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.
Scope:
See Abstract
In this part only the relevant descriptions of the format are included.
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.
____________________________
| Magic number |
----------------------------
| xpos |
----------------------------
! ypos |
----------------------------
| xsize |
----------------------------
| ysize |
----------------------------
| flags | ncolors |
----------------------------
| pixelbytes | ncmap |
----------------------------
| cmaplen | red bg |
----------------------------
| green bg | blue bg |
----------------------------
| color map entry 0 |
----------------------------
| color map entry 1 |
----------------------------
| . . . |
----------------------------
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:
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.
________________________
Short Operand |00| op-code | operand |
------------------------
Byte 0 Byte 1 Byte 2 Byte 3
____________________________________________
Long Operand |01| op-code | filler | operand | operand |
| | | |low byte |high byte|
--------------------------------------------
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 0x40 in the
opcode (this allows for 64 opcodes, of which 6 have been used so far.) The two
single operand formats are shown pictorially in Figure 2-2.The individual operations will now be discussed in detail. The descriptions are phrased in terms of the actions necessary to interpret the file. Three indices are necessary: the current channel, the scanline number, and the pixel index. The current channel is the channel to which data operations apply. The scanline number is just the Y position of the scanline in the image. The pixel index is the X position of the pixel within the scanline. The operations are: