RLE(5) 5 (9/14/82) RLE(5)
NAME
rle - Run length encoded file format produced by the rle
library
DESCRIPTION
The output file format is (note: all words are 16 bits, and
in PDP-11 byte order):
Word 0
A "magic" number 0xcc52. (Byte order 0x52, 0xcc.)
Words 1-4
The structure (chars saved in PDP-11 order)
{
short xpos, /* Lower left corner
ypos,
xsize, /* Size of saved box
ysize;
}
Byte 10
(flags) The following flags are defined:
H_CLEARFIRST
(0x1) If set, clear the frame buffer to background
color before restoring.
H_NO_BACKGROUND
(0x2) If set, no background color is supplied. If
H_CLEARFIRST is also set, it should be ignored (or
alternatively, a clear-to-black operation could be
performed).
H_ALPHA
(0x4) If set, an alpha channel is saved as color
channel -1. The alpha channel does not contribute
to the count of colors in ncolors.
H_COMMENT
(0x8) If set, comments will follow the color map
in the header.
Byte 11
(ncolors) Number of color channels present. 0 means
load only the color map (if present), 1 means a B&W
image, 3 means a normal color image.
Byte 12
(pixelbits) Number of bits per pixel, per color
channel. Values greater than 8 currently will not
work.
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RLE(5) 5 (9/14/82) RLE(5)
Byte 13
(ncmap) Number of color map channels present. Need not
be identical to ncolors. If this is non-zero, the
color map follows immediately after the background
colors.
Byte 14
(cmaplen) Log base 2 of the number of entries in the
color map for each color channel. I.e., would be 8 for
a color map with 256 entries.
Bytes 15-...
The background color. There are ncolors bytes of
background color. If ncolors is even, an extra padding
byte is inserted to end on a 16 bit boundary. The
background color is only present if H_NO_BACKGROUND is
not set in flags. IF H_NO BACKGROUND is set, there is
a single filler byte. Background color is ignored, but
present, if H_CLEARFIRST is not set in flags.
If ncmap is non-zero, then the color map will follow as
ncmap*2^cmaplen 16 bit words. The color map data is
left justified in each word.
If the H_COMMENT flag is set, a set of comments will
follow. The first 16 bit word gives the length of the
comments in bytes. If this is odd, a filler byte will
be appended to the comments. The comments are
interpreted as a sequence of null terminated strings
which should be, by convention, of the form name=value,
or just name.
Following the setup information is the Run Length
Encoded image. Each instruction consists of an opcode,
a datum and possibly one or more following words (all
words are 16 bits). The opcode is encoded in the first
byte of the instruction word. Instructions come in
either a short or long form. In the short form, the
datum is in the second byte of the instruction word; in
the long form, the datum is a 16 bit value in the word
following the instruction word. Long form instructions
are distinguished by having the 0x40 bit set in the
opcode byte. The instruction opcodes are:
SkipLines (1)
The datum is an unsigned number to be added to the
current Y position.
SetColor (2)
The datum indicates which color is to be loaded with
the data described by the following ByteData and
RunData instructions. Typically, 0->red, 1->green,
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RLE(5) 5 (9/14/82) RLE(5)
2->blue. The operation also resets the X position to
the initial X (i.e. a carriage return operation is
performed).
SkipPixels (3)
The datum is an unsigned number to be added to the
current X position.
ByteData (5)
The datum is one less than the number of bytes of color
data following. If the number of bytes is odd, a
filler byte will be appended to the end of the byte
string to make an integral number of 16-bit words. The
X position is incremented to follow the last byte of
data.
RunData (6)
The datum is one less than the run length. The
following word contains (in its lower 8 bits) the color
of the run. The X position is incremented to follow
the last byte in the run.
EOF (7)
This opcode indicates the logical end of image data. A
physical end-of-file will also serve as well. The EOF
opcode may be used to concatenate several images in a
single file.
SEE ALSO
librle(3)
AUTHOR
Spencer W. Thomas, Todd Fuqua
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