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RLE(5) FreeBSD File Formats Manual RLE(5)

rle - Run length encoded file format produced by the rle library

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
            xsize,                      /* Size of saved box
Byte 10
(flags) The following flags are defined:
(0x1) If set, clear the frame buffer to background color before restoring.
(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).
(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.
(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.
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, 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.


Spencer W. Thomas, Todd Fuqua
9/14/82 4th Berkeley Distribution

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