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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