Provided by: sng_1.1.0-3_amd64 
NAME
sng - compiler/decompiler for Scriptable Network Graphics
SYNOPSIS
sng [-vV] [file...]
DESCRIPTION
The sng program translates between PNG (Portable Network Graphics) format and SNG (Scriptable Network
Graphics) format. SNG is a printable and editable minilanguage for describing PNG files. With sng, it is
easy to view and edit exotic PNG chunks not supported by graphics editors; also, since SNG is easy to
generate from scripts, sng may be useful at the end of a pipeline that programmatically generates PNG
images.
An SNG description consists of a series of chunk specifications in a simple editable text format. These
generally correspond one-for-one to PNG chunks. There is one exception; the IMAGE chunk specification is
automatically translated into an IDAT chunk (doing appropriate interlacing, compression, etcetera).
Given no file arguments, sng translates stdin to stdout. In this mode, it checks the first character. If
that character is printable, the input stream is assumed to contain SNG; sng looks for an #SNG leader and
tries to translate the file to PNG. If the character is non-printable, the input stream is assumed to
contain PNG; sng tries to translate it to SNG.
For each file that sng operates on, it does its conversion according to the file extension (.png or
.sng). The result file has the same name left of the dot as the original, but the opposite extension and
type.
The -V option makes sng identify itself and its version, then exit. The -v option makes sng report on
what files it is converting.
SNG LANGUAGE SYNTAX
In general, the SNG language is token-oriented with tokens separated by whitespace. Anywhere that
whitespace may appear, a `#' comment leader causes all characters up to the next following newline to be
ignored. The characters `:' and `;' are treated as whitespace, except the `;' terminates a data element
(see below).
In the syntax descriptions below, lines between {} may occur in any order. Elements bracketed in [] are
optional; a sequence bracketed by []* may be repeated any number of times. Elements separated by | are
alternatives. Elements separated by plus signs are an attribute set; any sequence of one or more of those
element tokens is valid.
The elements <byte>, <short>, <long>, <float>, <string> are byte numeric, short-integer numeric,
long-integer numeric, and float numeric literals respectively (all unsigned). The <slong> element is a
signed long-numeric literal. All numerics use C conventions; that is, they are decimal unless led by 0x
(hex) or 0 (octal).
The element <string> is any number of doublequote-delimited character string literals. C-style escapes
(\n, \t, \b, \r or \ followed by octal or hex digits) are interpreted. The result is the concatenation of
all the literals.
The element <keyword> is a doublequote-delimited PNG keyword; that is, a string of no more than 79
printable Latin-1 characters or spaces, with no leading and no trailing and no consecutive spaces.
A <data> element consists of a sequence of byte specifications in any of the following formats. Either
'}' or ';' ends a data literal; `}' also ends the enclosing chunk.
1. string format is an SNG string literal or sequence of string literals (see above). The bytes of data
are the string contents.
2. base64 format is signaled by the leading token `base64'. This encoding can only be used when the
values of all bytes is less than 64. It encodes each byte as a single character, with decimal digits
representing values 0-9, followed by A-Z for 10-35, followed by a-z for 36-61, followed by + for 62 and /
for 63. Base64 format can be used if the image either has total (color plus alpha) bit depth of four or
less, or it is a spaletted image with 64 or fewer colors. Whitespace is ignored. Note that this encoding
is only very loosely related to RFC2045 base-64 encoding, which uses a different mapping of bytes to
values, and supports encoding of arbitrary binary data.
3. hex format is signaled by the leading token `hex'. In hex format, each byte is specified by two hex
digits (0123456789abcdef), most significant first. Whitespace is ignored.
4. P1 format is Portable Bit Map (PBM) format P1. A decimal height and width follow; it is a fatal error
for them to fail to match the IHDR dimensions. Following this, the only non-whitespace characters are
expected to be `0' and `1', with the obvious values. Whitespace is ignored.
5. P3 format is Portable Pixel Map (PPM) format P3. A decimal height and width follow; it is a fatal
error for them to fail to match the IHDR dimensions. A maximum channel value in decimal follows; it is a
fatal error for any following channel value to exceed this value. Following this are triples of decimal
channel values representing RGB triples. Whitespace separates decimal channel values but is otherwise
ignored.
An <rgb> element may be expanded to:
(<byte>, <byte>, <byte>) | <string>
That is, it is either a paren-enclosed list of RGB values or a string naming a color named in the X RGB
database. Note that color names are not necessarily portable between hosts or even displays, due to
different screen gammas and colorimetric biases. For this reason, the SNG decompiler generates color
names in comments.
IMAGE segments contain unpacked and uninterlaced raster data. There will be exactly one IMAGE per SNG
dump, containing the pixel data from all IDAT chunks, unless the -i option is on. In that case, there
will be multiple IDAT chunks containing raw (compressed) image data.
The options member of an IMAGE chunk (if present) sets image write transformations, supplying the third
argument of the png_write_png() call used for output. Note that for images with a bit depth of less than
8, there is a default `packing' transformation. Consult the libpng(3) manual page for details.
Every SNG file must begin with the string "#SNG", followed by optional SNG version information, followed
by a colon (`:', ASCII 58) character. The remainder of the first line is ignored by SNG.
Comments in the syntax diagram describe intended semantics. This specification should be read in
conjunction with the PNG standard.
IHDR {
height <long>
width <long>
bitdepth <byte>
[using grayscale+color+palette+alpha]
[with interlace] # Adam7 assumed if interlacing on
}
PLTE {
[<rgb>]* # RGB triples or X color names
}
IDAT {
<data>
}
gAMA {<float>}
cHRM {
white (<float>,<float>) # White point x and y
red (<float>,<float>)
green (<float>,<float>)
blue (<float>,<float>)
}
sRGB {<byte>} # Colorimetry intent, range 0-3
iCCP { # International Color Consortium profile
name <keyword>
profile <data>
}
sBIT {
red <byte> # Color images only
blue <byte> # Color images only
green <byte> # Color images only
gray <byte> # Grayscale images only
alpha <byte> # Images with alpha only
}
bKGD {
red <short> # Color images only
blue <short> # Color images only
green <short> # Color images only
gray <short> # Grayscale images only
index <byte> # Paletted images only
}
hIST {
<short> [, <short>]* # Count must match palette size
}
tRNS {
[gray <short>] # Grayscale images only
[red <short>] # True-color images only
[green <short>] # True-color images only
[blue <short>] # True-color images only
[<byte>]* # Paletted images only
}
pHYs {
xpixels <long>
ypixels <long>
[per meter]
}
tIME {
year <short>
month <byte>
day <byte>
hour <byte>
minute <byte>
second <byte>
}
tEXt { # Ordinary text chunk
keyword <keyword>
text <string>
}
zTXt { # Compressed text chunk
keyword <keyword>
text <string>
}
iTXt { # International UTF-8 keyword
language <keyword>
keyword <keyword>
translated <keyword> # Translation of the keyword
text <string>
[compressed]
}
oFFs {
xoffset <slong>
yoffset <slong>
[unit pixels|micrometers]*
}
sPLT {
name <keyword>
depth <byte>
[<rgb>, <short>, <short>]* # Color followed by alpha and frequency
}
pCAL {
name <keyword>
x0 <slong>
x1 <slong>
mapping linear|euler|exponential|hyperboli unit <string>
[parameters <string>]
}
sCAL {
unit meter|radian
width <string>
height <string>
}
IMAGE {
options identity+packing+packswap+invert_mono
+shift+bgr+swap_alpha+invert_alpha+swap_endian+strip_filler
pixels <data>
}
gIFg {
disposal <byte>
input <byte>
delay <short>
}
gIFx {
identifier <string> # Must be 8 characters
code <string> # Must be 3 characters
data <data>
}
private <string> { # Private chunk declaration
<data>
}
BUGS
The -i option doesn't work yet, and won't until libpng's ability to suppress special handling of IDATs is
working. See the distribution TODO file for other minor problems.
FILES
rgb.txt
The X colorname database, used for RGB-to-name mappings in the decompiler and name-to-RGB mappings in
the compiler.
SEE ALSO
pbm(5), ppm(5).
AUTHOR
Eric S. Raymond <esr@snark.thyrsus.com> December 1999. The SNG home page is at
http://sng.sourceforge.net/.
For more information about PNG, see the PNG website at <http://www.libpng.org/pub/png/>.
The W3C recommendation is Portable Network Graphics (PNG) Specification (Second Edition)[1]. The PNG
specification is also ISO/IEC 15948.
NOTES
1. Portable Network Graphics (PNG) Specification (Second Edition)
http://www.w3.org/TR/2003/PR-PNG-20030520/index-noobject.html