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