1SNG(1) User Commands SNG(1)
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6 sng - compiler/decompiler for Scriptable Network Graphics
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9 sng [-vV] [file...]
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12 The sng program translates between PNG (Portable Network Graphics)
13 format and SNG (Scriptable Network Graphics) format. SNG is a printable
14 and editable minilanguage for describing PNG files. With sng, it is
15 easy to view and edit exotic PNG chunks not supported by graphics
16 editors; also, since SNG is easy to generate from scripts, sng may be
17 useful at the end of a pipeline that programmatically generates PNG
18 images.
19 An SNG description consists of a series of chunk specifications in a
21 simple editable text format. These generally correspond one-for-one to
22 PNG chunks. There is one exception; the IMAGE chunk specification is
23 automatically translated into an IDAT chunk (doing appropriate
24 interlacing, compression, etcetera).
25 Given no file arguments, sng translates stdin to stdout. In this mode,
27 it checks the first character. If that character is printable, the
28 input stream is assumed to contain SNG; sng looks for an #SNG leader
29 and tries to translate the file to PNG. If the character is
30 non-printable, the input stream is assumed to contain PNG; sng tries to
31 translate it to SNG.
32 For each file that sng operates on, it does its conversion according to
34 the file extension (.png or .sng). The result file has the same name
35 left of the dot as the original, but the opposite extension and type.
36 The -V option makes sng identify itself and its version, then exit.
38 The -v option makes sng report on what files it is converting.
39
41 In general, the SNG language is token-oriented with tokens separated by
42 whitespace. Anywhere that whitespace may appear, a `#' comment leader
43 causes all characters up to the next following newline to be ignored.
44 The characters `:' and `;' are treated as whitespace, except the `;'
45 terminates a data element (see below).
46 In the syntax descriptions below, lines between {} may occur in any
48 order. Elements bracketed in [] are optional; a sequence bracketed by
49 []* may be repeated any number of times. Elements separated by | are
50 alternatives. Elements separated by plus signs are an attribute set;
51 any sequence of one or more of those element tokens is valid.
52 The elements <byte>, <short>, <long>, <float>, <string> are byte
54 numeric, short-integer numeric, long-integer numeric, and float numeric
55 literals respectively (all unsigned). The <slong> element is a signed
56 long-numeric literal. All numerics use C conventions; that is, they are
57 decimal unless led by 0x (hex) or 0 (octal).
58 The element <string> is any number of doublequote-delimited character
60 string literals. C-style escapes (\n, \t, \b, \r or \ followed by octal
61 or hex digits) are interpreted. The result is the concatenation of all
62 the literals.
63 The element <keyword> is a doublequote-delimited PNG keyword; that is,
65 a string of no more than 79 printable Latin-1 characters or spaces,
66 with no leading and no trailing and no consecutive spaces.
67 A <data> element consists of a sequence of byte specifications in any
69 of the following formats. Either '}' or ';' ends a data literal; `}'
70 also ends the enclosing chunk.
71 1. string format is an SNG string literal or sequence of string
73 literals (see above). The bytes of data are the string contents.
74 2. base64 format is signaled by the leading token `base64'. This
76 encoding can only be used when the values of all bytes is less than 64.
77 It encodes each byte as a single character, with decimal digits
78 representing values 0-9, followed by A-Z for 10-35, followed by a-z for
79 36-61, followed by + for 62 and / for 63. Base64 format can be used if
80 the image either has total (color plus alpha) bit depth of four or
81 less, or it is a spaletted image with 64 or fewer colors. Whitespace is
82 ignored. Note that this encoding is only very loosely related to
83 RFC2045 base-64 encoding, which uses a different mapping of bytes to
84 values, and supports encoding of arbitrary binary data.
85 3. hex format is signaled by the leading token `hex'. In hex format,
87 each byte is specified by two hex digits (0123456789abcdef), most
88 significant first. Whitespace is ignored.
89 4. P1 format is Portable Bit Map (PBM) format P1. A decimal height and
91 width follow; it is a fatal error for them to fail to match the IHDR
92 dimensions. Following this, the only non-whitespace charaters are
93 expected to be `0' and `1', with the obvious values. Whitespace is
94 ignored.
95 5. P3 format is Portable Pixel Map (PPM) format P3. A decimal height
97 and width follow; it is a fatal error for them to fail to match the
98 IHDR dimensions. A maximum channel value in decimal follows; it is a
99 fatal error for any following channel value to exceed this value.
100 Following this are triples of decimal channel values representing RGB
101 triples. Whitespace separates decimal channel values but is otherwise
102 ignored.
103 An <rgb> element may be expanded to:
105 (<byte>, <byte>, <byte>) | <string>
107 That is, it is either a paren-enclosed list of RGB values or a string
109 naming a color named in the X RGB database. Note that color names are
110 not necessarily portable between hosts or even displays, due to
111 different screen gammas and colorimetric biases. For this reason, the
112 SNG decompiler generates color names in comments.
113 IMAGE segments contain unpacked and uninterlaced raster data. There
115 will be exactly one IMAGE per SNG dump, containing the pixel data from
116 all IDAT chunks, unless the -i option is on. In that case, there will
117 be multiple IDAT chunks containing raw (compressed) image data.
118 The options member of an IMAGE chunk (if present) sets image write
120 transformations, supplying the third argument of the png_write_png()
121 call used for output. Note that for images with a bit depth of less
122 than 8, there is a default `packing' transformation. Consult the
123 libpng(3) manual page for details.
124 Every SNG file must begin with the string "#SNG", followed by optional
126 SNG version information, followed by a colon (`:', ASCII 58) character.
127 The remainder of the first line is ignored by SNG.
128 Comments in the syntax diagram describe intended semantics. This
130 specification should be read in conjunction with the PNG standard.
131 IHDR {
133 height <long>
134 width <long>
135 bitdepth <byte>
136 [using grayscale+color+palette+alpha]
137 [with interlace] # Adam7 assumed if interlacing on
138 }
139 PLTE {
141 [<rgb>]* # RGB triples or X color names
142 }
143 IDAT {
145 <data>
146 }
147 gAMA {<float>}
149 cHRM {
151 white (<float>,<float>) # White point x and y
152 red (<float>,<float>)
153 green (<float>,<float>)
154 blue (<float>,<float>)
155 }
156 sRGB {<byte>} # Colorimetry intent, range 0-3
158 iCCP { # International Color Consortium profile
160 name <keyword>
161 profile <data>
162 }
163 sBIT {
165 red <byte> # Color images only
166 blue <byte> # Color images only
167 green <byte> # Color images only
168 gray <byte> # Grayscale images only
169 alpha <byte> # Images with alpha only
170 }
171 bKGD {
173 red <short> # Color images only
174 blue <short> # Color images only
175 green <short> # Color images only
176 gray <short> # Grayscale images only
177 index <byte> # Paletted images only
178 }
179 hIST {
181 <short> [, <short>]* # Count must match palette size
182 }
183 tRNS {
185 [gray <short>] # Grayscale images only
186 [red <short>] # True-color images only
187 [green <short>] # True-color images only
188 [blue <short>] # True-color images only
189 [<byte>]* # Paletted images only
190 }
191 pHYs {
193 xpixels <long>
194 ypixels <long>
195 [per meter]
196 }
197 tIME {
199 year <short>
200 month <byte>
201 day <byte>
202 hour <byte>
203 minute <byte>
204 second <byte>
205 }
206 tEXt { # Ordinary text chunk
208 keyword <keyword>
209 text <string>
210 }
211 zTXt { # Compressed text chunk
213 keyword <keyword>
214 text <string>
215 }
216 iTXt { # International UTF-8 keyword
218 language <keyword>
219 keyword <keyword>
220 translated <keyword> # Translation of the keyword
221 text <string>
222 [compressed]
223 }
224 oFFs {
226 xoffset <slong>
227 yoffset <slong>
228 [unit pixels|micrometers]*
229 }
230 sPLT {
232 name <keyword>
233 depth <byte>
234 [<rgb>, <short>, <short>]* # Color followed by alpha and frequency
235 }
236 pCAL {
238 name <keyword>
239 x0 <slong>
240 x1 <slong>
241 mapping linear|euler|exponential|hyperboli unit <string>
242 [parameters <string>]
243 }
244 sCAL {
246 unit meter|radian
247 width <string>
248 height <string>
249 }
250 IMAGE {
252 options identity+packing+packswap+invert_mono
253 +shift+bgr+swap_alpha+invert_alpha+swap_endian+strip_filler
254 pixels <data>
255 }
256 gIFg {
258 disposal <byte>
259 input <byte>
260 delay <short>
261 }
262 gIFx {
264 identifier <string> # Must be 8 characters
265 code <string> # Must be 3 characters
266 data <data>
267 }
268 private <string> { # Private chunk declaration
270 <data>
271 }
272
274 The -i option doesn't work yet, and won't until libpng's ability to
275 suppress special handling of IDATs is working. See the distribution
276 TODO file for other minor problems.
277
279 rgb.txt
280 The X colorname database, used for RGB-to-name mappings in the
281 decompiler and name-to-RGB mappings in the compiler.
282
284 pbm(5), ppm(5).
285
287 Eric S. Raymond <esr@snark.thyrsus.com> December 1999. The SNG home
288 page is at http://sng.sourceforge.net/.
289 For more information about PNG, see the PNG website at
291 <http://www.libpng.org/pub/png/>.
292 The W3C recommendation is Portable Network Graphics (PNG) Specification
294 (Second Edition)[1]. The PNG specification is also ISO/IEC 15948.
295
297 1. Portable Network Graphics (PNG) Specification (Second Edition)
298 http://www.w3.org/TR/2003/PR-PNG-20030520/index-noobject.html
299Open Source 01/12/2016 SNG(1)