Inside SUN Rasterfile
			    Jamie Zawinski
			jwz@teak.berkeley.edu

The manpage for rasterfile(5) doesn't say anything about the format of
byte-encoded images, or  about plane/scanline  ordering in multi-plane
images.

The first thing in the file is

	struct rasterfile {
		int ras_magic;
		int ras_width;
		int ras_height;
		int ras_depth;
		int ras_length;
		int ras_type;
		int ras_maptype;
		int ras_maplength;
		};

The ras_magic field always contains the following constant:

	#define RAS_MAGIC 0x59a66a95

The ras_length field  is the  length of the image  data (which  is the
length of  the file minus   the  length  of the header  and colormap).
Catch: this is sometimes zero  instead, so you  can't really depend on
it.

The ras_type  field is  ras_old=0, ras_standard=1, ras_byte_encoded=2,
or  ras_experimental=FFFF.  There  doesn't seem to   be any difference
between OLD and STANDARD except that the ras_length  field is always 0
in OLD.

I didn't deal with cmaps, so from the  man page: "The  ras_maptype and
ras_maplength  fields contain  the type and   length in   bytes of the
colormap values, respectively.  If ras_maptype is not RMT_NONE and the
ras_maplength is not 0, then the colormap values are the ras_maplength
bytes    immediately  after  the   header.   These   values are either
uninterpreted bytes (usually  with the ras_maptype  set to RMT_RAW) or
the equal length red, green and blue vectors, in  that order (when the
ras_maptype is RMT_EQUAL_RGB).  In the  latter case, the ras_maplength
must be three  times the size  in bytes of  any  one of the  vectors."
Regardless of width, the stored scanlines are  rounded up to multiples
of 16 bits.

I found the following description of byte-length encoding in Sun-Spots
Digest, Volume 6, Issue 84:

> From:    jpm%green@lanl.gov (Pat McGee)
> Subject: Re: Format for byte encoded rasterfiles (1)
> 
> The format is composed of many sequences of variable length records.
> Each record may be 1, 2, or 3 bytes long.
> 
>  o  If the first byte is not 0x80, the record is one byte long, and 
>     contains a pixel value.  Output 1 pixel of that value.
>  o  If the first byte is 0x80 and the second byte is zero, the record
>     is two bytes long.  Output 1 pixel with value 0x80.
>  o  If the first byte is 0x80, and the second byte is not zero, the 
>     record is three bytes long.  The second byte is a count and the 
>     third byte is a value.  Output (count+1) pixels of that value.
> 
> A run is not terminated at the end of a scan line.  So, if there are 
> three lines of red in a picture 100 pixels wide, the first run will 
> be 0x80 0xff 0x<red>, and the second will be 0x80 0x2b 0x<red>.
> 
> 	Pat McGee, jpm@lanl.gov