics) as input and
23 produces a Netpbm image as output. The type of the output file depends
24 on the input file - if it's black & white, pngtopam creates a PBM file.
25 If it's grayscale, pngtopam creates a PGM file. Otherwise, it creates
26 a PPM file. Except that with the -alphapam option, it always creates a
27 PAM file. That file has tuple type GRAYSCALE_ALPHA or RGB_ALPHA
28 depending on whether the input has color or not.
29 To convert in the other direction, use pamtopng or pnmtopng. The for‐
31 mer is the more modern of the two and can recognize transparency infor‐
32 mation in a PAM file, as you might generate with pngtopam -alphapam.
33 It has existed only since June 2015. The latter has more features, but
34 probably not ones that matter in the modern world.
35
39 -verbose
40 Display various information about the input PNG image and the
41 conversion process.
42 If you want even more information about the PNG image, use
44 pngcheck (not part of Netpbm).
45 -alphapam
48 Produce a single output image containing the main image (fore‐
49 ground) and the transparency channel or transparency mask. This
50 image is in the PAM format with tuple type of either
51 GRAYSCALE_ALPHA (which has a depth of 2 channels) or RGB_ALPHA
52 (which has a depth of 4 channels).
53 You can specify only one of -alphapam, -alpha, and -mix. With
55 none of them, pngtopam produces an image of the foreground of
56 the input image and discards transparency information.
57 This option was new in Netpbm 10.44 (September 2008).
59 -alpha Output the transparency channel or transparency mask of the
62 image. The result is either a PBM file or a PGM file, depending
63 on whether different levels of transparency appear.
64 pngtopam discards the main image (the foreground).
66 You can specify only one of -alphapam, -alpha, and -mix. With
68 none of them, pngtopam produces an image of the foreground of
69 the input image and discards transparency information.
70 -mix Compose the image with the transparency or transparency mask
73 against a background. The background color is determined by the
74 bKGD chunk in the PNG, except that you can override it with
75 -background. If the PNG has no bKGD chunk and you don't specify
76 -background, the background color is white.
77 You can specify only one of -alphapam, -alpha, and -mix. With
79 none of them, pngtopam produces an image of the foreground of
80 the input image and discards transparency information.
81 -background=color
84 This option specifies the background color with which to mix the
85 image when you specify -mix.
86 color is as described for the argument of the ppm_parsecolor()
88 library routine ⟨libppm.html#colorname⟩ .
89 Examples:
91 · -background=rgb:01/ff/80
95 · -background=rgbi:1/255/128
97 If you don't specify -background, the background color is what
100 is specified in the PNG image, and if the PNG doesn't specify
101 anything, white.
102 You cannot specify -background unless you also specify -mix.
104 Before Netpbm 10.27 (March 2005), you could specify -background
105 without -mix and it was just ignored. (This caused a usability
106 problem).
107 -gamma=value
111 This option causes pngtopam to respect the image gamma informa‐
112 tion in the PNG file (from the gAMA chunk). Probably by histor‐
113 ical accident, pngtopam ignores that information by default,
114 assuming the image uses the same gamma transformation as a
115 Netpbm image, so the output image has different colors than the
116 PNG file actually represents if the PNG doesn't actually do
117 that. (However, it is rare for a PNG file to use a gamma trans‐
118 formation different from what the Netpbm formats specify, or if
119 it does, to specify with a gAMA chuck what that is).
120 But when you do specify -gamma, you get a rather strange addi‐
122 tional function, probably a historical mistake: pngtopam incor‐
123 porates the specified screen gamma value into the output pixels,
124 so that the samples in the Netpbm output deviate from the Netpbm
125 format specifications and are appropriate raw intensity values
126 to send to the display. This function essentially just exer‐
127 cises the ability of the PNG library to make gamma corrections
128 to the pixels as it reads them from the PNG file to produce val‐
129 ues appropriate for sending to a certain display in certain
130 viewing conditions. It's a strange function because it has
131 nothing to do with PNG and because in Netpbm, the normal way to
132 make gamma corrections appropriate for sending to a ceratin dis‐
133 play in certain viewing conditions is with the program pngtopam,
134 applied to the normal output of pngtopam.
135 If you specify -gamma, but the PNG image does not specify what
137 gamma transformation it uses (there is no gAMA chunk), pngtopam
138 assumes a simple power transformation with an image gamma of
139 1.0. That is probably not not the actual image gamma; it is
140 much more likely to be .45.
141 Because the gammas of uncompensated monitors are around 2.6,
143 which results in an image-gamma of 0.45, some typical situations
144 are: when the image-gamma is 0.45 (use -verbose to check) and
145 the picture is too light, your system is gamma-corrected, so
146 convert with "-gamma 1.0". When no gAMA chunk is present or the
147 image-gamma is 1.0, use 2.2 to make the picture lighter and 0.45
148 to make the picture darker.
149 One oddity to be aware of when using -gamma on an image with
151 transparency: The PNG image specifies that a certain color is
152 transparent, i.e. every pixel in the image of that color is
153 transparent. But pngtopam interprets this as applying to the
154 gamma-corrected space, and there may be less precision in that
155 space than in the original, which means multiple uncorrected
156 colors map to the same corrected color. So imagine that the
157 image contains 3 shades of white (gray) and specifies that one
158 of them is transparent. After gamma correction, those three
159 shades are indistinguishable, so pngtopam considers pixels of
160 all three shades to be transparent.
161 -text=file
165 Writes the tEXt and zTXt chunks to a file, in a format as
166 described in the pnmtopng user manual. These chunks contain
167 text comments or annotations.
168 -time Prints the tIME chunk to stderr.
171 -byrow This option can make pngtopam run faster or in environments
174 where it would otherwise fail.
175 pngtopam has two ways to do the conversion from PNG to PAM,
177 using respectively two facilities of the PNG library:
178 Whole Image
183 Decode the entire image into memory at once, using
184 png_read_image(), then convert to PAM and output row by row.
185 Row By Row
188 Read, convert, and output one row at a time using
189 png_read_row().
190 Whole Image is generally preferable because the PNG library does
194 more of the work, which means it understands more of the PNG
195 format possibilities now and in the future. Also, if the PNG is
196 interlaced, pngtopam does not know how to assemble the rows in
197 the right order.
198 Row By Row uses far less memory, which means with large images,
200 it can run in environments where Whole Image cannot and may also
201 run faster. And because Netpbm code does more of the work, it's
202 possible that it can be more flexible or at least give better
203 diagnostic information if there's something wrong with the PNG.
204 The Netpbm native code may do something correctly that the PNG
206 library does incorrectly, or vice versa.
207 In Netpbm, we stress function over performance, so by default
209 pngtopam uses Whole Image. You can select Row By Row with
210 -byrow if you want the speed or resource requirement improve‐
211 ment.
212 -byrow was new in Netpbm 10.54 (March 2011).
214 -orientraw
218 A TIFF stream contains raster data which can be arranged in the
219 stream various ways. Most commonly, it is arranged by rows,
220 with the top row first, and the pixels left to right within each
221 row, but many other orientations are possible.
222 The common orientation is the same on the Netpbm formats use, so
224 tifftopnm can do its jobs quite efficiently when the TIFF raster
225 is oriented that way.
226 But if the TIFF raster is oriented any other way, it can take a
228 considerable amount of processing for tifftopnm to convert it to
229 Netpbm format.
230
235 pamtopng(1), pnmtopng(1), pngtopnm(1), ptot, pnmgamma(1), pnm(1)
236 For information on the PNG format, see http://schaik.com/png
238 ⟨http://schaik.com/png⟩ .
239
242 A PNG image contains a lot of information that can't be represented in
243 Netpbm formats. Therefore, you lose information when you convert to
244 another format with "pngtopam | pnmtoxxx". If there is a specialized
245 converter that converts directly to the other format, e.g. ptot to con‐
246 vert from PNG to TIFF, you'll get better results using that.
247
250 There could be an option to include PNG comment chunks in the output
251 image as PNM comments instead of putting them in a separate file.
252 The program could be much faster, with a bit of code optimizing. As
254 with any Netpbm program, speed always takes a back seat to quick
255 present and future development.
256
259 pngtopam was new in Netpbm 10.44, as a replacement for pngtopnm. The
260 main improvement over pngtopnm was that it could generate a PAM image
261 with a transparency channel, whereas with pngtopnm, you would have to
262 extract the transparency channel as a separate file, in a separate run.
263 pngtopnm was new in Netpbm 8.1 (March 2000), the first big change to
265 the package in Netpbm's renaissance. It and pnmtopng were simply
266 copied from the pnmtopng package" (1) by Greg Roelofs. Those were
267 based on simpler reference applications by Alexander Lehmann
268 <alex@hal.rhein-main.de> and Willem van Schaik <willem@schaik.com> and
269 distributed with their PNG library.
270 Nearly all of the code has changed since it was copied from the pnm‐
272 topng package, most of it just to improve maintainability.
273
277 Copyright (C) 1995-1997 by Alexander Lehmann and Willem van Schaik.
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280 This manual page was generated by the Netpbm tool 'makeman' from HTML
281 source. The master documentation is at
282 http://netpbm.sourceforge.net/doc/pngtopam.html
284netpbm documentation 22 July 2008 Pngtopam User Manual(0)