DESIGN OF THE UTAH RLE FORMAT
Spencer W. Thomas
University of Utah, Department of Computer Science
ABSTRACT
The Utah RLE (Run Length Encoded) format is designed to provide an
efficient, device independent means of storing multi-level raster
images. Images of arbitrary size and depth can be saved. The design
of the format is presented, followed by descriptions of the library
routines used to create and read RLE format files.
1. Introduction
The Utah RLE (Run Length Encoded) format is designed to provide an efficient,
device independent means of storing multi-level raster images. It is not
designed for binary (bitmap) images. It is built on several basic concepts.
The central concept is that of a channel. A channel corresponds to a single
color, thus there are normally a red channel, a green channel, and a blue
channel. Up to 255 color channels are available for use; one channel is
reserved for "alpha" data. Although the format supports arbitrarily deep
channels, the current implementation is restricted to 8 bits per channel.
Image data is stored in an RLE file in a scanline form, with the data for
each channel of the scanline grouped together. Runs of identical pixel values
are compressed into a count and a value. However, sequences of differing
pixels are also stored efficiently (not as a sequence of single pixel runs).
The file header contains a large amount of information about the image,
including its size, the number of channels saved, whether it has an alpha
channel, an optional color map, and comments. The comments may be used to add
arbitrary extra information to the saved image.
A subroutine interface has been written to allow programs to read and write
files in the RLE format. Two interfaces are available, one that completely
interprets the RLE file and returns scanline pixel data, and one that returns a
list of "raw" run and pixel data. The second is more efficient, but more
difficult to use, the first is easy to use, but slower.
The Utah RLE format has been used to save images from many sources, and to
display saved images on many different displays and from many different
computers.
2. Description of RLE Format
All data in the RLE file is treated as a byte stream. Where quantities
larger than 8 bits occur, they are written in PDP-11 byte order (low order byte
first).
The RLE file consists of two parts, a header followed by scanline data. The
header contains general information about the image, while the scanline data is
a stream of operations describing the image itself.
2.1. The Header
Figure 2-1: RLE file header
The header has a fixed part and a variable part. A diagram of the header is
shown in Figure 2-1. The magic number identifies the file as an RLE file.
Following this are the coordinates of the lower left corner of the image and
the size of the image in the X and Y directions. Images are defined in a first
quadrant coordinate system (origin at the lower left, X increasing to the
right, Y increasing up.) Thus, the image is enclosed in the rectangle
[xpos,xpos+xsize-1]X[ypos,ypos+ysize-1].
The position and size are 16 bit integer quantities; images up to 32K square
may be saved (the sizes should not be negative).
A flags byte follows. There are currently four flags defined:
ClearFirst If this flag is set, the image rectangle should first be cleared
to the background color (q.v.) before reading the scanline data.
NoBackground If this flag is set, no background color is supplied, and the
ClearFirst flag should be ignored.
Alpha This flag indicates the presence of an "alpha" channel. The
alpha channel is used by image compositing software to correctly
blend anti-aliased edges. It is stored as channel -1 (255).
Comments If this flag is set, comments are present in the variable part
of the header, immediately following the color map.
The next byte is treated as an unsigned 8 bit value, and indicates the number
of color channels that were saved. It may have any value from 0 to 254
(channel 255 is reserved for alpha values).
The pixelbits byte gives the number of bits in each pixel. The only value
currently supported by the software is 8 (in fact, this byte is currently
ignored when reading RLE files). Pixel sizes taking more than one byte will be
packed low order byte first.
The next two bytes describe the size and shape of the color map. Ncmap is
the number of color channels in the color map. It need not be identical to
ncolors, but interpretation of values of ncmap different from 0, 1, or ncolors
may be ambiguous, unless ncolors is 1. If ncmap is zero, no color map is
saved. Cmaplen is the log base 2 of the length of each channel of the color
map. Thus, a value for cmaplen of 8 indicates a color map with 256 entries per
channel.
Immediately following the fixed header is the variable part of the file
header. It starts with the background color. The background color has ncolors
entries; if necessary, it is filled out to an odd number of bytes with a filler
byte on the end (since the fixed header is an odd number bytes long, this
returns to a 16 bit boundary).
Following the background color is the color map, if present. Color map
values are stored as 16 bit quantities, left justified in the word. Software
interpreting the color map must apply a shift appropriate to the application or
to the hardware being used. This convention permits use of the color map
without knowing the original output precision. The channels of the map are
stored in increasing numerical order (starting with channel 0), with the
entries of each channel stored also in increasing order (starting with entry
0). The color map entries for each channel are stored contiguously.
Comments, if present, follow the color map. A 16 bit quantity giving the
length of the comment block comes first. If the length is odd, a filler byte
will be present at the end, restoring the 16 bit alignment (but this byte is
not part of the comments). The comment block contains any number of
null-terminated text strings. These strings will conventionally be of the form
"name=value", allowing for easy retrieval of specific information. However,
there is no restriction that a given name appear only once, and a comment may
contain an arbitrary string. The intent of the comment block is to allow
information to be attached to the file that is not specifically provided for in
the RLE format.
2.2. The Scanline Data
The scanline data consists of a sequence of operations, such as Run,
SetChannel, and Pixels, describing the actual image. An image is stored
starting at the lower left corner and proceeding upwards in order of increasing
scanline number. Each operation and its associated data takes up an even
number of bytes, so that all operations begin on a 16 bit boundary. This makes
the implementation more efficient on many architectures.
Figure 2-2: RLE file operand formats
Each operation is identified by an 8 bit opcode, and may have one or more
operands. Single operand operations fit into a single 16 bit word if the
operand value is less than 256. So that operand values are not limited to the
range 0..255, each operation has a long variant, in which the byte following
the opcode is ignored and the following word is taken as a 16 bit quantity.
The long variant of an opcode is indicated by setting the bit 0x
