sng - compiler/decompiler for Scriptable Network Graphics

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 maskes sng identify itself and its version, then exit. The -i option causes IDAT chunks in a PNG to be dumped in raw form as IDAT chanks rather than as a reassembled IMAGE. The -v option makes sng report on what files it is converting.

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, float numeriliterals 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'. It is RFC2045 base-64 encoding, 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.

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 charaters 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> }

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.

rgb.txt

The X colorname database, used for RGB-to-name mappings in the decompiler and name-to-RGB mappings in the compiler.

pbm(5), ppm(5).

Eric S. Raymond <esr@snark.thyrsus.com> December 1999. The SNG home page is at http://sng.sourceforge.net/: http://sng.sourceforge.net/.

For more information about PNG, see the PNG website at <http://www.libpng.org/pub/png/: http://www.libpng.org/pub/png/>.

The W3C recommendation is Portable Network Graphics (PNG) Specification (Second Edition): http://www.w3.org/TR/2003/PR-PNG-20030520/index-noobject.html. The PNG specification is also ISO/IEC 15948.