sng - Online in the Cloud

PROGRAM:

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>
}

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